WO2019200468A1

WO2019200468A1 - Gas-based reactivity control system

Info

Publication number: WO2019200468A1
Application number: PCT/CA2019/050467
Authority: WO
Inventors: Ibrahim Khalil ATTIEH; Adam Eckhardt; Pamela Darlene TUME; Julian William Francis MILLARD; Zane Harry WALKER; Zachary Todd DEMERS; Mei Chen; Anna KARMANOVA; Jonathan Andrew TYO; Michael SOULARD; Martin Walter Macdonald; Maliha MASRORR; Smeena QAZI
Original assignee: Candu Energy Inc
Current assignee: Candu Energy Inc
Priority date: 2018-04-16
Filing date: 2019-04-16
Publication date: 2019-10-24
Anticipated expiration: 2020-10-16
Also published as: AR115057A1; CA3097438A1

Abstract

A gas-based reactivity control system for a nuclear reactor. The system control reactivity of a nuclear reactor by injecting pressurized neutron absorbing gas into gas tubes that are positioned inside a reactor core. Injector tanks are filled with the neutron absorbing gas. Upon detection of a reactivity control trigger, the fluid communication channels between the injector tanks and the gas tubes are opened. The pressure differential between the injection tanks and the gas tubes causes the neutron absorbing gas to flow into the gas tubes, thereby introducing negative reactivity into the reactor core. The negative reactivity may be introduced into the core relatively quickly, compared to conventional systems. The gas may be left in the gas tube to keep the reactor shut down. The gas in the gas tubes may be vented in a controlled manner to control reactivity increase in the core when restarting the reactor.

Description

GAS-BASED REACTIVITY CONTROL SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims all benefit including priority to U.S. Provisional Patent Application 62/658,396, filed April 16, 2018, and entitled“GAS-BASED REACTIVITY CONTROL SYSTEM”. This application is hereby incorporated by reference.

FIELD

[0002] The following application relates to controlling the reaction rate in a nuclear reactor, and in particular to systems and methods for controlling the reaction rate in a nuclear reactor with a neutron absorbing gas.

BACKGROUND

[0003] A nuclear reactor, such as a CANDU™ reactor (“CANada Deuterium Uranium”), can generate electrical energy through a fission process. During a fission reaction, a neutron is absorbed by a material, such as uranium, which causes the uranium to split into fission products and release energy, radiation, and a number of free neutrons. These neutrons are slowed or moderated to improve the reaction. In turn, these free neutrons are absorbed by more uranium, thereby creating a fission chain reaction. The reaction rate may be controlled by absorbing the neutrons and varying the moderator. The generated heat is used to heat up water to make steam to power turbines and generators, which produce electrical energy.

[0004] CANDU™ reactors may have one or more shut down rods, adjuster rods, or mechanical control absorber rods to control reaction rate in the reactor. CANDU™ reactors may use on line fuelling to operate at a low positive reactivity, such that a relatively low reactivity may be introduced to impact control of the reaction rate of the reactor.

[0005] During operation of the reactor, there may be an increase in reactivity, which may be expected or may be unexpected.

[0006] Various systems have been developed for controlling the reaction rate of a nuclear reactor. For example, a first shutdown system comprises neutron absorbing rods that are automatically spring-accelerated into the reactor core and stop the fission chain reaction upon detection of a safety condition. However, this first shutdown system (hereinafter referred to as “SDS1”) may be relatively slow in injecting the neutron absorbing rods into the core. It takes some time for the neutron absorbing rods to be fully inserted into the reactor, and during this time, the reactivity may increase or continue to increase. Moreover, during the time that the rods are being inserted into the reactor core, certain components of the reactor may already be damaged. As another example, a second shutdown system (hereinafter referred to as“SDS2”) injects a liquid, or poison, which may comprise gadolinium nitrate, into the moderator. The liquid poison absorbs neutrons inside the reactor to stop the fission chain reaction. Poisoning the moderator may shut down the reactor, and may maintain the reactor in shut down in an over-poisoned state. However, when restarting the reactor, it may take an extended amount of time and labour to clean the reactor core and separate the poison and the moderator to remove the liquid poison and to restart the nuclear reactor. The cost of returning the reactor to operational condition may be a significant economic penalty, and may be exacerbated if the SDS2 is engaged by accident.

SUMMARY

[0007] According to an aspect of the invention a gas-based reactivity control system for a nuclear reactor is provided. The gas-based reactivity control system comprises: an injection tank for receiving and containing pressurized neutron absorbing gas; a gas tube at least a portion of which is positioned inside a reactor core; a valve system coupled to the injection tank and the gas tube providing a controllable fluid communication channel between the injection tank and the gas tube; and a controller for selectively actuating the valve system to control fluid communication between the injection tank and the gas tube.

[0008] According to another aspect, a method for reducing reactivity in a nuclear reactor is provided. The method comprises pressurizing neutron absorbing gas in an injection tank; and upon detection of a reactivity control trigger, opening fluid communication channel between the injection tank and a gas tube at least a portion of which is positioned inside the reactor core.

[0009] According to another aspect, a method of purging a gas rod filled with neutron absorbing gas is provided. The method comprises suctioning the neutron absorbing gas from the gas rod through a coiled pipe at a flow rate, wherein a transit time through the coiled pipe at the desired flowrate is greater than the half-life of a radioactive material in the gas rod; circulating an inert gas through the gas rod; and reducing pressure within the gas rod to vacuum pressure. [0010] Many further features and combinations thereof concerning embodiments described herein will appear to those skilled in the art following a reading of the instant disclosure.

BRIEF DESCRIPTION OF DRAWINGS

[0011] In the figures which illustrate example embodiments:

[0012] Figure 1 is a system diagram of a gas system for controlling the reaction rate in a nuclear reactor;

[0013] Figure 2 is a system diagram of the gas system of Figure 1 ;

[0014] Figure 3A is a schematic of the gas system of Figure 1 ;

[0015] Figure 3B is another embodiment of a schematic of the gas system of Figure 1 ;

[0016] Figure 4 is a schematic of a current reactivity deck layout and a proposed reactivity deck layout of a reactor having the gas system of Figure 1 ;

[0017] Figure 5 is a schematic of a controller of the system of Figure 1 ;

[0018] Figure 6 is a graph depicting reactor power after a postulated accident without activation of a shutdown system as a function of time;

[0019] Figure 7 is a graph depicting a bounding postulated accident case having excess reactivity insertion without activation of a shutdown system as a function of time;

[0020] Figure 8 is a graph depicting reactor power after a postulated bounding accident with and without a shutdown system activation as a function of time;

[0021] Figure 9 is a graph depicting shut-off rods percent insertion into the nuclear reactor core as a function of time;

[0022] Figure 10 is a graph depicting excess or negative reactivity as a function of krypton density for a system of gas shut-off rods;

[0023] Figure 11 is a graph depicting excess or negative reactivity as function of borane density in a system of gas shut-off rods; [0024] Figure 12 is a graph depicting krypton density in the gas tube as a function of time; and

[0025] Figure 13 is a flow chart depicting a method of using the gas system of Figure 1.

DETAILED DESCRIPTION

[0026] A gas-based reactivity control system for a nuclear reactor using a neutron absorbing gas and a method for its use are disclosed. Pressurized neutron absorbing gas (e.g. krypton or isotopically separated krypton gas Kr-83) in storage tanks may be injected into voided tubes in the moderator region of the reactor core. Due to the relatively high pressure of the gas stored in the storage tank compared to the voided tubes, the speed of the krypton gas insertion into the voided tube may be 10 to 20 times faster than that of a solid rod (e.g. of SDS1). Conventional shutdown systems using solid rods comprising neutron absorbers are able to neutralize reactivity in the core. In some situations, due to the higher speed of the injection of the gas relative to the insertion of the solid rods, in some embodiments, the gas-based reactivity control system can neutralize reactivity in the core more quickly.

[0027] In some embodiments, the speed of the gas injection and/or the control of the amount of neutron absorbing gas can moderate increases in reactivity without necessarily completely neutralizing reactivity. In some situations, this can provide a more moderated consistent reactor power output.

[0028] A number of options for the neutron absorbing gas (also referred to as poison gas herein) may exist to be used in the system, including krypton, xenon, or boron trihydride (Borane). These options may include mixtures and in use of different gases for different functions (for example, boron for shutdown transient and Krypton for hold-down and start-up). In some embodiments, the neutron absorbing gas of the system is natural Krypton gas or isotopically separated Krypton gas (e.g. ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³KR, ⁸⁴Kr, ⁸⁶Kr). While the present applicant is not limited to a single type of neutron absorbing gas, or isotopes thereof, certain isotopes have improved neutron capture cross section. As such, one or more isotopes of neutron absorbing gases, such as Krypton, may be selected based on their neutron capture cross section. In an embodiment, the isotopes of Kypton has a higher neutron capture cross section than natural krypton gas, e.g. ⁷⁸Kr, or ⁸³Kr.

[0029] Purity of the neutron absorbing gases is also influences the ability of the gases to absorb neutrons as neutron. A purer neutron absorbing gas will have a higher gas density and capability of absorbing neutron. In an embodiment, ultra high purity neutron absorbing gas, i.e. 99.999% to 99.995% pure.

[0030] The system may hold down or increase reactivity in a nuclear reactor. The system may hold pressurized neutron absorbing gas in a tube, at least a portion of which is in the reactor core. When desired, the gas in the tube may be removed into a purge tank.

[0031] In some embodiments, a system with sufficient pressure (gas density) is needed to obtain a rapid negative reactivity insertion of the order of 10 mk to 12 mk. The 10 mk to 12 mk of negative reactivity may be enough to suppress the transient excess reactivity until the conventional shutdown systems (e.g. SDS1 or SDS2), in particular, the rods of SDS1 , are fully inserted to the core to keep the reactor in a shutdown state. In some embodiments, the required system pressure is impacted by the purity and neutron capture cross section of the neutron absorbing gas - the higher the purity and/or neutron capture cross section of the neutron absorbing gas, the lower system pressure is required to achieve a desired reduction in reactivity.

[0032] In some embodiments, the system is configured to shut down or otherwise control the reactivity of the nuclear reactor, for example, in case of an increase of reactivity or power of the reactor core.

[0033] In some embodiments, the system may reduce the amount of time required to restart the nuclear reactor after a shutdown by adjusting reactivity during reactor restart.

[0034] The system may mitigate the size, strength, or rate of an increase in reactivity or power of the reactor. The speed of negative reactivity insertion into the reactor core may be a factor for controlling the reactivity or power of the reactor as the reactivity is increasing. In some embodiments, the system injects neutron absorbing gas into hollow tubes within the core with a greater speed than that of existing shutoff rods.

[0035] In some embodiments, the system acts as a supplementary safety feature to existing reactivity control or shutdown systems, such as SDS1 and SDS2. Accordingly, the system may provide early suppression of the increase in power or reactivity while the shutoff rods of SDS1 or the liquid poison of SDS2 provide the reactivity depth needed for a large margin of subcriticality in the long term. In some embodiments, the system may provide a means of evenly increasing reactivity into the range of the normal reactor control. [0036] During an increase in reactivity or power of the reactor, the amount of positive reactivity insertion may be small relative to the negative reactivity insertion by shut-off rods of SDS1 , which may range from -40 mk to -60 mk. From the perspective of the magnitudes of the reactivity, these gas rods may introduce sufficient negative reactivity to compensate for the increase in reactivity (e.g. approximately 8 mk or more). However, the insertion rate of the shut off rods, which may be relatively slow (e.g. about 1.4 seconds for full insertion of the shut off rods into the reactor core), may allow the reactivity to increase and develop with reactor power also increasing while the shut-off rods are entering the core before suppressing the increase in reactivity and shutting down the reactor. The insertion of more negative reactivity to

compensate for the increase in reactivity may control the reaction rate of the reactor relatively quickly and suppress the increase in reactivity.

[0037] The system may reduce the required start up time after a prolonged reactor shutdown. Reactor start up after a Guaranteed Shutdown State (GSS) may require an extended time period to remove neutron poison from the moderator. In some embodiments, the system maintains a concentration of neutron absorbing gas within hollow tubes in the reactor core to reduce the required poison concentration and shorten the start-up time of the reactor.

[0038] The system may control the reaction rate of a nuclear reactor during an increase in reactivity at a faster rate than existing methods, such as SDS1 and SDS2. The system’s control of the reaction rate of the reactor may increase the safety margin of operating the reactor, which may reduce the possibility of fuel failure and increase the safety of the operation of the reactor. By using the system, the process for preparing the reactor for restart may be simplified, as the neutron absorbing gas used by the system may not be mixed or come into contact with the fuel or moderator of the reactor, and may be removed in a relatively even manner.

[0039] In some embodiments, the gas reaction rate control system comprises gas tanks (fire tanks, injection tanks) that are filled with a neutron absorbing gas. The gas tanks are coupled to voided guide tubes (gas rods) that are placed inside the reactor core. In some embodiments, the gas tubes are inserted into existing adjuster rod tubes of the reactor. Once a reactor safety system sends a signal to the gas reaction rate control system, quick opening valves of the gas reaction rate control system open, and allows the neutron absorbing gas to flow from the gas tanks into the gas tubes inside the reactor core. As the gas rod may at least be partially located with the reactor core, once the gas rod is filled with neutron absorbing gas, reactivity in the nuclear reactor may be reduced as neutrons in the reactor are absorbed by the neutron absorbing gas. In some embodiments, the gas flows at high speed from the gas tanks into the gas guide tubes (gas rods) inside the core. The gas density of the neutron absorbing gas is sufficiently high to control the reaction rate of the reactor and bring the reactor to subcritical state. The gas tanks (fire tanks, injection tanks) are connected to refilling lines that replenish the tanks after the system activation. The tanks may be replenished with the neutron absorbing gas after the tanks have fired. The tanks may be replenished relatively quickly after they have fired. The tanks, after being replenished, may be ready to be re-fired. In addition, the pressure of the tanks increase after they are replenished with the neutron absorbing gas, which may prevent the return of gas from the gas tubes. The gas tanks (fire tanks, injection tanks) are connected to purge lines to void the gas guide tubes of neutron absorbing gas to transition the reactor from a shutdown state and return to full power operations in a slow and controlled manner.

[0040] Injecting a negative reactivity early in the transient may reduce the reactivity of the reactor as the reactivity is increasing. The system described herein may inject a neutron absorbing gas (e.g. krypton gas) into one or more gas tubes, at least a portion of which is in the reactor core, to insert sufficient negative reactivity into the reactor core. The system may be used with existing shutdown systems (e.g. SDS1 and SDS2), such that after the neutron absorbing gas is inserted in the gas tubes, the other shutdown systems may be activated (e.g. shutdown rods of SDS1 are inserted into the reactor core to provide adequate reactivity depth for reactor shutdown).

[0041] Figure 1 is a system diagram of a gas-based reactivity control system 100 for a nuclear reactor. The nuclear may be a CANDU™ reactor or another nuclear reactor. In some embodiments, the system 100 comprises an injection tank 102 for receiving and containing pressurized neutron absorbing gas, a gas tube 104 at least a portion of which is positioned inside a reactor core, a valve system 120 coupled to the injection tank 102 and the gas tube 104 providing a controllable fluid communication channel between the injection tank 102 and the gas tube 104, and a controller 500 for selectively actuating the valve system 120 to control fluid communication between the injection tank 102 and the gas tube 104.

[0042] As depicted in Figure 1 , the injection tank 102 is coupled to a top portion of the gas tube 104, such that upon injection of the neutron absorbing gas from the injection tank 102 to the gas tube 104, the gas is first introduced into the top portion of the gas tube 104, and then flows towards the middle portion of the gas tube 104 and the bottom portion of the gas tube 104. [0043] In some embodiments, the injection tank 102 is a tank that is configured to receive, contain, and house a neutron absorbing gas that is pressurized. As depicted in Figure 1 , the injection tank 102 is coupled to a high pressure absorber gas tank or cylinder bank 106. The tank 106 stores the neutron absorbing gas to be injected into the gas tube 104. The neutron absorbing gas stored in the tank 106 is used to fill or refill the injection tank 102 with the neutron absorbing gas. In some embodiments, the tank 106 is coupled to the injection tank 102 using pipes, conduits, hoses, and the like. As depicted in Figure 1 , a regulator and valve is interposed between the tank 106 and the injection tank 102 to control filling of the injection tank 102 with the gas that is contained in the tank 106.

[0044] The injection tank 102 may be refilled after the neutron absorbing gas is injected into the gas tube 104. The pressurization of the injection tank 102 may act to prevent back flow of gas from gas tube 104.

[0045] In some embodiments, the gas tube 104 is configured to receive and contain the neutron absorbing gas. The gas tube 104 extends generally vertically into the reactor core. When the gas is injected into the gas tube 104, in some embodiments, the gas should be proximate to the reactivity mechanism. Accordingly, in such embodiments, the gas tubes 104 are positioned where there is high flux or the most flux. For example, the gas tubes 104 are placed inside the adjuster tubes of the reactor. In some examples, there are 21 or more adjuster tubes in a reactor.

[0046] In some embodiments, where the gas tubes 104 are placed into reactivity sites with existing guide tubes, the guide tubes are removed and the gas tubes 104 are connected to the same locator fittings.

[0047] In some embodiments, the injection tank 102 or gas tube 104 is configured to fit within a reactivity control site (e.g. booster rod or adjuster rod location) of the nuclear reactor.

[0048] As depicted in Figure 1 , the system 100 comprises a central low pressure dump tank 108. The tank 108 is coupled to the bottom portion of the gas tube 104, using pipes, conduits, hoses, and the like. In some embodiments, a vent valve system 140 is coupled to the gas tube 104 and the tank 108 for controlling fluid communication between the gas tube 104 and the tank 108. As depicted in Figure 1 , the gas tube 104 can be vented through one or more lines (e.g. two lines) for redundancy, each isolated with a valve of the vent valve system 140. The gas tube 104 can be vented by controlling the vent valve system 140. Accordingly, the vent valve system 140 controls venting of the gas tube 104. The gas tube 104 may be vented through a line with no vacuum pump 110, or through a line with a vacuum pump 110. In some

embodiments, the gas tube 104 is vented through the line with no vacuum pump 110, and the rate of venting through that line can be controlled by actuation of the vent valve system 140. In such embodiments, the pressure differential between the gas tube 104 and the tank 108 drives gas flow from the gas tube 104 to the tank 108. In some embodiments, the gas tube 104 is vented through the line with the vacuum pump 110. In such embodiments, the vacuum pump 110 pumps the gas in the gas tube 104 to the tank 108. In some embodiments, gases in the tank 108 are vented out through active ventilation into a designated ventilation area configured to receive the gases. Fluid flow from the tank 108 to the active ventilation is controlled by a valve, as depicted in Figure 1.

[0049] When neutron absorbing gas is in the gas tube 104, the vent valve system 140 is controllable to vent the neutron absorbing gas from the gas tube 104 to reduce negative reactivity caused by the neutron absorbing gas in the reactor core at a controlled rate.

[0050] As depicted in Figure 1 , the high pressure absorber gas tank or cylinder bank 106 is coupled to the central low pressure dump tank 108. In some embodiments, fluid communication between the tank 106 and the tank 108 is controlled by the fill pump and a valve, as depicted in Figure 1. Accordingly, the tank 106 can be directly vented to the tank 108 and through the active ventilation using the fill pump and a valve. In some embodiments, the gases in the tank 108 can be pumped into the tank 106 using the fill pump, such that the gases vented from the gas tube 104 can be reused and filled into the injection tank 102. Where the gases vented from the gas tube 104 are reused, such gases may be processed using one or more processing steps such that the processed gas may be reused by the system 100 and be injected into the gas tube 104.

[0051] As depicted in Figure 1 , the system 100 comprises a central purge gas tank or cylinder bank 112. As an example, the purge gas is helium. The system 100 can be purged after operation of the system 100 to flush the system 100, such as to remove neutron absorbing gas from the gas tubes 104 and to transition the reactor from a reduced reactivity mode (e.g.

shutdown mode) to an operational mode. In some embodiments, the tank 112 is coupled to the gas tube 104 using a purge 130, which is piping, conduits, hoses, and the like. In some embodiments, as depicted in Figure 1 , there is a check valve and valve installed on the purge line 130 to prevent purge gas from flowing back into the tank 112. [0052] As depicted in Figure 1 , the tank 106, tank 108, and tank 112 are connected to one injection tank 102 and gas tube 104, one valve system 120 and one vent valve system 140. In some embodiments, the tank 106, tank 108, and tank 112 are connected to more than one injection tank 102 and gas tube 104, valve system 120 and vent valve system 140. For example, the tank 106, tank 108, and tank 112 are connected to each of the five injection tanks 102 and gas tubes 104, valve systems 120 and vent valve systems 140 as depicted in Figure 1. In some embodiments, there are one or more tanks 106, tanks 108, and tanks 112 that are connected to the one or more injection tanks 102 and gas tubes 104, valve systems 120 and vent valve systems 140 of the system 100.

[0053] Figure 2 is a system diagram of the gas system 100. Figure 2 depicts an example valve system 120. During regular operation of the reactor, the valve system 120 is closed, such that there is no fluid communication between the tank 102 and the gas tube 104. When the valve system 120 is closed, the injection tank 102 can be filled with the neutron absorbing gas from tank 106. As depicted in Figure 2, the purge line 130 is coupled to the gas tube 104 to purge the gas tube 104 of neutron absorbing gas. In some embodiments, the valve and purge line 130 may be used as a test line to measure pressure and check if any gas is leaking.

[0054] In some embodiments, the vent valve system 120 comprises one or more valves for providing a controllable fluid communication channel between the injection tank 102 and the gas tube 104. In some embodiments, as depicted in Figure 2, the valve system 120 comprises a first valve 122 positioned along the fluid communication channel between the injection tank 102 and the gas tube 104. In some embodiments, the valve system 120 comprises a first valve 122 positioned in series to a second valve 124 along the fluid communication channel between the injection tank 102 and the gas tube 104. In such embodiments, the first valve 122 is coupled to the injection tank 102, and the second valve 124 is coupled to the gas tube 104. In some embodiments, the valve system 120 comprises a first valve 122 coupled in parallel to another valve (e.g. valve 126) along the fluid communication channel between the injection tank 102 and the gas tube 104. In such embodiments, the first valve 122 and valve 126 are coupled to the injection tank 102 and the gas tube 104. In some embodiments, as depicted in Figure 2, the valve system 20 comprises a first valve 122 coupled in series to a second valve 124 and defining a first valve series; a third valve 126 and a fourth valve coupled in series to define a second valve series, and the first and second valves series are coupled in parallel. [0055] In some embodiments, the valves of the valve system 120 are quick opening automatic valves. In such embodiments, the quick opening valves are powered to close, and open upon failure. In some embodiments, the valves of the valve system 120 are ball valves with a flow area that matches that of the gas tube 104.

[0056] While Figure 2 depicts the valve system 120 having four valves 122, 124, 126, and 128, other embodiments of the valve system 120 comprise one or more valves, connected in series, parallel, or a combination thereof.

[0057] Where the valve system 120 comprises a plurality of valves, such as that described herein or depicted in Figure 2, the valves act as redundant backups in case one or more of the other valves fail.

[0058] In some embodiments, the one or more valves of the valve system 120 may comprise valve trim, such that the one or more valves of the valve system 120 does not open suddenly. The valve trim of the one or more valves of the valve system 120 may restrict flow and control flow of the neutron absorbing gas into the gas tube 104.

[0059] As depicted in Figure 2, the gas tube 104 is coupled to the vent valve system 140 to vent the gas tube 104 when transitioning the reactor from a shutdown mode to an operational mode. The vent valve system 140 is generally similar to the valve system 120 described herein. In some embodiments, the vent valve system 140 comprises one or more valves, connected in series, parallel, or a combination thereof. Where the vent valve system 140 comprises a plurality of valves, such as that described herein or depicted in Figure 2, the valves act as redundant backups in case one or more of the other valves fail.

[0060] In some embodiments, the vent valve system 140 comprises one or more orifice plates for controlling the flow of the vented gas from the gas tube 104. There may be an orifice plate positioned upstream or downstream of the one or more valves of the vent valve system 140.

The orifice plate may be a component of the one or more valves of the vent valve system 140 (e.g. part of the valve trim or integrally formed with the one or more valves) or may be a separate component that is connected to the one or more valves. The orifice plate may control the flow and pressure change of the vented gas from the gas tube 104 such that the vented gas does not experience a step change in pressure. [0061] As depicted in Figure 2, an example line connecting the valve system 120 to the gas tube 104 is a 2” line, and an example line connecting the gas tube 104 to the vent valve system 140 is a ¼” line. In some embodiments, the line connecting the valve system 120 to the gas tube 104 is larger than the line connecting the gas tube 104 to the vent valve system 140. In such embodiments, more neutron absorbing gas can be introduced into the gas tube 104 to reduce the reactivity quickly, and neutron absorbing gas can be slowly vented from the gas tube 104 to control the increase in reactivity during venting to bring the reactor back to critical state.

[0062] As depicted in Figure 1 , the system 100 comprises a controller 500. The controller 500 may be a relay logic circuit. In some embodiments, the controller 500 is in data communication with one or more components of the system 100. The controller 500 can send a control command to the one or more components of the system 100 to actuate the one or more components of the system 100 and control operation and function of the system 100. For example, to fill the injection tank 102, the controller 500 can send a control command to the valve system 120 to close fluid communication between the injection tank 102 and the gas tube 104, and open the valves and regulators between the tank 106 and the injection tank 102 for the pressurized neutron absorbing gas to flow from the tank 106 to the injection tank 102.

[0063] As another example, to vent the gas tube 104, the controller 500 sends a control command to close the valve system 120 and to open the vent valve system 140 for the gas to vent from the gas tube 104 to the tank 108.

[0064] In some embodiments, the controller 500 is configured to, upon detection of a reactivity control trigger, actuate the valve system 120 to open fluid communication between the injection tank 102 and the gas tube 104 causing the pressurized neutron absorbing gas to flow from the injection tank into at least the portion of the gas tube positioned inside the reactor code.

[0065] In some embodiments, the controller 500 is configured to detect a reactivity control trigger based on one or more sensors in and/or around the core. Sensors can include, for example, neutronic in-core flux detectors, temperature sensors, pressure sensors, and the like. In some embodiments, a reactivity control trigger can be triggered based on one or more algorithms executed by the controller 500 to monitor changes, absolute values, and/or rates of change in sensors readings.

[0066] In some embodiments, the controller 500 detects a reactivity control trigger when a trigger signal is received from an input device such as a user input received at a user input device connected to a computer or other device, a fire alarm, an earthquake alarm, and/or any other safety or emergency input signal.

[0067] Figure 3A a schematic of the gas system 100. As depicted in Figure 3A, a connecting pipe 142 is coupled to the injection tank 102 and the valve system 120. In Figure 3A, the valve system 120 is depicted as having one valve, which may be a quick opening automatic valve. In some embodiments, the valve system 120 has more than one valve. A connecting pipe 144 is coupled to the valve system 120 and a divergence cone 146. The cone 146 is coupled to the connecting pipe 144 and the gas tube 104. The system 100 may comprise features to control velocity of the gas, such as, for example, the relative diameters of pipe 144 relative to pipe 142 along with valve trims, or other features like orifices.

[0068] As depicted in Figure 3A, the connecting pipe 144 is generally cylindrical in shape. In an example, when valve 120 is opened, connecting pipe 144 fluidly connects injection tank 102 to gas tube 104 where gas tube 104 is located inside a nuclear reactor (not shown).

[0069] Figure 3B is another schematic of gas system 100. As depicted in Figure 3B, a connecting pipe 142 is coupled to the injection tank 102 and the valve system 120. In Figure 3A, the valve system 120 is depicted as having one valve, which may be a quick opening automatic valve. In some embodiments, the valve system 120 has more than one valve. In some embodiments, at least a portion of the fluid communication channel between the gas rod 104 and the injection tank 102 is provided by a non-linear pipe that prevents direct line of sight between gas rod 104 and injection tank 102. In such embodiments, connecting pipe 144 may have any non-linear shape such as a helical pipe or generally serpentine arrangement (as shown in FIG 3B). During operation, when valve 120 is opened, non-linear piping serves to shield or block the path of neutrons between gas rod 104 and injection tank 102. In other words, the walls of piping 144 obstruct the linear path travelled by neutrons in gas rod 104 toward injection tank 102. In some embodiments, one or more shielding elements are positioned at locations between the non-linear piping to additionally block the path of neutrons between gas rod 104 and injection tank 102.

[0070] Gas system 100 may also comprise one or more extraction pipe(s) 147 which are connected to one or more dump tank(s) 108 to extract poisons gases from gas rod 104. In some embodiments, there is a central dump tank or other piping or receptacle for receiving the extracted gases from one or more gas rods. In some embodiments, the dump tank(s) are at a low pressure, e.g. vacuum pressure. In such embodiments, for example, the extraction pipe 147 can have a helical shape or similar shape to promote low velocity of the poison gas extracted from gas tube 104 and/or to increase the distance of travel between the gas rod and the dump tank(s). The helical or other non-linear shape of the connecting pipe 144 may slow gas from flowing upwards from the gas tube 104. By slowing the gas from flowing upwards from the gas tube 104, this may allow decay of radioactive materials having a relatively short half-life. In some embodiments, the helical shape of the connecting pipe 144 may prevent or mitigate neutron and gamma radiation from streaming towards the reactivity mechanism deck and exposing workers at the reactivity mechanism deck to radiation hazards. In some

embodiments, extraction pipe(s) 147 may be sized to have a length and diameter based on the transit time from gas rod 104 through the extraction pipe 147, mass of one or more radioactive isotopes in the gas rod 104, and half-life of one or more radioactive isotopes in the neutron absorbing gas. In an example, the extraction pipe 147 is sized such that when neutron absorbing gas is withdrawn (e.g. suctioned by pumping) from gas rod 104 through the extraction pipe 147 at a desired flow rate, the transit time through the extraction pipe at the desired flowrate is greater than the half-life of a radioactive isotope in the gas rod.

[0071] As shown in Figure 3B, extraction pipe 147 has a helical shape which surrounds connecting pipe 144. In other embodiments, the extraction pipe 147 does not surround the connecting pipe and/or the gas rod 104. In some embodiments, the extraction pipe 147 can include one or more helical or non-linear portions and one or more linear portions. In other embodiments, control of the speed of extraction can enable a linear extraction pipe 147 to be used.

[0072] The helical coil 147 may be a delay coil. The helical shape of the extraction pipe 147 may cause a pressure drop over a longer distance as the gas flows through the connecting pipe 144. In an embodiment, connecting pipe 142, 144 may have a radius of about 2.54 cm whereas extraction pipe 147 has a radius of about 0.635 cm. In some embodiments , the generally helical shape of extraction pipe 147 may prevent or reduce the amount of radioactive material that can flow from the gas tube 104 into the fluid connection channel between the gas tube 104 and the injection tank 102. In some reactors, portions of the fluid connection channel may not be fully insulated from radioactive material, so the helical shape and/or diameter of the connecting pipe 144 or other component may, in some situations, allow time for decay of radioactive material. [0073] When gas rod 104 is no longer required to reduce reactivity in the nuclear reactor, gas rod 104 may be purged of neutron absorbers within said rod. A method of purging a gas rod filled with neutron absorbing gas is provided. In an embodiment, the neutron absorbing gas may be suctioned from the gas rod through a coiled pipe (e.g. a helical coiled pipe) at a desired flow rate. The desired flowrate may be selected to provide a transit time through the coiled pipe greater than the half-life of a radioactive material in the gas rod. Slowly pumping the neutron absorbing gas from gas rod 104 permits radioactive material in the gas to decay before it is withdrawn to dump tank 108. The neutron absorbing gas may be suctioned to a dump tank by a vacuum pump. The method also comprises circulating an inert gas through the gas rod which may flush out any residual neutron absorbing gas left in gas rod 104. In an embodiment, the inert gas is helium. In a further embodiment, the inert gas is circulated through the gas rod from a first end 105 of the gas rod to a second end 107 of the gas rod. The method may further comprise reducing pressure within the gas rod to vacuum pressure by closing a first end 105 of the gas rod and suctioning out gas within the gas rod to reduce pressure within the gas rod to vacuum pressure.

[0074] As depicted in Figures 3A and 3B, cone 146 is coupled to the connecting pipe 144 and the gas tube 104, and interposed between the connecting pipe 144 and the gas tube 104. The cone 146 has a first opening and a second opening. In the embodiment as depicted in Figure 3A, the cone 146 is oriented such that the neutron absorbing gas flows through the first opening into the cone 146, and out through the second opening into the gas tube 104. In some embodiments, the first opening that the gas flows through to flow into the cone 146 is smaller than the second opening through which the gas flows to enter the gas tube 104. The diameter of the cross-section of the cone 146 may increase from the first end having the first opening to the second end having the second opening. The shape, length, and dimension of the cone 146 may be such that the cone 146 defines a gradual divergence. In an embodiment, cone 146 may have a generally frustoconical shape. In an embodiment, cone 146 may have a first opening circle radius of 2.54 cm, a second opening circle radius of 5.5 cm, and a height of 11cm. As the neutron absorbing gas flows through the cone 146, the cone 146 may mitigate or prevent sudden expansion of the gas, which may reduce instability of the gas, and may minimize turbulent flow in the pipe. Turbulent flow creates eddies in the piping which reduces the velocity with which poison gas may travel from injection tank 102 to gas rod 104. Cone 146 may be configured to minimize turbulence in the flow through connection pipe 144 and gas rod 104. Turbulent flow may further be minimized by maintaining gas rod 104 at vacuum pressure with respect to injection tank 102. In an embodiment, cone 146 is configured such that the Reynolds number of the neutron absorbing gas travelling through the cone 146 is less than 10000. In another embodiment, cone 146 is configured such that the Reynold number of the neutron absorbing gas travelling through the cone 146 is between 4000 and 10000. In another embodiment, cone 146 is configured such that the Reynold number of neutron absorbing gas travelling through the cone 146 is at transition flow (i.e. Reynolds number between 2100 and 4000). In another embodiment, the Reynold number neutron absorbing gas travelling between injection tank 102 and gas rod 104 is laminar flow (i.e. Reynold number less than 2100).

[0075] Figure 4 is a schematic of a current reactivity deck layout and a proposed reactivity deck layout of a reactor having the gas system of Figure 1. Schematic 402 depicts the current reactivity layout of a reactor, and schematic 404 depicts a possible new layout with shutdown system enhancement (SDSE) and adjuster rods (ADJ).

[0076] In some embodiments, the system 100 may be installed or retrofitted on an existing reactor.

[0077] As an example, in regards to controller absorbers, repurpose the two centre of core sites as cobalt adjusters and move them to positions around Shutoff 27 and 29.

[0078] As an example, in regards to adjusters, apart from taking over CA 3 and 4 sites, one may consider taking over sites in the Shut off 19, 23 and 14 triangle area that cause trips when testing on the pair of vertical ion chambers and the symmetric shut off 12, 16, 21 position. Options of changing core power rather than just flux shape using reactivity control sites should be checked though due to the partial use over time these would not be expected to be cobalt sites.

[0079] As an example, in regards to SDS1 , select 8 to 10 reactivity control sites (half on each side of the core) and investigate how SDS1 performance would change with them as either converted poison sites or conventional shutoff sites.

[0080] As an example, work on other systems may be used to see how detectors could be repositioned in this reconfiguration.

[0081] As depicted in schematic 402, the reactivity layout comprises a number of SDR, ADJ, and MCA. An existing reactor may be retrofit, such that some of the SDR, ADJ, or MCA may be replaced with the gas tube 104 of the system 100. [0082] As depicted in Figure 4, MCA is mechanical control absorber rods which reactor control uses to make big power changes; LZC is liquid zone control which does fine spatial and smaller power controls; ADJ is adjuster rod which is put in for longer term spatial power shaping across the core; SDR or SOR is a shut off rod which is the first and primary shutdown system; and SDSE is shut down system enhancement which is a special case of a third shutdown system to enhance an older reactor. In some embodiments, the first and second shutdown systems are SDS1 and SDS2. In some embodiments, the second shutdown system is a liquid injection shutdown system LISS in more recent reactor designs and a moderator dump in earlier reactor designs.

[0083] In some embodiments, the system 100 comprises a number of vertical gas tubes 104 to be installed in the reactor core. These tubes 104 are connected to a number of high pressure neutron poison gas storage / injection tanks 102 that are mounted on the reactor deck above each of the tubes 104. In some embodiments, the controller 500 can generate a control command to open the valves of the valve system 120. In some embodiments, the system 100 is used as a shutdown system alongside SDS1 or SDS2, or both. In such embodiments, a trip signal is initiated by either SDS1 or SDS2 and received by the controller 500, and, in response, generates a control command to open the valves of the valve system 120, or either SDS1 or SDS2 initiates a trip signal, and a trip relay causes the valves of the valve system 120 to open to fluidly communicate the injection tanks 102 and the gas tubes 104 to inject neutron poison gas from the injection tanks 102 into the voided gas tubes 104. The negative reactivity inserted into the core by the neutron poison gas may be sufficient to control the reaction rate of the reactor. As per the normal trip progression the neutron absorbing rods of SDS1 , or the liquid poison of SDS2 enters the core to provide additional negative reactivity depth to maintain the core in a subcritical state.

[0084] In some embodiments, after the neutron absorbing gas has been injected into the gas tube 104, the injection tank 102 of the system 100 is re-poised or refilled, for example, immediately, to keep the system 100 available through any subsequent guaranteed shutdown or start-up operations. When the reactor is to return to critical state, the neutron absorbing rods of SDS1 rods may be retrieved (if they have been left in as part of a Rod-based guaranteed shutdown), and then pressure is slowly decreased in the system 100 by venting the system 100 through the tank 108 to lower its negative reactivity worth. Then, each gas tube 104 of the system 100 is voided, purged with helium using tank 112, and revoided. [0085] In some embodiments, the system 100 comprises a relatively large pipe connecting injection tank 102 and the gas tube 104, and a relatively small tube for gas removal from the gas tube 104, as depicted in Figure 1 and Figure 2. In such embodiments, the relatively small scavenge tube coupled to the bottom of the gas tube 104 allows the gas in the gas tube 104 to be removed to the maximum degree through vacuum and/or flushing. Where the injection tanks 102 are quickly re-poised (i.e. re-pressurized over tube pressure), the return path for gas out of the gas tubes 104 is limited to the relatively small tube. With a relatively large tube for providing gas into the gas tube 104 and a relatively small tube for venting the gas tube 104, the injection rate for the gas into the gas tube 104 may be relatively high, and the rate of venting the gas tube 104 may be controlled at a relatively low rate.

[0086] In some embodiments, other features of the system 100 include: a bank of gas cylinders 106 to recharge or refill the injection tank 102 shutdown tank/cylinder; redundant or duplicated shutoff valves of the valve system 120 with each tube 120 that may be mounted at the reactor deck with a monitored space between the duplicated valves to allow leakage to be detected in either direction; a purge tank 108 nominally in the reactor vault hold up any partly activated gas from each tube at a slow controlled rate; a vacuum pump 110 to void the tubes 104, and a purge gas supply cylinder set 112 (the purge gas is, for example, helium) to be used to refill through the gas extraction line in a purge cycle as required before re-voiding.

[0087] Figure 5 is a schematic of the controller 500 of the system 100.

[0088] In some embodiments, the controller 500 is configured to receive a reactivity control trigger signal, and is configured to actuate the valve system 120 to open fluid communication between the injection tank 102 and the gas tube 104, causing the pressurized neutron absorbing gas to flow from the injection tank 102 into at least the portion of the gas tube 104 positioned inside the reactor core. In some embodiments, the controller 500 is in data communication with the components of the system 100, and generates control commands to control the configuration of the components of the system 100. In some embodiments, the controller 500 is a relay logic circuit.

[0089] For simplicity, only one controller 500 is depicted in Figure 1 , but the system 100 may include one or more programmable controllers, relay logic, or a combination thereof. In some embodiments, the controller 500 includes at least one processor, a data storage device

(including volatile memory or non-volatile memory or other data storage elements or a combination thereof), and at least one communication interface. The computing device components may be connected in various ways including directly coupled.

[0090] For example, and without limitation, the computing device may be a server, network appliance, set-top box, embedded device, computer expansion module, personal computer, laptop, or computing devices capable of being configured to carry out the methods described herein.

[0091] As depicted in Figure 5, the controller 500 may include a processor 502, an interface API 510, memory 504, an I/O interface 506, or a network interface 508.

[0092] The processor 502 may process the data received from the components of the system 100, or from other systems of the nuclear reactor, which may include a reactivity control trigger signal, an emergency signal, and so on. In some embodiments, each processor 502 is, for example, a microprocessor or microcontroller, a digital signal processing (DSP) processor, an integrated circuit, a field programmable gate array (FPGA), a reconfigurable processor, a programmable read-only memory (PROM), or any combination thereof.

[0093] In some embodiments, memory 504 includes a suitable combination of computer memory that may be located either internally or externally such as, for example, random-access memory (RAM), read-only memory (ROM), compact disc read-only memory (CDROM), electro- optical memory, magneto-optical memory, erasable programmable read-only memory

(EPROM), and electrically-erasable programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM) or the like.

[0094] Each I/O interface 506 enables the processor 502 to interconnect with one or more input devices, such as a keyboard, mouse, touch screen and a microphone, or with one or more output devices such as a display screen and a speaker. The I/O interface 506 may be isolatable or removable when not required.

[0095] Where the controller 500 comprises the network interface 508, the network interface 508 enables the processor 502 to communicate with other components, to exchange data with other components, to access and connect to network resources, to serve applications, and perform other computing applications by connecting to a network (or multiple networks) capable of carrying data. [0096] Application programming interface (API) 510 is configured to connect with a front end interface to provide interface services when required.

[0097] In some embodiments, the system 100 comprises a front end interface to transmit processed data, and receive data from different interfaces. The front end interface may reside on different types of devices, such as a computer, a personal digital assistant, a laptop, or a smart phone. The front end interface provides different reporting services and graphical renderings of processed data for user devices. Graphical renderings of processed data that was captured from the system 100, can be used, for example, by various parties and/or stakeholders in analyzing or monitoring the status of the nuclear reactor (e.g. operators in a control room).

[0098] The front end interface provides an interface to the controller 500 for user devices and third-party systems. The front end interface may, for example, generate, assemble and transmit interface screens.

[0099] The front end interface may include a historical data page, which may display historical data captured by the system and processed by the controller 500.

[00100] The processor 502 may be operable to register and authenticate user and user devices (using a login, unique identifier, and password for example) prior to providing access to applications, network resources, and data. The processor 502 may serve one user/customer or multiple users/customers.

[00101] As depicted in Figure 1 and Figure 2, the system 100 features vertical gas tubes 104, at least a portion of which is positioned inside the reactor core through substantially the full core height. During normal operation of the reactor, these gas tubes 104 are at below atmospheric pressure (i.e. vacuum pressure) and filled with helium. The gas tubes 104 are connected to high pressure injection tanks 102 that are filled with a neutron poison gas (e.g., natural krypton, isotopically separated krypton (Kr-83), xenon, BF₃ or BH₃). The neutron poison gas or gas blend may be selected based on a number of factors, which may include their relatively high neutron absorption cross sections.

[00102] During normal operation of the reactor, the system 100 is in a standby state, where the fluid communication channel between the injection tank 102 and the gas tube 104 is closed. In the standby state, the injection tank 102 is filled with the pressurized neutron absorbing gas such that a pressure differential between the injection tank 102 and the gas tube 104 enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 when the fluid communication channel is opened.

[00103] In some embodiments, in the standby state, the injection tank 102 is filled with a quantity of the pressurized neutron absorbing gas such that the neutron absorbing gas flowing into the gas tube 104 provides 10 to 12 mk of negative reactivity into the reactor core when the fluid communication channel is opened.

[00104] In the standby state, the gas tube 104 may be filled with gas at a sub- atmospheric pressure. For example, as depicted in Figure 1 , the gas tube 104 may contain helium supplied from the tank 112 at a sub-atmospheric atmosphere.

[00105] The system 100 may be used as a standalone system or alongside other shutdown systems, such as SDS1 or SDS2. Once a reactor trip signal is received by the system 100 (e.g. generated by SDS1 , SDS2, or by the controller 500), the valves of the valve system 120 (e.g. high pressure tank valves) are opened and the neutron absorbing gas is injected into the gas tubes 104. The injection tanks 102 have sufficient pressure for the gas to fill the tubes 104 at a sufficient concentration to arrest the reactivity increase and shut down the reactor until the conventional solid rods of SDS1 enter into the core for shutting down the reactor.

[00106] When the fluid communication channel between the injection tank 102 and the gas tube 104 is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 before a fluid or solid rod injection mechanism (such as SDS1 or SDS2), configured to insert a neutron absorbing media into the reactor core in response to the reactivity control trigger, inserts the neutron absorbing rod into a fully-inserted position.

[00107] When the fluid communication channel between the injection tank 102 and the gas tube 104 is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 before a neutron absorbing rod, being inserted into the reactor core in response to the reactivity control trigger (e.g. by SDS1), absorbs enough neutrons to suppress an increase in reactivity of the reactor. [00108] During a guaranteed shutdown state of the reactor, the gas tubes 104 may be filled with compressed, neutron absorbing gas. In some embodiments, each gas tube 104 is connected to its own injection tank 102. In such embodiments, since each gas tube 104 is connected to its own injection tank 102 and is independent from the other gas tubes 104, gas leakage is not expected to significantly impair the reactivity depth. In some embodiments, redundant or duplicate valves may be used to control or mitigate gas leakage.

[00109] The neutron absorbing gas remains in the gas tube 104 until the gas tube 104 is vented. For reactor restart, the neutron absorbing gas is gradually removed, decreasing neutron absorption in a uniform manner within the core. The vent valve system 140 is opened for gas in the gas tubes 104 to vent to the tank 108. The gas in the gas tubes 104 may also be pumped out using the vacuum pump 110.

[00110] In some embodiments, the gas tubes 104 are vented after the shut-off rods of SDS1 are removed. After the gas tubes are vented 104, the system 100 is flushed with helium from the tank 112 such that no neutron absorbing gas is left in the gas tubes 104 to leave a parasitic reactivity loss.

[00111] In some embodiments, the system 100 injects the neutron absorbing gas into the gas tubes 104, at least a portion of which extends into the reactor core. The gas is generally separated from the moderator by the gas tube 104, such that the neutron absorbing gas may not mix with the moderator. If the gas was mixed with the moderator, then the gas would have to be separated from the moderator during the start-up of the reactor. This may require taking additional steps and using other components, such as using an ion exchanger, to separate the gas and the moderator. This may require additional resources, such as time and labour. By using the system 100 to control the reaction rate of the reactor, using such additional resources may be avoided. For example, by using the system 100, the moderator may not have to be processed by ion exchange to separate the neutron absorbing gas and the moderator.

Accordingly, the system 100 may reduce the amount of time for restarting the reactor, namely, by reducing the amount of time for analyzing the quality of the moderator or purifying the moderator.

[00112] In some embodiments, the injection tanks 102 are refilled immediately after the neutron absorbing gas has been injected into the gas tubes 104 to return to a standby state. In some situations, this may enable a second injection of neutron absorbing gas to be injected into the gas tubes in the event of another trigger condition, including an accidental venting of the first injection of neutron absorbing gas.

[00113] As described herein, the system 100 uses a neutron absorbing gas to control the reaction rate of the reactor. When the gas flows into the gas tube 104, the gas distributes throughout the gas tube 104 and along the length of the gas tube 104. As depicted in Figure 12, the density of the gas may vary while in the tube 104, but the neutron absorbing gas is distributed throughout the gas tube 104 and along the length of the gas tube 104. Accordingly, the negative reactivity may be introduced throughout the gas tube 104 and along the length of the gas tube 104, and the negative reactivity may be generally consistent throughout the gas tube 104 and along the length of the gas tube 104. As the neutron absorbing media of the system 100 is a gas, stratification may be mitigated or reduced, such that the gas tube 104 having the neutron absorbing gas is not a stratified column. With a non-gas media, such as a liquid, the poison of the neutron absorbing media may precipitate to the top of the gas tube 104 and the liquid component of the neutron absorbing media and be at the bottom of the gas tube 104, such that the negative reactivity is only introduced at the top of the gas tube 104.

[00114] The system 100 may allow the neutron absorbing gas to have a relatively high flow rate when flowing into the gas tube 104, and a relatively low flow rate when venting out of the gas tube 104. The size of the inlet line into the gas tube 104 (e.g. 2”) may be larger than the size of the outlet line out of the gas tube 104 (e.g. ¼”) may allow for a relatively high flow rate into the gas tube 104, and a relatively low flow rate out of the gas tube 104. The helical shape of the inlet tube 144 may also allow for a relatively high flow rate into the gas tube 104, and a relatively low flow rate out of the gas tube 104. In addition, the orifice plates upstream or downstream of the one or more valves of the vent valve system 140 may control the vent flow rate out of the gas tube 104, such that the vent flow rate is lower than the flow rate into the gas tube. The relatively high flow rate into the gas tube 104, and the relatively low flow rate out of the gas tube 104 may also mitigate or prevent stratification inside the gas tube 104.

[00115] After the tank 102 has been fired and the gas flows into the gas tube 104, the tank 102 may be refilled with the neutron absorbing gas. With the tank 102 filled with the pressurized gas, if there is a leakage at the top of the gas tube 104, the relatively high pressure of the refilled tank 102 may prevent upward flow of the leaked gas. In addition, the valve system 120 may delay or prevent upward flow of the leaked gas. [00116] In some embodiments, the system 100 is not a closed loop that recycles the neutron absorbing gas that is introduced into the gas tube 104. After the gas tube 104 is vented, the neutron absorbing gas may be vented by active ventilation to a designated ventilation area configured to receive the vented gas. In some embodiments, the system 100 may recycle the neutron absorbing gas, but may not be a continuous closed loop. The gas vented from the gas tube 104 may be processed (e.g. filtered) prior to being reused by the system 100.

[00117] In some embodiments, the system 100 can be retrofitted into or installed on an existing Candu™ reactor (e.g. CANDU6 reactor). In such embodiments, the function of system 100 is to inject high pressure neutron absorbing gas (e.g. Krypton gas) into the reactor core upon generation and receipt of the shutdown trip signal. In some embodiments, the system 100 comprises ten solid wall zirconium gas tubes 104 located in place of six adjusters and four shutoff rods (SORs) of the SDS1 in the current CANDU™ design. In some embodiments, 10 adjusters are removed and all of the SORs are kept in the SDS1 system. In some

embodiments, one or more reactivity devices, such as booster rods, rod sites, etc. may be retrofitted with the system 100.

[00118] The system 100 can be used together as a reaction rate control system with the conventional SDS1 system, albeit with a reduced number of SORs (e.g. twenty four instead of twenty eight). In some embodiments, the gas tubes 104 of the system 100 replace adjuster numbers 2, 4, 6, 16, 18, and 20, and SOR numbers 10, 12, 17, and 19. The listed adjusters and SORs would be replaced together with their guide tubes. The guide tubes for the gas tubes 104 would need to be solid, rather than perforated, to mitigate gas leakage out of the gas tubes 104.

[00119] In some embodiments, the configuration of the system 100 includes ten injection tanks 102 with pressurized neutron absorbing gas (e.g. Krypton gas). In some embodiments, each injection tank 102 is associated with their own gas tube 104. As an example, the pressure of the injection tank 102 with the neutron absorbing gas is 20 MPa. The valve or valves of the valve system 120 (e.g. a quick opening ball valve) isolates the high pressure gas from each injection tube 104, as depicted in Figure 1 and Figure 2. Under normal operating conditions, the gas tubes 104 would be filled with helium below atmospheric pressure.

[00120] Upon the trip signal, the valves of the valve system 120 open and the neutron absorbing gas fills the gas tubes 104. The gas density increases quickly and so does the absorption of thermal neutrons by the neutron absorbing gas molecules. As an example, Krypton density of about 0.08 g/cm³ yields about -6 mk of dynamic reactivity decrease. The 24 remaining SORs of the SDS1 are inserted into the core at the speed dictated by the insertion curve.

[00121] To re-poise the system 100, the valve system 120 are closed, and the vent valve system 140 downstream of the gas tubes 104 are opened. Also, a valve in the flushing line 130 that connects the gas tube 104 to the tank 112 is opened (not shown), and a gas such as C0₂ or helium with low neutron absorption cross-sections is pushed into the gas tubes 104 to remove the existing neutron absorbing gas. After the neutron absorbing gas is flushed out of the gas tubes 104, the flushing gas may be pumped out (e.g. by a vacuum pump 110) to yield a partial vacuum in the gas tubes 104.

In some embodiments, the system 100 may be able to withdraw the neutron poison gas from the shut-off gas tubes 104 in a controlled manner and direct the used gas into either a storage tank, a radioactive filtering system, or if within emissions limits to vent it through a safe and approved path. System 100 may also be able to withdraw the neutron poison gas from the shut-off gas tubes 104 in a controlled manner to keep the reactor under the controllability of the reactor regulating system and to directly withdraw the used neutron poison gas to storage tank or radioactive filtering system. Note that this shall also involve controlling failure modes to limit the possible gas pressure decrease rate.

[00122] It is allowable to fulfill the above functions through a single system 100 or through complementary systems.

[00123] In some embodiments, the performance of the system 100 are as follows:

[00124] A. Insertion of -12 mk reactivity into the core within 0.25 seconds after trip signal is received.

[00125] B. Insertion of -24 mk to -30 mk reactivity into the core to be ready for guaranteed shutdown and restart.

[00126] C. Control the reactor through the xenon transient.

[00127] D. The withdrawal rate of poison from the core is targeted at 3 hours. [00128] E. Be capable to be re-poised during shut down to ensure the system is available for shutdown duty during restart.

[00129] In some embodiments, the operation of the system 100 shall have no impact on the operation of any interfacing system, except for the RRS system due to removal of some adjuster rods and adding some adjuster rods, for retrofitting or installing the system 100 to existing reactors.

[00130] In some embodiments, implementation of the system 100 will use the reactivity mechanism deck and in core available space originally intended for other systems.

[00131] In some embodiments, the system 100 is used as a shutdown enhancement, and is used with SDS1 and SDS2. In such embodiments, the system 100 is triggered to operate by relays for both SDS1 and SDS2.

[00132] In some embodiments, for start-up support, the system 100 is used in parallel to RRS under operator control as is the current moderator poison system.

[00133] In some embodiments, the gas bottle supplies for the system 100 shall be collocated with AGS supply bottles.

[00134] In some embodiments, purge tanks of the system 100 are placed in an existing shielded access controlled area like the upper areas of the reactor vault.

[00135] In some embodiments, all discharges from the system 100 are routed via active filtration connections.

[00136] For system 100, in some embodiments, a number of options for the neutron absorbing gas exist, including krypton, xenon, or boron trihydride (Borane). These options include mixtures and in use of different gases for different functions (for example boron for shutdown transient and Krypton for hold-down and start-up). Of the three named choices there are absorption advantages to a boron based gas, but one or more factors, such as economics and potential activity transport considerations, may determine the choice of neutron absorbing gas used in the system 100. Furthermore, if it is economically feasible, certain xenon or krypton isotopes may be used with the system 100 to increase the negative reactivity depth of the core or reduce the required pressure for the system 100. [00137] In some embodiments, the neutron absorbing gas of the system 100 is natural Krypton gas or isotopically separated Krypton gas.

[00138] In some embodiments, one parameter for functioning of the system 100 is the insertion speed of the neutron absorbing gas (e.g. krypton, isotopically separated krypton, xenon, Borane, Boron Trifluoride) into the gas tubes 104. By increasing the speed of the gas injection, the system 100 may be able to introduce negative reactivity to the reactor more quickly and control the reaction rate of the reactor. In some embodiments, the system 100 can reach the desired pressure ten times faster than the full insertion of the regular SORs. For example, the current full insertion time for the existing solid shut off rods (SOR) is 1.4 seconds.

[00139] For example, the system 100 pressure of approximately 5 MPa is required to obtain the desired depth for GSS if krypton is used. It is possible that this pressure may be different for other poison gases based on their negative reactivity worth.

[00140] In some embodiments, the gas tubes 104 should have a sufficiently large volume to contain sufficient amount of neutron absorbing gas to achieve the desired negative reactivity depth for reactivity control, shutdown, and GSS operations modes. A small tube is coupled to the bottom of the gas tube 104 and coupled to the vent valve system 140 to withdraw the neutron absorbing gas from the gas tube 104 to bring the core back to normal operations. In some embodiments, the pressure in the gas tube 104 is kept at below atmospheric pressure during normal operations. In some embodiments, the system 100 comprises a larger central purge tank and fill tank mounted on or near to the reactivity mechanism deck with extractor pump to pull back gas gradually from the fuel channel annuli, and then may replace with low partial pressure inert purge gas. As an example, the gas tube 104 is made with a Zirconium Niobium alloy with a diameter of about 4’.

[00141] The following are features of an example embodiment of the system 100:

• High purity krypton gas is the neutron poison gas.

• The system diagram of the example system 100 is depicted in Figure 1 and Figure 2.

• The system 100 is installed on an existing CAN DU™ reactor, particularly at surplus reactivity rod sites of the reactor.

• A series of canisters 106 are mounted vertically in the reactor core connected to the reactivity deck by a support, a small return tube and a larger fill pipe. • The in-core gas tube 104 volume is normally maintained in a voided state with the outside of the gas tube 104 touching moderator fluid.

• Pressure transducers monitor canister pressure, pressure above the first valve of the valve system 120 connected to the tube 104 and pipe (to monitor leakage) and in the small fire tank cylinder mounted above each canister at deck level.

• The duplicated fire valves connected to each canister are spring loaded open so a loss of power will cause them to open and have limiting switches to monitor if they are open or closed. The valves on the small return tube are fail closed and also have position sensors.

• For availability testing in the poised state individual of the duplicated valves can be opened and closed and the change in pressure on the interspace between the valves then confirmed to change as the valve position changes.

• A trip signal from the Shut-Down System 1 or 2 trip logic causes the valves of the valve system 120 to open which allows the pressure to equalise between the injection tank 102 and tube 104.

• The system 100 is sized so shutdown is obtained part way through pressure equalisation on the faster slope portion of that roughly exponential curve. Once pressure is equalised the valves of the valve system 120 are closed, trapping the gas in the tube 104. This allows the injection tank 102 above each rod at deck level to be recharged to full pressure again ready for a future event (and providing a prevention for gas moving back up the large injection pipes).

• When reactivity worth is required to be dropped the valves of the vent valve system 140 in the small return tubes are opened allowing gas pressure in the in core gas tube 104 to slowly drive out gas into a remote mounted purge tank 108. Once pressure has dropped to a very low value the remaining gas is extracted using a vacuum pump 110 through that return line. Once the in-core gas tube 104 is measured to be fully voided, a remote valve connection allows a low pressure volume of helium to be put into the in core canister as a purge. That helium is then vented and the canister revoided using the vacuum pump 110. At that point the system 100 is in its normal state. Should a reactor trip happen while the canister is being emptied at any stage the valves of the vent valve system 140 close and the injection sequence happens normally (although potentially raising canister pressure to a slightly higher value).

[00142] As depicted in Figure 1 and Figure 2, the system 100 comprises a vertically oriented voided gas tube 104 to deliver a neutron absorbing gas to the reactor core, similar to the liquid zone control. In some embodiments, the system 100 allows for a delivery method of a neutron poison that is faster than the existing SDS1.

[00143] In some embodiments, the system 100 can be installed on existing CAN DU™ reactors, such as using unused reactivity rod locations and the associated locations upon the reactivity mechanism deck. There may be available locations on the reactivity mechanism deck to position the neutron poison gas injection tanks 102. The components of the system 100 should not be installed and interfere with existing components of the reactor.

[00144] Based on the gas pressures for injection of the neutron absorbing gas, it is feasible to use either pressure tube or calandria tube materials for the body of the gas tubes 104.

[00145] The number of neutron poison gas injection tanks 102 can be decided once the effectiveness of the system 100 has been assessed from density point of view and reactivity worth point of view. For example, the system 100 may have 8 to 12 gas tubes 104.

[00146] As an example the size and operating pressure of the gas injection tanks 102 are estimated to be approximately 25 liters at 20 MPa based on the bounding Krypton case.

[00147] The size of supply lines to tanks and for removal of gas from tubes and to tanks may be the same as that used in the liquid zone control systems for the outlet. This may be because there is an option to route outlet lines in the shielded liquid zone control trenches on the deck and from there into less accessible areas lower in the building where a purge tank and other systems could be placed. The outlet tubing will need to be sized based on the required reactivity control and the radiation physics assessment of activated neutron poison. As an example, the outlet tube is a ¼” outlet tube, with addition of delay coils and orifices as required. As an example, a 2” line is used an in inlet. In some embodiments, both limiting orifices to control maximum gas speed and a different line size will be used, in part to prevent seat erosion in the injection valves.

[00148] In some embodiments, the system 100 injects negative reactivity into the core during shutdown initiation. For example, krypton is used as the neutron absorber and is injected at high pressure into gas tubes 104 that are inserted into the core. In some embodiments, the system 100 comprises 10 injection tanks 102 connected to ten individual gas tubes 104. The krypton injection tanks 102 may be poised at 20 MPa (g).

[00149] As shown in Figure 1 , the system 100 comprises a helium tank 112 provides inert gas to blanket the gas tubes 104 and purge the lines, a bank of gas cylinders or feed tank 106 provides the source of pressurized krypton gas to the injection tanks 102, and a receiver tank 108, equipped with a vacuum pump 110, to draw gas from the tubes 104 when returning the reactor to critical. In some embodiments, the receiver tank 108 is used to recirculate the gas or is purged to active ventilation.

[00150] As an example, the injection tanks 102 are located above the reactivity mechanisms deck at elevation 666’ approximately. The purge 112, feed 106 and receiver 108 tanks will be located on the C side, approximately at floor elevation 649’.

[00151] In an embodiment of the design of the system 100, as depicted in Figure 1 and Figure 2, the injection tanks 102 are filled with high pressure krypton from the feed tank 106 and the gas tubes 104 are blanketed with helium, which when SDS1 or SDS2 initiates, the helium is immediately displaced into the receiver tank 108 as high pressure krypton is injected. When the system 100 is re-poised, krypton is drawn out using the vacuum pump 110 into the receiver tank 108. In some embodiments, the contents (now active) from the receiver tank 108 are then transferred to the feed tank 106 and repressurized, ready to feed into the injection tanks 102. Accordingly, the gases vented from the gas rod 104 may be recirculated and reused by the system 100. Helium from the helium tank 112 is reintroduced into the gas tubes 104 to purge any remaining krypton to avoid neutron absorption during normal operation.

[00152] In some embodiments, the recirculation option has the benefit of reusing the gas of the system 100 and reducing the amount of gas required to re-fire the system 100. However, the system 100 may need to be configured for recirculation of gases. The gas in the receiver tank 108 will be a mixture of helium and krypton, such that diluted krypton is fed into the injection tanks 106 from the receiver tank 108, which may mean reduced reactivity depth available in the system 100. In some embodiments, the helium and krypton can be separated using a molecular sieve. In some embodiments, the recirculation concept will also introduce active components such as a compressor to pressurize the contents transferred from receiver tank 108 to the feed tank 106. Another approach would be to have a passive, once through purge system and venting to the off gas management system. The off gas management system may need to be configured to handle the helium and krypton gas purge. The major equipment may include:

• gas cylinders to provide gas source (helium as the purge gas, and krypton as the absorber gas), equipped with pressure regulators,

• helium purge tank,

• krypton feed tank, • a receiver tank (if storing gas is required prior to purging through the off gas management system),

• a vacuum pump,

• pressure regulator to ensure that system 100 is not overpressurized, and

• check valve to ensure there is no back flow from the reactor to the storage tanks.

[00153] In some embodiments, where the system 100 is configured to reuse the neutron absorbing gas or a portion of the neutron absorbing gas, the system 100 may comprise a filtration system to filter the vented gas prior to reusing the gas.

[00154] In some embodiments, the system 100 comprises two main supply lines from the feed tank 106 to provide krypton gas to the injection tanks 102 to ensure reliability of the system. Each main supply line is equipped with an isolation valve. After these valves, each of the two lines branches to each of the injection tanks 102. Each injection tank 102 has two flow paths into the gas tubes 104 to meet single failure criteria, and two isolation valves in series are provided in each line to ensure isolation, avoiding spurious addition of negative reactivity to the core, as depicted in Figure 2.

[00155] In some embodiments, the helium purge tank 112 requires a check valve such that backflow does not enter into the tank 112 when the injection tanks 102 fire two isolation valves in series. An embodiment of the krypton feed tank 106 configuration is to use gas cylinders to fill the feed tank, which may reduce minimize flow disruptions and variations to the injection tanks 102. Feeding directly from the gas cylinders may not provide consistent gas flow.

[00156] In some embodiments, the system 100 may be configured such that sufficient flow rates, pressures, etc. are met, and choked flow is avoided in the injection lines. Choked flow is a phenomenon that limits the mass flow rate of a compressible fluid through nozzles, contractions and expansions, which could otherwise limit the flow rate required when injection is initiated. In some embodiments, the gas feed tank pressure will be sufficient to inject krypton into the injection tanks, and a vacuum pump for the receiver tank 108 may not be required, but may be present to vent the gas tube 104. [00157] In some embodiments, instrumentation and control monitoring and alarms will be required to provide low gas cylinder pressure, low feed tank pressure, low injection tank pressure in the main control and secondary control rooms.

[00158] In some embodiments, the tanks and all isolable lines of the system 100 comprises overpressure protection devices (e.g., relief valves or rupture discs). Alternatively, the system 100 may be designed to withstand the pressures expected in the system 100.

Rupture discs are considered passive, but there will be downtime operationally when a rupture disc bursts, which may be considered when selecting the type of overpressure protection used in the system 100 (i.e. the implication should be weighed against the safety and operational goals). The system 100 should be designed to ensure that an actuation of an overpressure protection device does not depressurize and impair the system 100. In addition, a burst rupture disc of the system 100 should not have any impact on personnel (i.e., exposure hazards).

Spring loaded relief valves of the system 100 may be exhausted to the active ventilation or filtration system. Both rupture discs and relief valves of the system 100 may be subject to routine inspection and maintenance activities.

[00159] As described herein, the system 100 may comprise a helium purge tank 112, a krypton feed tank 106 or bank of krypton cylinders, a receiver tank 108, a vacuum pump 110 to withdraw the gases from the gas tubes 104, a compressor to transfer contents from the receiver tank 108 to the feed tank 106 for recirculation, and associated valves and piping.

[00160] In some embodiments, the system 100 may have a minimum number of valves and active components, while still addressing single failure criteria and reliability of the system 100.

[00161] Gas cylinders should be located in accessible areas for personnel to monitor gas pressures and allow easy change out of cylinders.

[00162] In some embodiments, the system 100 comprises one or more pressure and temperature sensors to detect pressure and temperature in the injector tanks 102 and the gas tubes 104, and to confirm that the injection valves are not leaking such that the containment function or the system 100 function is jeopardized. This equipment may be generally similar to sensors that are used for SDS1 or SDS2 in CANDU™ reactors. Activation of the shut off rods is expected to follow the same parallel connection logic used in existing CANDU™ when

Shutdown System Enhancement (SDSE) was implemented. [00163] In some embodiments, space that may be originally allocated to other systems of the reactor (e.g. boosters) may be allocated to the system 100 in the control room and control equipment room. Pressure loss sniffing and local gas sniffing may be used to detect leaks in the system 100. In some embodiments, leakage techniques used in SDS1 or SDS2 may be adapted and used in the system 100, such as leakage techniques for monitoring leakage in a space between serial valves.

[00164] Figure 6 is a graph depicting reactor power during an increase in reactivity without activation of a shutdown system as a function of time.

[00165] As shown in graph 600, without activation of any reaction rate control system, such as SDS1 , the reactor power increases. Graph 600 depicts the reactor power increasing over time.

[00166] Figure 7 is a graph depicting reactivity insertion without activation of a shutdown system as a function of time.

[00167] During an increase in reactivity, positive reactivity may be inserted into the core. As depicted in Figure 7, during such an increase in reactivity, where no shutdown systems are activated, approximately 7.25 mk of positive reactivity was inserted into the core in 1.5 seconds. As reactivity is increased, the reactor power could increase beyond the reactor’s initial power.

[00168] During the increase in reactivity, the amount of positive reactivity insertion may be small relative to the negative reactivity insertion by the shut-off rods, which may range from - 40 mk to -60 mk. These rods may introduce sufficient negative reactivity to compensate for the increase in reactivity (e.g. 8 mk or more). However, the insertion rate of the shut-off rods, which may be relatively slow (e.g. about 1.4 seconds for full insertion of the shut off rods into the reactor core), may allow the reactor power to increase while the shut-off rods are entering the core before suppressing the increase of reactivity and shutting down the reactor. In some embodiments, CANDU™ reactors may use the same rods to give fast transient response and longer term hold down response. The insertion of more negative reactivity to compensate for the increase in reactivity control the rate of reactivity of the reactor. For example, the insertion of more negative reactivity to compensate for the increase in reactivity may shut down the reactor quickly. [00169] The system 100 may insert negative reactivity into the core relatively quickly compared to SDS1 or SDS2. SDS1 may have more than sufficient negative reactivity to shut down the core fully. However, the system 100 may be able to insert sufficient negative reactivity into the core rapidly enough to sufficiently suppress excess reactivity quickly enough and control the reaction rate of the reactor. As shown in the Figure 7, only 7.25 mk are inserted into the core during the first 1.6 seconds. If the system 100 is able to insert around -12 mk into the core rapidly, then it may be sufficient to suppress the excess reactivity and control the rate of reaction of the reactor (e.g. shutdown the reactor fully). If the system 100 is used alongside existing shutdown systems, such as SDS1 , then the SDS1 shut-off rods would then enter the core as the system 100 is triggered to ensure that the reactor stays in shutdown mode as other positive reactivity is inserted into the core due to fuel cooling down, xenon decay, and other parameters.

[00170] Figure 8 is a graph depicting reactor power after an increase in reactivity with and without a shutdown system activation as a function of time. Similar to Figure 6, the graph 800 shows that after the increase in reactivity and without activation of any shutdown system, such as SDS1 , the reactor power increases. Graph 800 depicts the reactor power increasing over time, and the reactor power measured at about 1.6 seconds is greater than the reactor power measured at about 0.0 seconds.

[00171] Consider graph 800 between the 0.0 second and 1.0 second time period. As depicted in Figure 8, whether or not a shutdown system is activated, the reactor power continues to increase in generally the same manner.

[00172] In some embodiments, it takes about 1.0 seconds or longer to see a decrease in the reactor power as it may take about that order of magnitude of time for the shut-off rods of SDS1 to be inserted into the reactor core, as depicted in Figure 9.

[00173] Between 0.0 seconds and 1.0 seconds, the increase in reactor power may have already caused damage to the fuel or other components of the reactor.

[00174] Figure 9 is a graph depicting shut-off rods percent insertion into the nuclear reactor core as a function of time. As depicted in graph 900, it takes about 0.8 seconds for the shut-off rod to be about half-way inserted into the reactor, and it takes about 1.4 seconds for the shut-off rod to be fully inserted into the reactor. As depicted in graph 900, during the 0.0 second to 0.4 second time period, less than 10 percent of the shut-off rod may be inserted into the core. [00175] Figure 10 is a graph depicting excess reactivity as a function of krypton density for a system of gas shut-off rods. As depicted in graph 1000, the excess reactivity of krypton becomes progressively, but slowly, more negative as the krypton density increases in the gas shut-off tubes, reaching -20 mk at 0.25 g/cm³, as shown in Figure 10. Figure 11 is a graph depicting excess reactivity as function of ¹⁰BH₃ density in a system of gas shut-off rods. As depicted in graph 1100, the excess reactivity of borane reaches -30 mk at ¹⁰BH₃ density of 0.002 g/cm³, for a density that may be lower than that of krypton, as shown in Figure 11.

[00176] In some embodiments, with the view of retrofitting existing CAN DU™ type reactors to accommodate the system 100, certain adjusters and SORs for a CANDU™-6 core have been selected for conversion to Krypton injection tubes 104. For example, the following six adjusters and four SORs may be replaced with gas tubes 104: Adjusters number 2, 4, 6, 16, 18, and 20; SORs number 10, 12, 17, and 19.

[00177] Figure 12 is a graph depicting krypton density in the gas tube 104 as a function of time. After the system 100 is triggered, and as the neutron absorbing gas flows into the gas tube 104, the density of the gas in the gas tube 104 increases.

[00178] The transient gas density is shown in Figure 12 for the first 150 ms. In some embodiments, first, the inlet valve is fully opened during a 0.2 second interval, while the outlet valve remains closed. The resulting density transients are plotted for three of a number (e.g. 10 nodes) of equally spaced nodes, as depicted in Figure 12. The data of plane 1 represents data captured at a node that is closer to the top of the gas tube 104. The data of plane 2 represents data captured at a node that is generally around the middle of the gas tube 104. The data of plane 3 represents data captured at a node that is closer to the bottom of the gas tube 104. As depicted in Figure 12, the gas density is generally higher in the upper portion of the gas tank 104 compared to the middle or lower portion of the gas tank 104.

[00179] The gas of the system 100 may be introduced to the reactor more quickly than the shutoff rods of conventional systems. Accordingly, as compared to a system having conventional shut off rods, in the event of an increase in reactivity of the reactor, the system 100 may cause a reduction the reactivity of the reactor more quickly. Similarly, as compared to a system having shut off rods, in the event of an increase in power of the reactor, the system 100 may cause a reduction the power of the reactor more quickly.

[00180] The system 100 may also increase the margin to prompt criticality. [00181] The system 100 may have the potential to improve safety shutdown performance when mitigating events having fast reactivity increase. The power peak may be reduced, and the integrated energy deposited in components of the reactor (e.g. the fuel) may be reduced. Accordingly, damage or failure of a component of the reactor due to the increase in power or reactivity may be reduced or eliminated. Peak fuel sheath temperatures may be reduced, precluding the generation of hydrogen gas due to high temperature Zirc-water reaction.

[00182] During an increase in reactivity of the reactor, the peak power of a reactor having the system 100 may be lower than the peak power of a reactor having a system having conventional shutoff rods. The power of a reactor having the system 100 may begin to decrease at a moment sooner than a reactor having a system having conventional shutoff rods.

[00183] In addition, during an increase in reactivity of the reactor, the peak excess reactivity of a reactor having the system 100 may be lower than the peak excess reactivity of a reactor having a system having conventional shutoff rods. The excess reactivity of a reactor having the system 100 may begin to decrease at a moment sooner than a reactor having a system having conventional shutoff rods.

[00184] The system 100 may be installed on reactors to introduce negative reactivity into the core during the event of an increase in reactivity or power of the reactor to control the reaction rate of the reactor.

[00185] The components of the system 100, such as the injection tanks 102, piping, and the gas tubes 104, may be designed to fit into existing unused reactor reactivity control sites. The pressures and volumes of the injection tanks 102 and piping may readily available in the market. It may be feasible to use either pressure tube or calandria tube materials for the body of the rods 104.

[00186] The instrumentation and control equipment used as part of the system 100 may be similar to that used for existing shutdown systems, such as SDS1 or SDS2 in CAN DU™ reactors. Activation of the shut off rods may follow the same parallel connection logic used in existing CANDU™ reactors when shutdown system enhancements are implemented. The space for the control equipment may use the same space that was used for the replaced reactivity controls. [00187] To monitor the system for leaks, techniques used for other equipment may be utilized, in particular pressure loss and local gas sniffing equipment.

[00188] Neutron poison gas can reach acceptable reactivity at reasonable pressure ranges. The isotopes produced during irradiation of the neutron absorbing gas or the products produced from the neutron absorbing gas may be in low concentrations and may be mitigated through the design of the system 100.

[00189] The system 100 may inject neutron poison at a 10 to 20 times faster rate than the current SDS1 design, which may provide a faster response to an increase in power or reactivity of a nuclear reactor. The reactivity or power increase is turned around earlier, limiting the short term reactivity required to be effective. In some embodiments, a small number of gas tubes 104 may be sufficient to provide a performance change with conventional rods retained for guaranteed hold down.

[00190] As noted herein, the neutron absorbing gas used in the system 100 may be borane or boron trifluoride. For these gases, a low density is needed to obtain a large negative worth for the gas shutdown system, have low system pressure, and may have a small footprint on the reactivity mechanism deck.

[00191] In some embodiments, the system 100 may be installed on CANDU™ reactors that have smaller or lower reactivity management margins (e.g. older CANDU™ reactors). The system 100 may be installed on older CANDU™ reactors, the control absorbers may be relocated, and adjusters may be added. These changes may flatten the power distribution in the core, reduce the power or reactivity in the event of an increase in power or reactivity of the reactor, and may generate a revenue stream with cobalt production.

[00192] Figure 13 is a flow chart depicting a method S1300 of using the gas system 100.

[00193] At block S1302, the injection tank 102 may be pressurized with a neutron absorbing gas, such as krypton, isotopically separated krypton, xenon, borane, or born trifluorine. The injection tank 102 may be coupled to the high pressure absorber gas tank or cylinder bank 106. The valve system 120 may be closed, and the valves and regulators interposed between the tank 106 and the injection tank 102 may be opened such that the neutron absorbing gas flows from the tank 106 to the injection tank 102. When the tank 102 is filled, the tank 102 is sealed and isolated with the pressurized neutron absorbing gas, and the gas is ready to flow into the gas tube 104 upon detection of a reactivity control trigger.

[00194] At block S1304, upon detection of a reactivity control trigger, fluid communication channel between the injection tank 102 and a gas tube 104, at least a portion of which is positioned inside the reactor core, is opened. The controller 500, or existing shutdown systems, such as SDS1 or SDS2, may generate the reactivity control trigger. The valve system 120 interposed between the injection tank 102 and the gas tube 104 is opened to open fluid communication between the injection tank 104 and the gas tube 104.

[00195] In some embodiments, before detection of the reactivity control trigger, the system 100 is maintained a standby state, wherein the neutron absorbing gas in the injection tank 102 is pressurized, and the fluid communication channel between the injection tank 102 and the gas tube 104 is closed.

[00196] In some embodiments, the neutron absorbing gas in the injection tank 102 is pressurized such that a pressure differential between the injection tank 102 and the gas tube 104 enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 when the fluid communication channel is opened.

[00197] In some embodiments, when the fluid communication channel between the injection tank 102 and the gas tube 104 is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 before a rod injection mechanism, configured to insert a neutron absorbing rod into the reactor core in response to the reactivity control trigger, inserts the neutron absorbing rod into a fully-inserted position.

[00198] In some embodiments, when the fluid communication channel between the injection tank 102 and the gas tube 104 is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank 102 to flow into the gas tube 104 before a neutron absorbing rod, being inserted into the reactor core in response to the reactivity control trigger, absorbs enough neutrons to control reactivity of the reactor.

[00199] In some embodiments, before the reactivity control trigger is detected, the pressure within the gas tube 104 is reduced to a sub-atmospheric pressure. This may promote steady state during operation of the system 100. [00200] In some embodiments, the pressurizing of the neutron absorbing gas in the injection tank 102 comprises filling the injection tank 102 with a quantity of the pressurized neutron absorbing gas such that the neutron absorbing gas flowing into the gas tube 104 provides 10 to 12 mk of negative reactivity into the reactor core when the fluid communication channel between the injection tank 102 and the gas tube 104 is opened.

[00201] In some embodiments, when neutron absorbing gas is in the gas tube 104, the venting of the gas tube 104 is controlled to vent the neutron absorbing gas from the gas tube 104 to reduce negative reactivity caused by the neutron absorbing gas in the reactor core at a controlled rate.

[00202] In some embodiments, the neutron absorbing gas is natural krypton gas or isotopically separated krypton gas.

[00203] One advantage of the system 100 may include controlling the reaction rate of the reactor core (e.g. shutting down the reactor more) quickly than existing reactor shutdown systems, such as SDS1 and SDS2. For example, the system 100 shuts down the reactor at 10 to 20 times faster than the SDS1 and SDS2. This relatively fast reaction rate control of the reactor may increase the safety margin of the operating reactor, which may reduce the possibility of fuel failure or damage to components of the reactor during a postulated accident scenario.

[00204] A second advantage of the system 100 may be that it may allow for the insertion of negative reactivity into the core during reactor shutdown for the core to reach Guaranteed Shut Down State, and may reduce the amount of negative reactivity that may be added into the core during the Guaranteed Shutdown state. This may have financial benefits as it may reduce the amount of time to transition out of the Guaranteed Shutdown State to full reactor operation, which may have positive economic benefits for the reactor operator.

[00205] By using the system 100 as a shutdown system, the system 100 may be an alternative to SDS2 or may be used with SDS2, such that gadolinium may not have to be introduced to the moderator to introduce negative reactivity to the reactor. This may reduce the amount of time for analyzing or monitoring the moderator, and purifying the moderator.

[00206] By using the system 10, the gas shut-off rods may minimize, reduce, or negate the need to use poison in the moderator during reactor shutdown. The neutron absorbing gas is separated from the moderator by the gas tubes 104. After the activation of the system 100, the system 100 may be re-poised for another activation if needed during reactor restart and normal operation. The neutron absorbing gas may be maintained in the gas tubes 104 at high pressure and is withdrawn from the tube 104 (e.g. by actuation of the vent valve system 140) to vent the gas from the tube 104 and bring the reactor out of guaranteed shutdown state. This also allows for fine reactivity control spread evenly over the gas tubes 104.

[00207] The system 100 may introduce a negative reactivity into the core relatively quickly (e.g. around 50 ms), such that the system 100 may control the reaction rate of the reactor during an increase in reactivity. The system 100 may be used with SDS1 or SDS2. Accordingly, the system 100 may control the reaction rate of the reactor during an increase in reactivity.

[00208] The system 100 may or may not be needed during the power ramp up to 100% full power. The system 100 may or may not be needed to get out of guaranteed shutdown state mode.

[00209] The system 100 may reduce the start-up period of the reactor, which may bring cost savings to the plant operators. The start-up period may be reduced as less time is required to purify the moderator and remove poison from the moderator. Rather, the neutron absorbing gas has to be vented from the gas tubes 104, which may take less time than purifying the moderator.

[00210] The system 100 described herein may use fewer pumps and valves compared to a system that is controlling the particular reactivity of a reactor core. The system 100 may be simpler than such systems that control the particular reactivity of the reactor more.

[00211] The embodiments of the devices, systems and methods described herein may be implemented in a combination of both hardware and software. These embodiments may be implemented on programmable computers, each computer including at least one processor, a data storage system (including volatile memory or non-volatile memory or other data storage elements or a combination thereof), and at least one communication interface.

[00212] Throughout the foregoing discussion, references may be made regarding servers, services, interfaces, portals, platforms, or other systems formed from computing devices. It should be appreciated that the use of such terms is deemed to represent one or more computing devices having at least one processor configured to execute software instructions stored on a computer readable tangible, non-transitory medium. For example, a server can include one or more computers operating as a web server, database server, or other type of computer server in a manner to fulfill described roles, responsibilities, or functions.

[00213] The system 100 described herein may shut down a nuclear reactor by injecting pressurized neutron absorbing gas into gas tubes 104, at least a portion of which are positioned inside a reactor core. Injector tanks 102 are filled with the neutron absorbing gas. Upon detection of a reactivity control trigger, the fluid communication channel between the injector tanks 102 and the gas tubes 104 are opened. The pressure differential of the injection tanks 102 and the gas tubes 104 causes the neutron absorbing gas to flow into the gas tubes 104, thereby introducing negative reactivity into the reactor core. The neutron absorbing gas may be introduced into the core relatively quickly, compared to the time required to fully insert a neutron absorbing rod of conventional shutdown systems. The gas in the gas tubes 104 may be vented in a controlled manner to control reactivity in the core when restarting the reactor.

[00214] Various example embodiments are described herein. Although each

embodiment represents a single combination of inventive elements, all suitable combinations of the disclosed elements include the inventive subject matter. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[00215] The term“connected” or "coupled to" may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[00216] The technical solution of embodiments may be in the form of a software product. The software product may be stored in a non-volatile or non-transitory storage medium, which can be a compact disk read-only memory (CD-ROM), a USB flash disk, or a removable hard disk. The software product includes a number of instructions that enable a computer device (personal computer, server, or network device) to execute the methods provided by the embodiments.

[00217] The embodiments described herein may be implemented by physical computer hardware, which may include computing devices, servers, receivers, transmitters, processors, memory, displays, or networks. The embodiments described herein provide useful physical machines and particularly configured computer hardware arrangements. The embodiments described herein are directed to electronic machines and methods implemented by electronic machines adapted for processing and transforming electromagnetic signals which represent various types of information.

[00218] The embodiments described herein pervasively and integrally relate to machines, and their uses; and the embodiments described herein have no meaning or practical applicability outside their use with computer hardware, machines, and various hardware components.

[00219] Substituting the physical hardware particularly configured to implement various acts for non-physical hardware, using mental steps for example, may substantially affect the way the embodiments work. Such computer hardware limitations are clearly essential elements of the embodiments described herein, and they cannot be omitted or substituted for mental means without having a material effect on the operation and structure of the embodiments described herein. The computer hardware is essential to implement the various embodiments described herein and is not merely used to perform steps expeditiously and in an efficient manner.

[00220] Although the embodiments have been described in detail, it should be

understood that various changes, substitutions and alterations can be made herein without departing from the scope as defined by the appended claims.

[00221] Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

[00222] The examples described above and illustrated are intended to be examples only.

Claims

WHAT IS CLAIMED IS:

1. A gas-based reactivity control system for a nuclear reactor, comprising: an injection tank for receiving and containing pressurized neutron absorbing gas; a gas tube at least a portion of which is positioned inside a reactor core; a valve system coupled to the injection tank and the gas tube providing a controllable fluid communication channel between the injection tank and the gas tube; and a controller for selectively actuating the valve system to control fluid communication between the injection tank and the gas tube.

2. The gas-based reactivity control system of claim 1 , wherein the controller comprises a processer configured to: upon detection of a reactivity control trigger; and actuate the valve system to open fluid communication between the injection tank and the gas tube causing the pressurized neutron absorbing gas to flow from the injection tank into at least the portion of the gas tube positioned inside the reactor core.

3. The gas-based reactivity control system of claim 1 , wherein the valve system comprises a first valve positioned along the fluid communication channel between the injection tank and the gas tube.

4. The gas-based reactivity control system of claim 1 , wherein the valve system comprises a first valve positioned in series with a second valve along the fluid communication channel between the injection tank and the second valve is coupled to the gas tube.

5. The gas-based reactivity control system of claim 1 , wherein the valve system comprises a first valve coupled in parallel to a second valve along the fluid communication channel between to the injection tank and the gas tube.

6. The gas-based reactivity control system of claim 1 , wherein at least a portion of the fluid communication channel between the gas rod and the injection tank is provided by a non linear pipe.

7. The gas-based reactivity control system of claim 1 , wherein the neutron absorbing gas is natural Krypton gas or isotopically separated Krypton gas.

8. The gas-based reactivity control system of claim 7, wherein the neutron absorbing gas is selected from the group consisting of ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³KR, ⁸⁴Kr, and ⁸⁶Kr.

9. The gas-based reactivity control system of claim 2 wherein in a standby state where the fluid communication channel between the injection tank and the gas tube is closed and the injection tank is filled with the pressurized neutron absorbing gas such that a pressure differential between the injection tank and the gas tube enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube when the fluid communication channel is opened.

10. The gas-based reactivity control system of claim 9, wherein when the fluid

communication channel is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube before a rod injection mechanism, configured to insert a neutron absorbing rod into the reactor core in response to the reactivity control trigger, inserts the neutron absorbing rod into a fully- inserted position.

11. The gas-based reactivity control system of claim 9, wherein when the fluid

communication channel is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube before a neutron absorbing rod, being inserted into the reactor core in response to the reactivity control trigger, absorbs enough neutrons to reduce reactivity of the reactor.

12. The gas-based reactivity control system of claim 9, wherein in the standby state, the gas tube is filled with gas at a vacuum pressure .

13. The gas-based reactivity control system of claim 9, wherein in the standby state, the injection tank is filled with a quantity of the pressurized neutron absorbing gas such that the neutron absorbing gas flowing into the gas tube provides 10 to 12 mk of negative reactivity into the reactor core when the fluid communication channel is opened.

14. The gas-based reactivity control system of claim 1 , wherein the injection tank or gas tube is configured to fit within a reactivity control location of the nuclear reactor.

15. The gas-based reactivity control system of claim 1 , wherein the gas tube is coupled to a vent valve system for controlling venting of the gas tube.

16. The gas-based reactivity control system of claim 15, wherein when neutron absorbing gas is in the gas tube, the vent valve system is controllable to vent the neutron absorbing gas from the gas tube to reduce negative reactivity caused by the neutron absorbing gas in the reactor core at a controlled rate.

17. The gas-base reactivity control system of claim 1 , wherein the valve system comprises a divergence cone for reducing turbulence in the flow of the pressurized neutron absorbing gas to the gas tube.

18. A method for reducing reactivity in a nuclear reactor, the method comprising: pressurizing neutron absorbing gas in an injection tank; and upon detection of a reactivity control trigger, opening fluid communication channel between the injection tank and a gas tube at least a portion of which is positioned inside the reactor core.

19. The method of claim 18, comprising: before detection of the reactivity control trigger, maintaining a standby state wherein the neutron absorbing gas in the injection tank is pressurized, and the fluid communication channel between the injection tank and the gas tube is closed.

20. The method of claim 18, comprising: pressuring the neutron absorbing gas in the

injection tank such that a pressure differential between the injection tank and the gas tube enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube when the fluid communication channel is opened.

21. The method of claim 20, wherein when the fluid communication channel is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube before a rod injection mechanism, configured to insert a neutron absorbing rod into the reactor core in response to the reactivity control trigger, inserts the neutron absorbing rod into a fully-inserted position.

22. The method of claim 20, wherein when the fluid communication channel is opened, the pressure differential enables the pressurized neutron absorbing gas from the injection tank to flow into the gas tube before a neutron absorbing rod, being inserted into the reactor core in response to the reactivity control trigger, absorbs enough neutrons to reduce reactivity of the reactor.

23. The method of claim 18, comprising: before detecting of the reactivity control trigger, reducing pressure within the gas tube to a sub-atmospheric pressure.

24. The method of claim 18, wherein pressurizing the neutron absorbing gas in the injection tank comprises: filling the injection tank with a quantity of the pressurized neutron absorbing gas such that the neutron absorbing gas flowing into the gas tube provides 10 to 12 mk of negative reactivity into the reactor core when the fluid communication channel is opened.

25. The method of claim 18, comprising: when neutron absorbing gas is in the gas tube, controlling venting of the gas tube to vent the neutron absorbing gas from the gas tube to reduce negative reactivity caused by the neutron absorbing gas in the reactor core at a controlled rate.

26. The method of claim 18, wherein the neutron absorbing gas is natural Krypton gas or isotopically separated Krypton gas.

27. The method of claim 25, wherein the neutron absorbing gas is selected from the group consisting of ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³KR, ⁸⁴Kr, and ⁸⁶Kr.

28. A method of purging a gas rod filled with neutron absorbing gas comprising: suctioning the neutron absorbing gas from the gas rod through a coiled pipe at a flow rate, wherein a transit time through the coiled pipe at the desired flowrate is greater than the half-life of a radioactive material in the gas rod; circulating an inert gas through the gas rod; and reducing pressure within the gas rod to vacuum pressure.

29. The method of claim 28, wherein the neutron absorbing gas is suction to a dump tank by a vacuum pump.

30. The method of claim 28, wherein the inert gas is helium.

31. The method of claim 28, wherein the inert gas is circulated through the gas rod from a first end of the gas rod to a second end of the gas rod.

32. The method of claim 28, wherein a first end of the gas rod is closed and gas within the gas rod is suctioned out to reduce pressure within the gas rod to vacuum pressure.