JPS637355B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a nuclear reactor shutdown device that discharges a neutron-absorbing liquid into a moderator. Conventionally, in pressure tube nuclear reactors, a moderator (heavy water) is generally circulated for cooling inside the reactor. FIG. 1 is an explanatory diagram of the above-mentioned circulation, and the control rod guide tube 2 fixedly installed through the top surface of the calandria tank 1 is originally a member for guiding the insertion and withdrawal of control rods (not shown). More than ten to several tens of calandrias are provided for one calandria tank 1, and one of them is illustrated in FIG. 1. The moderator is fed from above using the control rod guide tube 2, flows from the lower end of the control rod guide tube 2 into the calandria tank 1 in the direction of arrow A, rises inside the tank 1, and flows into the outflow tube 3. It overflows in the direction of arrow B. The aforementioned control rod guide tube has several discharge ports 4a, 4b, 4 on the tube wall, like the control rod guide tube 4 shown in FIG.
There are also types with holes c and 4d. In this case, the moderator fed from above the control rod guide tube 4 in the direction of arrow E is divided into discharge ports 4a to 4d, flows into the calandria tank 1, and overflows from the outflow tube 3. Conventionally, the setting of the above-mentioned moderator flow circulation path and the flow rate distribution of each hole in the case of a plurality of outflow holes have been designed and manufactured with the main aim of uniformly cooling the inside of the furnace. The above design concept is natural as far as temperature distribution and thermal balance are concerned in the operation of a nuclear reactor. However, if some abnormality occurs during reactor operation and it is necessary to stop the reaction urgently, one method of stopping the reaction is to inject neutron-absorbing liquid (poison) into the moderator, as follows: There is a problem. That is, when a neutron-absorbing liquid is injected into the circulating flow of the moderator being fed into the control rod guide tube 2 or 4 in the direction of arrow A or the direction of arrow E, this neutron-absorbing liquid flows into the calandria tank 1. must be incorporated into the moderator in a very short time to an effective concentration. Specifically, the content of the neutron absorbing liquid in each part of the calandria tank 1 must reach an effective concentration within a reference time. The reference time here is an allowable time that should be calculated in design according to legal safety standards, and is usually a short time of around 10 seconds in current nuclear reactors. In addition, the effective concentration of neutron-absorbing liquid that can stop the reaction is about a few ppm in normal current nuclear reactors. As mentioned above, from the viewpoint of mixing the neutron absorbing liquid into the calandria tank 1 as uniformly as possible within the shortest possible time, it is designed solely for the purpose of uniform cooling as explained with reference to FIGS. 1 and 2. Conventional circulating flow paths and flow distributions are not optimal. Briefly explaining the malfunction situation, when the control rod guide tube 2 of the type shown in Fig. 1 is used, the moderator containing the neutron-absorbing liquid flows into the tank 1 in the direction of arrow A. In C, the neutron absorption liquid concentration increases relatively early. However, until the moderator in section C rises with the steady flow of moderator in the tank and reaches section D near the top, the concentration of the neutron absorbing liquid in each part of the calandria tank 1 reaches an effective concentration that can stop the reaction. No. Generally, the total horizontal cross-sectional area of the control rod guide tube is significantly narrower than the horizontal cross-sectional area of the ascending flow path in the calandria tank 1, so the steady circulation flow rate in the calandria tank 1 is relatively slow, and the flow rate from section C to D The flow time to reach this point is just over 10 seconds. This wasted flow time is a significant loss compared to the standard time (allowable time until reactor shutdown), which is about 10 seconds as described above. Furthermore, as shown in Fig. 2, in the case of the control rod guide tube 4 in which discharge ports 4a, 4b, 4c, and 4d of equal diameter are bored at equal intervals, the following non-uniformity in the concentration of the neutron-absorbing liquid occurs. . That is, in the same figure, when comparing the flow path passing through the discharge port 4a and the flow path passing through the discharge port 4d among the flow paths of the moderator containing neutron absorbing liquid injected in the direction of arrow E, it is found that the control The length of the flow path from the top of the rod guide tube 4, through the discharge port 4a, to the outflow tube 3 in the direction of arrow F, is from the top of the control rod guide tube 4, through the discharge port 4d, to the outflow tube 3 in the direction of arrow J. It is extremely short compared to the length of the flow path. Therefore, at the time when the moderator containing the neutron-absorbing liquid descends inside the control rod guide tube 4 and starts flowing out from the discharge port 4d,
The moderator flow containing the neutron absorbing liquid that has flowed from the discharge port 4a in the direction of the arrow F has already reached the outflow pipe 3 and has started overflowing. At the above-mentioned time point, the moderator flow containing the neutron-absorbing liquid from the discharge port 4d has started to pass through the discharge port 4d, and the concentration of the neutron-absorbing liquid is still zero in the portion C shown in the figure. At this time, the neutron absorbing liquid contained in the moderator that has already overflowed from the outflow pipe 3 is a serious loss from the viewpoint of uniformly increasing the concentration in each part of the tank within the shortest possible time. The present invention was made in view of the above circumstances, and
The specifications (i.e., the shape and dimensions of each outlet) of the plurality of outlets drilled at equal intervals in the control rod guide tube are set appropriately, and the flow is carried out from each outlet into the calandria tank within the above-mentioned standard time. By keeping the amount of incoming neutron absorbing liquid equal, the purpose is to quickly and reliably stop the reaction by injecting the neutron absorbing liquid without significantly disturbing the cooling circulation during normal operation. be. Next, the configuration of an embodiment of the present invention will be explained based on FIG. 3. One of the several dozen control rod guide tubes 5 fixedly installed through the top surface of the calandria tank 1 is illustrated as an example. The guide tube 5 has five discharge ports 5a, 5b,...5e bored at equal intervals, and the hole spacing l dimension is 70 mm in this embodiment.
Made in cm. The neutron absorbing liquid supply device 6 is connected to a branch pipe 8 via a valve 7 . The branch pipes 8 have a number of branch pipes 9 _-1 , 9 equal to the number of control rod guide pipes provided in the calandria tank 1 .
_-2 ,...9 _-o , one of which, branch pipe 9 _-4 , is connected and communicated with the control rod guide pipe 5 illustrated in this figure, and when the valve 7 is opened, the neutron absorbing liquid supply device A neutron absorbing liquid (for example, ¹⁰ B) pumped from the control rod guide tube 5 is injected into the moderator circulation flow arrow K in the control rod guide tube 5. The main specifications in this example are as follows. Standard time…8 seconds Control rod guide tube cross-sectional area…5.568×10 ^-3 m ² Moderator flow rate per control rod guide…0.01642
m ³ /sec Neutron absorbing liquid concentration in control rod guide tube...
3000ppm ( ¹⁰ B conversion) Calandria tank capacity...200m ^3The most upstream discharge port 5a is named the first stage discharge port, the next discharge port 5b is named the second stage discharge port, and the most downstream discharge The outlet 5e is referred to as a fifth stage outlet. In this embodiment, the time _t1 from the injection operation of the neutron absorbing liquid until the discharge of the neutron absorbing liquid is started from the first stage discharge port 5a is 4.5 seconds.
The time t _o until the ejection of the neutron absorbing liquid starts from the ejection port of the n-th stage is calculated by the following equation. However, S: Control rod guide tube cross-sectional area (in this example, 5.568×10 _- ³ m ² as described above): Discharge port interval (in this example, as described above)
0.7 m) Q: Flow rate in the control rod guide tube (in this example, 0.01642 m ³ /sec as described above) After the reactor shutdown signal is sent, until the neutron absorbing liquid reaches the discharge port, the neutron absorbing liquid is not injected into the calandria tank, the reference time T (in this example, 8
The weight G _o of the neutron absorbing liquid discharged per second) is expressed by the following equation. G _o = (T - t _o ) x Q _o x γ However, T: After the reactor shutdown signal is sent, the time required to complete the reactivity injection necessary for safe shutdown of the reactor by injecting neutron absorbing liquid t _o : Reactor shutdown The time from when the signal is sent until the neutron absorbing liquid starts to be discharged from the n-th stage discharge port Q _o : The discharge flow rate from the n-th stage discharge port, and Q _o =
It is expressed as Q×η _o . Here, η is the flow rate ratio of the n-th stage discharge port, and is expressed as η _o =Q _o /Q. Note that η _o is determined by the flow rate balance depending on the specifications (diameter, number) of each discharge port. γ: Specific weight of neutron absorbing liquid (1100Kg/m ³ in this example) In this example (see Figure 3), each discharge was calculated as shown in the following table using the above calculation formula with η _o as a parameter. The outlet flow distribution was determined.

[Table] In other words, from the discharge port 5a, 14% of the flow rate in the pipe
, 16% from 5b, 18% from 5c,
The diameter of each hole was determined so as to discharge 21% from 5d and 31% from 5e. Note that the shape of each discharge port does not necessarily have to be circular, but in this example, a circular hole was designed to achieve the above flow rate distribution, and the above-mentioned shape was corrected experimentally based on rough calculations. The flow distribution was obtained as follows. Next, the effects of the configuration in which the flow rate distribution is determined as described above will be explained. The entire flow situation of the moderator containing the neutron-absorbing liquid flowing into the calandria tank 1 from the five discharge ports will be described later, but for the sake of explanation, first, the flow of the neutron-absorbing liquid from the most upstream discharge port 5a,
A comparative explanation will be given of the inflow of the neutron absorbing liquid from the most downstream discharge port 5e. The following circumstances can be understood from the numerical values shown in the first column 5a and fifth column 5e of the above table. The discharge start time of the first-stage discharge port 5a is 4.5 seconds after the valve 7 shown in FIG. 3 is operated and communicated (column _t1 ). Therefore, the remaining time T- _t1 after the start of ejection within the standard time of 8 seconds is 3.5 seconds. Similarly, the 5th stage discharge port
5e starts discharging 6.34 seconds after operation, so the remaining time T-t ₁ is only 1.66 seconds. However, the discharge port 5a discharges 14% of the total flow rate for 3.5 seconds, while the discharge port 5e discharges 31% of the total flow rate.
Since it is discharged for 1.66 seconds, the value of flow rate x discharge time is almost equal. As shown in the table, the discharge port 5a is 4.5
Flow rate discharged per second (G ₁ column) is 8.85Kg, discharge port 5
The flow rate e discharged in 1.66 seconds ( _G1 column) is 9.31Kg. To explain the above situation over time based on the graph of FIG. 4, the line _G1 indicating the cumulative flow rate of the moderator containing the neutron absorbing liquid from the first stage discharge port 5a is 4.5
Starting from seconds and increasing in proportion to time, the reference time T
It reached 8.85Kg in (8 seconds). In addition, the line _G5 indicating the cumulative flow rate of the moderator containing the neutron absorbing liquid from the fifth stage discharge port 5e starts from 6.34 seconds and reaches the reference time T.
It reached 9.31Kg. In addition, the flow rate cumulative line G ₂ of the second stage discharge port 5b,
The flow rate cumulative line G ₃ of the third stage discharge port 5c, and the fourth
The cumulative flow rate line _G4 of the stage discharge port 5d is also 4.78.
After the reference time of 8 seconds after starting from the time points of 5.12 seconds, 5.12 seconds, and 5.57 seconds, the weight of each weight reached approximately 9 kg. Next, the graph in FIG. 5, which depicts the above-mentioned situation in correspondence with each outlet position in the calandria tank, will be explained. The vertical axis of this graph indicates the effective distance in the axial direction of the core, where the 0 point corresponds to the bottom of the calandria tank, and the 3.70 m point corresponds to the top of the calandria tank. Then, the five points shown on the vertical axis, namely
3.25m point, 2.55m point, 1.85m point, 1.15m point, and
The 0.45m points are discharge ports 5a, 5b, and 5, respectively.
This corresponds to positions c, 5d, and 5e. The horizontal axis represents the discharge amount G (Kg) of the moderator containing the neutron absorbing liquid. In the graph of FIG. 5, a curve 6s indicates the discharge amount G (Kg) 6 seconds after the neutron absorbing liquid injection operation.
Similarly, the curve 7s shows the discharge amount G (Kg) after 7 seconds, and the curve 8s shows the discharge amount G (Kg) after 8 seconds. To briefly explain the meaning of the curve 6s showing the discharge amount after 6 seconds, at the 3.25 m position corresponding to the uppermost discharge port 5a, the amount represented by the length of the arrow 5a is being discharged. It can be seen that at a position of 1.15 m corresponding to the fourth discharge port 5d, only the amount represented by the length of the arrow 5d is still being discharged. Furthermore, it can be seen that after this 6 seconds, the ejection has not yet started at the 0.45 m position corresponding to the fifth stage ejection port 5e. The fact that this curve 6s intersects the 0 discharge rate line at a position of 0.8 m on the vertical axis of the graph means that the beginning of the downward flow of the moderator containing the neutron-absorbing liquid injected into the control rod guide tube is in the tank. This suggests that it has reached a point about 0.8m from the bottom. In the curve 7s showing the discharge amount after 7 seconds, discharge started at the 0.45 m point corresponding to the discharge port 5e, indicating that the amount represented by the length of the arrow 5e was discharged. . In this way, each point on the curve 8s indicating the discharge amount after 8 seconds, which is the reference time, is lined up almost vertically, which means that each equal amount of neutrons is distributed at equal intervals in the height direction within the reference time. This indicates that the moderator containing the absorbing liquid has been discharged. In this example, when the valve 7 shown in FIG. 3 is opened and the neutron absorbing liquid is injected into the moderator circulation flow arrow K in the control rod guide tube 5, its concentration is kept constant (3000 ppm). Therefore, the fact that an equal amount of the moderator containing the neutron absorbing liquid is discharged as described above can be said to mean that an equal amount of the neutron absorbing liquid has been injected and mixed. In this embodiment, the discharge ports 5a...5e are arranged at equal intervals, so that the concentration of the neutron-absorbing liquid in the calandria tank 1 does not exhibit any macroscopic bias in the height direction. Furthermore, when the present invention is applied to distribute the flow rate of each discharge port 5a...5e, the discharge ports in the lower stage start discharging the neutron absorbing liquid later than the discharge ports in the upper stage compared to the reference time. In order to discharge an equal amount within the tank, a large flow rate must necessarily be used. This means that even during normal operation, the circulating flow rate of the moderator passing through the discharge ports in the lower stage is larger than the circulating flow rate of the moderator passing through the discharge ports in the upper stage. There is no risk of significant adverse effects on cooling inside the calandria during normal operation. Due to the effects described in detail above, the plurality of discharge ports 5a, 5 formed in the control rod guide tube 5 by applying the present invention
b...5n are arranged at equal intervals and the flow rate distribution of each discharge port is set so that the amount of neutron absorbing liquid discharged from each discharge port within the standard time is reliably and uniformly distributed within the standard time. The neutron-absorbing liquid can be injected and mixed into each part of the calandria tank at a predetermined concentration, and it does not have any particular adverse effect on the cooling effect due to moderator circulation flow in the calandria tank during normal operation.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic cross-sectional views of main parts of conventional moderator circulation devices, and Figure 1 is a circulation device using control rod guide tubes without discharge ports in the tube wall. , FIG. 2 shows a circulation system using a control rod guide tube with a discharge port bored in the tube wall.
FIG. 3 is a schematic diagram showing a cross section of the main part of a moderator circulation system in an embodiment of the present invention, with hydraulic piping symbols indicating a neutron absorption liquid injection device added, and FIGS. 4 and 5 are diagrams according to the present invention. It is an explanatory chart of the action and effect concerning one example. DESCRIPTION OF SYMBOLS 1... Calandria tank, 2, 4, 5... Control rod guide tube, 6... Neutron absorption liquid supply device, 7... Valve,
8...Distribution pipe, _9-1 , _9-2 , _9-3 , _9-4 ...Branch pipe of distribution pipe, 5a, 5b, 5c, 5d, 5e...Discharge port.

Claims (1)

[Claims] 1. In a pressure tube type nuclear reactor shutdown system consisting of a control rod guide tube with a uniform cross section and a plurality of discharge ports, a neutron absorbing liquid supply device, and valves and piping attached to the neutron absorbing liquid supply device, the above-mentioned In addition to arranging the discharge ports drilled in the control rod guide tube at equal intervals in the axial direction of the tube, the standard time required for the reactor shutdown function as the specified reaction rate time by injecting the neutron absorbing liquid into the reactor body. A neutron-absorbing liquid characterized in that the specifications of each discharge port are individually set to determine the flow rate distribution so that the amount of the neutron-absorbing liquid discharged from the discharge port is equal for each discharge port. Nuclear reactor shutdown equipment.