JPH0544637B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a water supply system for a boiling water nuclear reactor that supplies cooling water to a reactor pressure vessel in a boiling water nuclear reactor.

[Technical Background of the Invention] FIG. 1 shows a conventional water supply system for a boiling water nuclear reactor, and in the figure, reference numeral 1 indicates a reactor pressure vessel housing a reactor core 1a. A main steam pipe 3 connected to a turbine 2 that drives an electric motor 2a is connected to the upper part of the reactor pressure vessel 1, and a main steam isolation valve 4 is inserted into the main steam pipe 3.
The turbine 2 is connected to a condenser 6 through a pipe 5, and a hot well 7 is formed in the condenser 6. The hot well 7 and the reactor pressure vessel 1 are connected by a water supply pipe 8, and the water supply pipe 8 includes, in order from upstream, a low pressure condensate pump 9, a condensate desalination device 11, a high pressure condensate pump 14, and an on-off valve 15. ,
A low-pressure feed water heater 17, an on-off valve 18, a turbine-driven water pump 19, a check valve 20, an on-off valve 21, a high-pressure feed water heater 23, an on-off valve 24, and a check valve 25 are inserted. A motor-driven water supply pump 26 and a check valve 27 are arranged in parallel with the turbine-driven water supply pump 19 and check valve 20 .
The reactor pressure vessel 1 is provided with a recirculation pipe 28 for recirculating the coolant in the reactor pressure vessel 1, and the recirculation pipe 28 is equipped with a recirculation pump 30.
is inserted. The reactor pressure vessel 1 also includes a core spray system 31 and a low-pressure core injection system 3 that supply cooling water into the reactor pressure vessel 1 in an emergency.
2 are arranged.

That is, in the nuclear power plant configured as described above, steam generated in the reactor pressure vessel 1 is guided to the turbine 2 through the main steam pipe 3, drives the turbine 2, and then is converted to condensate in the condenser 6. is,
This condensate is led out by a low-pressure condensate pump 9, passes through a condensate desalination device 11, is pressurized by a high-pressure condensate pump 14, passes through an on-off valve 15, and is then supplied to a low-pressure feedwater heater 17.

The condensate supplied to the low-pressure feed water heater 17 is heated to a predetermined temperature here, sucked into the turbine-driven supply pump 19 driven by the water supply turbine 22, pressurized here, and then further heated to high-pressure feed water. It is heated in the reactor pressure vessel 23 and recirculated to the reactor pressure vessel 1 through the on-off valve 24 and the check valve 25 at a temperature of about 220° C. during normal operation.

In a boiling water reactor configured in this way, if a loss of coolant accident such as a rupture of the main steam pipe 3 occurs, the water level in the reactor pressure vessel 1 will drop, and this water level will drop below a certain level. When this happens, the reactor scrams and the nuclear reaction is stopped. When the reactor water level further decreases and reaches a certain level, the main steam isolation valve 4 is closed, the recirculation pump 30 is also tripped, and the flow rate of coolant flowing through the reactor core is reduced naturally due to the difference in static water head between outside the shroud and inside the shroud. Only the circulating amount.

On the other hand, even if a loss of coolant accident occurs, there is no built-in electrical circuit that would stop the operation of the water supply system, and when the main steam isolation valve 4 closes, the rotation speed of the turbine-driven water supply pump 19 will drop. The electric motor 16 is activated by the water supply flow rate signal based on this, and as a result, the electric motor-driven water supply pump 26 is operated, thereby continuing water supply. Note that when water is supplied by this motor-driven water supply pump 26, the water supply flow rate is approximately 50% of the rated value.

If such a loss of coolant accident occurs, the coolant inside the reactor pressure vessel 1 will be lost, and the coolant flowing through the reactor core will decrease, so the reactor core 1a
There is a risk that the fuel rods disposed within the fuel rods will be exposed, causing a temperature increase between the fuel and the cladding. Therefore,
Generally, boiling water reactors are equipped with an emergency core cooling system that injects coolant into the reactor to maintain the surface temperature of the fuel cladding below a predetermined temperature in the event of a loss of coolant accident.

[Problems with the Background Art] As described above, in the event of a loss of coolant accident, the emergency core cooling system operates, but a large amount of steam is generated in the lower plenum and the core 1a due to the drop in reactor pressure and heat transfer within the reactor core 1a. This steam generation causes a gas-liquid counterflow limitation (CCFL) phenomenon at the core 1a inlet orifice and upper tie plate.
The cooling water of the emergency core cooling system flows out of the separator, and the injection of the cooling water of the emergency core cooling system into the core is delayed, and the core fuel may not be cooled quickly.

That is, in general, in the event of a loss of coolant accident, the water level in the reactor decreases, and the water level in the reactor rises again due to the operation of the emergency core cooling system, thereby cooling the core combustion. In order to quickly cool this core fuel, it is necessary to speed up the injection of coolant into the core by the emergency core cooling system.

Generally, during a loss of coolant accident, the reactor core 1a is completely exposed when the low-pressure core injection system is activated, and coolant accumulates from the lower plenum to the core 1a due to the operation of the low-pressure core injection system.
However, due to the upward flow of steam generated in the lower plenum and core 1a due to the drop in reactor pressure, a gas-liquid counterflow restriction phenomenon occurs at the upper tie plate and core inlet orifice, causing the core to re-flood. may be delayed.

[Object of the Invention] The present invention has been made in response to such conventional circumstances, and it reduces the amount of steam generated in the reactor pressure vessel in the event of a loss of coolant accident in a boiling water reactor, and improves the efficiency of the emergency core cooling system. The present invention aims to provide a water supply system for a boiling water reactor that can speed up the injection of coolant into the reactor core.

[Summary of the Invention] That is, the present invention provides a water supply pipe including a low-pressure feedwater heater and a high-pressure feedwater heater in order from upstream for supplying condensate from a condenser into a reactor pressure vessel, and bypass piping equipped with on-off valves provided on the inlet and outlet sides of the high-pressure feedwater heater, and on-off valves arranged in parallel with the high-pressure feedwater heater; and a low-pressure reactor core that opens into the reactor pressure vessel. an output device that outputs an activation signal when the injection system is activated; and an output device that inputs the activation signal to open the on-off valve of the bypass piping and close the on-off valves provided on the inlet side and the outlet side of the high-pressure feed water heater, respectively. This is a water supply system for a boiling water reactor, characterized in that it is equipped with a control device.

[Embodiment of the Invention] The details of the present invention will be described below with reference to an embodiment shown in the drawings.

FIG. 2 shows a water supply system for a boiling water reactor according to an embodiment of the present invention. An on-off valve 21 provided,
Bypass piping 36 is arranged in parallel to bypass 24 and into which an on-off valve 34 and a check valve 35 are inserted.

Furthermore, a flow meter 37 is provided in the low-pressure core injection system 32 that opens into the reactor pressure vessel 1 to measure the flow rate of the coolant flowing through this pipe. In the figure, reference numeral 38 indicates a control device, and this control device 38 inputs a flow rate signal from the flow meter 37 and opens and closes the on-off valves 21, 24, and 34.

That is, this control device 38 inputs the signal from the flow meter 37, and when this value exceeds a predetermined value, determines that the low-pressure core injection system 32 has been activated, opens the on-off valve 34, and closes the on-off valve 21. , 24 are closed. The temperature of the feed water during normal operation is approximately 30°C at the inlet of the low-pressure feedwater heater 17, approximately 150°C at the outlet, and approximately 220°C at the outlet of the high-pressure feedwater heater 23.

In the boiling water reactor water supply system configured as described above, in the unlikely event that a loss of coolant accident occurs, the reactor water level and reactor pressure drop, and the low-pressure core injection system 32 is activated, the on-off valves 21, 24 is closed, the on-off valve 34 is opened, and the temperature is about 150 ℃ from the water supply pipe 8.
Feed water is supplied into the reactor pressure vessel 1 at a temperature slightly lower than that during normal operation at .

As a result, the reactor pressure decreases to approximately 5 atmospheres (saturation pressure of 150°C), and this low-temperature feed water undergoes reduced-pressure boiling. As a result, as shown by curve a in Figure 3, the decrease in reactor pressure is somewhat relaxed, the amount of steam generated due to depressurization is significantly reduced, and a gas-liquid counterflow restriction phenomenon occurs at the upper tie plate and core inlet orifice. As a result, the emergency core cooling system cooling water that had been accumulated in the upper plenum and core bypass suddenly flows into the core and lower plenum.

As a result, the re-flooding of the core is accelerated, and as shown by curve b in FIG. 4, the cooling capacity of the core can be improved.

In other words, Figure 3 shows the change in reactor pressure during a loss of coolant accident. The horizontal axis shows the time after the accident, the vertical axis shows the reactor pressure, and the solid line curve a is In the case of the embodiment shown in FIG.
A curve c shown by a broken line shows the conventional case. As is clear from the figure, in the case of the embodiment shown in FIG. 2, the sudden decrease in reactor pressure due to the operation of the low-pressure core injection system 32 can be alleviated.

Figure 4 shows the change in the maximum temperature of the cladding during a loss of coolant accident.The horizontal axis shows the time after the accident, and the vertical axis shows the maximum temperature of the cladding.The solid line curve b is In the case of the embodiment shown in FIG.
A curve d shown by a broken line shows the conventional case shown in FIG. As is clear from the figure, in the case of the embodiment shown in FIG. 2, the re-submersion time of the reactor core is accelerated, so that the maximum temperature of the cladding tube can be significantly reduced compared to the conventional method.

Furthermore, since the flow rate of the emergency core cooling system depends on the reactor pressure, it is possible that injection of the emergency core cooling system may be hindered by mitigating the decrease in reactor pressure due to depressurized boiling of low-temperature feed water. As is clear from Figure 5, when the reactor pressure is below 5 kg/cm ^{2 ,} the flow rate of each emergency core cooling system is approximately the rated flow rate, and no particular problem arises.

In other words, Figure 5 shows the relationship between the reactor pressure and the flow rate of each emergency core cooling system.The horizontal axis shows the flow rate, the vertical axis shows the reactor pressure, and the curves e and f , g indicate the cases of a low-pressure core injection system, a low-pressure spray system, and a high-pressure spray system, respectively. As is clear from the figure, when the reactor pressure is below 5 kg/cm ² , the flow rate is approximately the rated flow rate.

[Effects of the Invention] As described above, according to the boiling water reactor water supply system of the present invention, it is possible to hasten the re-flooding of the reactor core in the event of a loss of coolant accident, and as a result, the cooling performance of the reactor core is improved. , it is possible to maintain a larger safety margin for the reactor.

The present invention is also economically advantageous because it can be easily implemented by adding fewer valves, piping, and controls to existing systems.

[Brief explanation of the drawing]

FIG. 1 is a piping system diagram showing an embodiment of a water supply system for a conventional boiling water reactor, and FIG. 2 is a piping system diagram showing a water supply system for a boiling water reactor according to an embodiment of the present invention. Figure 3 is a graph showing changes in reactor pressure during a loss of coolant accident, Figure 4 is a graph showing changes in maximum cladding temperature during a loss of coolant accident,
FIG. 5 is a graph showing the relationship between reactor pressure and flow rate of each emergency core cooling system. 1...Reactor pressure vessel, 17...Low pressure feed water heater, 21, 22, 34...Opening/closing valve, 23...High pressure feed water heater, 32...Low pressure core injection system, 36...
Bypass piping, 37...flow meter, 38...control device.

Claims (1)

[Claims] 1. Water supply piping equipped with a low-pressure feedwater heater and a high-pressure feedwater heater in order from upstream for supplying condensate from a condenser into the reactor pressure vessel, and the inlet side and outlet side of the high-pressure feedwater heater of the water supply piping. an on-off valve provided in each of the high-pressure feedwater heaters, a bypass pipe provided with an on-off valve disposed in parallel with the high-pressure feedwater heater, and a low-pressure core injection system that opens into the reactor pressure vessel, and outputs an activation signal when the low-pressure core injection system opens into the reactor pressure vessel. and a control device that inputs the actuation signal and opens the on-off valve of the bypass piping and closes the on-off valves provided at the inlet and outlet sides of the high-pressure feed water heater, respectively. Features of boiling water reactor water supply system.