JPH01214796A - Emergency core cooling system of nuclear reactor - Google Patents

Abstract

PURPOSE:To urge a natural circulation and to enhance a long term cooling capability of a reactor core and also to enable a maintained flooding of the core, by concentrating a water injection outside a shroud by using an emergency core cooling system. CONSTITUTION:An emergency core cooling system consists of a high pressure spray system 11, a low pressure injection system 8, a residual heat removal system 12 and a cooling system during a reactor isolation. All openings of water injections are located outside of a reactor core shroud 5 which is placed inside of a reactor pressure vessel 1, and are located higher than an upper end of a reactor core 3. An emergency core cooling water is injected in between the shroud 5 in the vessel 1 and the vessel 1 itself to urge a flowing along with a flowing direction of a natural circulation in the vessel 1 and also to urge a core cooling for a long term cooling at a loss of coolant accident. Moreover, because openings of injection for an emergency core cooling water are located higher than an upper part of the reactor core, a water level outside the shroud 5 by the injected water can be maintained at a high level and a two phases water level inside the shroud 5 can be maintained high accordingly, therewith a flooding of the core can be firmly maintained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a boiling water nuclear reactor, and in particular to an emergency reactor suitable for promoting natural circulation capability and ensuring effective core cooling in the event of a loss of coolant accident. Regarding core cooling system.

[Conventional technology]

FIG. 2 shows the reactor pressure vessel l and emergency core cooling system of a conventional boiling water reactor (BWR).

In conventional boiling water reactors, the recirculation system consists of two loops each having one recirculation pump 2 outside the pressure vessel 1 . Approximately 1/2 of the coolant circulating in the core 3
3 is taken out to this recirculation system, pressurized by the recirculation pump 2, and then supplied to the jet pump 4 as a driving fluid to its nozzle. The remaining approximately 273 is absorbed by jet pump 4 and mixed with the driving stream before flowing through core 3.

The jet pump 4 is disposed in the annular portion between the core shroud 5 and the pressure vessel 1 in the pressure vessel 1 and is connected to a recirculation system to circulate coolant to the core 3 .

In addition, the jet pump 4 pumps the reactor core 3 by about 273 cm with a static water head.
The height of its throat is designed to allow it to be submerged in water.

In this conventional example, the emergency core cooling system is a low pressure core spray system7. It consisted of a low-pressure water injection system 8 and a high-pressure core spray system 9, and the injection position was inside the core shroud 5.

In conventional boiling water reactors, the inlet of the recirculation system is located below the core part outside the core shroud 5.
If a guillotine rupture of the recirculation piping is assumed as a design basis accident, the reactor core will be exposed temporarily, but after the reactor pressure has sufficiently decreased, the water level in the reactor core will reach the level of the jet pump throat 6.

That is, 2/3 or more of the reactor core is kept flooded. Thereafter, cooling of the core 3 is achieved by natural circulation.

The emergency core cooling system adopted in conventional boiling water reactors takes into consideration the behavior in the event of such a loss of coolant accident, and uses an i-pressure core spray system7 to ensure cooling when the core is exposed.
A high-pressure core spray system 9 was also installed.

[Problem to be solved by the invention]

In the above conventional technology, the emergency core cooling system is all about injecting water into the inside of the shroud, and this idea is for temporary exposure of the core in the event of a loss of coolant accident, and for long-term cooling after re-flooding, natural circulation is used. It worked in the direction of inhibiting the

The object of claim 1 of the present invention is to provide an emergency core cooling system that has a water injection opening between a core shroud and a pressure vessel in a reactor pressure vessel and promotes natural circulation during long-term cooling. It is in.

In addition to the object of the invention as claimed in claim 1, the object of the invention as claimed in claim 2 is to maintain the water level contributing to core flooding at a high level to reliably maintain the flooding state.

[Means to solve the problem]

The object of the invention as set forth in claim 1 is achieved by the first means of providing the injection port of the emergency core cooling system in the area between the core shroud and the pressure vessel within the reactor pressure vessel.

Further, the object of the invention of claim 2 is achieved by, in addition to the first means, a second means in which all the injection ports of the emergency core cooling system are provided at a position higher than the level of the upper part of the core. Ru.

[Effect]

In the first method, emergency core cooling water is injected between the core shroud and the pressure vessel in the reactor pressure vessel, and the flow direction of natural circulation within the pressure vessel (downcomer → lower plenum → core → upper plenum), and works to accelerate core cooling during long-term cooling in the event of a loss of coolant accident.

In the second method, the emergency core cooling water inlet is located higher than the top of the core, so the water level outside the shroud due to the injected water can be maintained high, and the two-phase water level inside the shroud is also higher than the top of the core. This will ensure that the reactor core is maintained at a high level of flooding.

〔Example〕

A first embodiment of the present invention shown in FIG. 1 relates to a boiling water nuclear reactor employing an internal pump.

In this embodiment, the emergency core cooling system is a high pressure spray system 11.
Low pressure water injection system8. Residual heat removal system 12 and isolation cooling system 1
3, all of whose water injection openings are located outside the core shroud 5 within the reactor pressure vessel 1 and at a height higher than the upper end of the reactor core 3.

In the boiling water reactor employing the internal pump 10, there is no external recirculation loop compared to the conventional boiling water reactor illustrated in FIG.

Since there are no large-diameter pipes below the top of the core, there is no need to assume a pipe rupture below the top of the core as a design basis accident. Therefore, if the water injection opening of the emergency core cooling system is installed above the upper end of the core 3, even if the piping is ruptured, there is a small possibility that the core 3 will be exposed.

FIG. 3 shows the relationship between the water level outside the core shroud 5 and the water level inside.

Further, the two-phase water level in the core 3 is determined by the balance of the water heads inside and outside the core shroud 5 as follows.

6191g: h2ρ2. g hl: Shroud outer water level h2: Core two-phase water level ρ□: Shroud outer liquid phase density ρ2: Core coolant average density g: Gravitational acceleration Here, shroud 5 outer liquid phase density ρ1 is the core coolant average density Since it is larger than ρ2, when the above relationship holds, the two-phase water level h2 in the core 3 portion is maintained higher than the water level hl on the outside of the shroud 5.

Therefore, by maintaining the water level outside the shroud 5 high, the submergence of the three parts of the reactor core is maintained.

Furthermore, by raising the position of the injection pipe to the outside of the shroud 5 where there is a possibility of breakage, it is possible to maintain a high water level outside the shroud even if the pipe should break.
Therefore, it is possible to avoid exposing the reactor core 3.

FIG. 4 shows another embodiment of the invention.

In this embodiment, a case will be explained in which the present invention is applied to a natural circulation nuclear reactor.

A natural circulation reactor has a reactor pressure vessel (1) containing a reactor core (3) that generates heat due to a nuclear fission reaction. Main steam pipe 14 that sends steam generated in the reactor core 3 to the turbine. It is composed of a water supply pipe 15 that sends water condensed in the condenser to the reactor pressure vessel 1 again. The flow of water in the reactor pressure vessel 1 is as follows. Water supply pipe 1
The water supplied from 5 flows through the core shroud 5 through natural circulation.
It passes through the outside of the reactor core 3 and reaches the lower part of the reactor core, absorbs heat in the reactor core 3 and becomes steam, which is supplied to the turbine through the main steam pipe 14.

In this embodiment, an accumulator injection system 16 is also provided as an emergency core cooling system in the event of a loss of coolant accident, but the injection position of this system is also in the area between the reactor pressure vessel 1 and the core shroud 5. .

With this emergency core cooling system, even in events where the emergency core cooling system is expected to operate, such as during a loss of coolant accident, the coolant injection direction is the same as the normal flow direction of the coolant, which is natural. Circulation is not obstructed, but rather its natural circulation is promoted.

Any of the embodiments has the following effects.

(1) Because the emergency core cooling system is entirely injected outside the core shroud, in conventional systems it was necessary to remove the shroud head when maintaining the spray sparger, etc., but with the present invention, such work is eliminated. This greatly improves maintainability.

(2) In boiling water reactors and natural circulation reactors that employ internal pumps without external recirculation loops, rupture of emergency core cooling system piping is considered as a design standard assumption for loss of coolant accidents. However, by locating the injection position of the emergency core cooling system outside the shroud,
It is not affected by the shroud head and standpipe, and the injection position can be significantly raised compared to the core position. For this reason, the assumed rupture location can be raised higher, ensuring that the core remains submerged in the event of a loss of coolant accident.

In addition, during long-term cooling after an accident, the two-phase water level in the core is determined by the water level outside the shroud, but by raising the injection position of the emergency core cooling system, the water level outside the shroud can be maintained high. The two-phase water level inside the shroud, which contributes to core flooding, is also maintained higher than the top of the core.

(3) Long-term heat removal from the core in the event of a loss of coolant accident is achieved through natural circulation, but in order to inject all of the emergency core cooling system water to the outside of the shroud, it is necessary to ) → The natural circulation force of the reactor core is promoted and effective core cooling is achieved.

〔Effect of the invention〕

According to the invention set forth in claim 1, since the water injected by the emergency core cooling system is concentrated on the outside of the shroud, natural circulation is promoted and the long-term cooling capacity of the core can be improved.

According to the invention of claim 2, in addition to the effect of the invention of claim 1, the submergence of the reactor core can be maintained reliably.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of the emergency core cooling system of a nuclear reactor according to the first embodiment of the present invention, FIG. 2 is a system diagram of the emergency core cooling system of a conventional boiling water reactor, and FIG. A schematic explanatory diagram of the water level in the pressure vessel of the nuclear reactor according to the invention, and FIG. 4 is a system diagram of the emergency core cooling system of the nuclear reactor according to another embodiment of the invention. 1...Reactor pressure vessel, 2...Recirculation pump, 3.
...Reactor core, 4...Jet pump, 5...Core shroud, 6...Jet pump throat section, 7...
Low pressure core spray system, 8...Low pressure water injection system, 9...High pressure core spray system, 10...Internal pump, 1
1...High pressure spray system, 12...Residual heat removal system, 1
3... Isolation cooling system, 14... Main steam pipe.

Claims (1)

[Scope of Claims] 1. In a boiling water nuclear reactor that includes a reactor pressure vessel and a series of shrouds that surround the reactor core and extend upward, the inside of the reactor pressure vessel and the outside of the shroud include: An emergency core cooling system for a nuclear reactor is formed by opening a water injection opening for the emergency core cooling system in the area thus formed. 2. The emergency core cooling system for a nuclear reactor according to claim 1, wherein all water injection openings of the emergency core cooling system are provided at positions higher than the height of the upper part of the reactor core.