JPH0312280B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an overflow system for a fast breeder reactor that uses liquid metal as a coolant. A bypass pipe is installed that leads to the overflow return pipe, and a portion of the liquid metal pumped up from the overflow is constantly flowed into the overflow return pipe to prevent a sudden temperature drop when the overflow is interrupted and to alleviate the thermal shock applied to the pipe when restarting. It is related to overflow system.

[Prior Art] A liquid metal such as liquid sodium is used as a coolant in a fast breeder reactor. Low-temperature liquid sodium flows into the reactor vessel through an inlet pipe at the bottom, is heated in the reactor core, and flows out through an exit pipe at the top. Here, an overflow system is provided to control changes in sodium capacity due to operating conditions of the primary main circulation pump, temperature changes in the primary cooling system, etc., and to maintain a constant sodium level in the reactor vessel.

This overflow system consists of a reactor vessel, an overflow return pipe drawn out from the reactor vessel, an overflow tank connected to it, an overflow pumping pipe leading from the overflow tank into the reactor vessel, and an overflow pipe attached to the pumping pipe. It consists of an electromagnetic pump, etc. Liquid sodium in the overflow tank is pumped up by an electromagnetic pump and supplied into the reactor vessel, and excess liquid sodium above the overflow level is returned to the overflow tank through the overflow return pipe.

[Problems to be solved by the invention] The temperature of liquid sodium during reactor rated power operation in a fast breeder reactor is extremely high, and the temperature of liquid sodium in the primary cooling system is extremely high.
The temperature will exceed 500℃. In this state, if a reactor scram (such as an off-site power loss accident) occurs with the primary main circulation pump tripping, the sodium level in the reactor vessel will decrease due to the loss of the primary main circulation pump's ability to push up the liquid level and the sodium level. It decreases rapidly due to heat shrinkage. As a result, the flow of sodium in the overflow return pipe is interrupted, and the inside of the pipe becomes a gas space in a short time, causing a rapid temperature drop phenomenon. According to actual measurements, the temperature drop rate is as high as 6 to 7° C./min at and around the safety vessel penetration portion of the overflow return piping, which has a particularly large heat dissipation effect.

Thermal stratification (a phenomenon in which high-temperature sodium stagnates in the upper layer) occurs due to a decrease in the circulation rate of liquid sodium in the reactor vessel due to a trip in the primary main circulation pump, and the temperature of sodium at the reactor outlet decreases to the level of the reactor scram. At the same time, there is a rapid decline in
The upper sodium temperature is as high as during reactor power operation. From this state, when the sodium level in the reactor vessel recovers by pumping with the electromagnetic pump of the overflow system and reaches the overflow level,
High-temperature sodium from the upper layer flows into the overflow return pipe, causing a large thermal shock. This causes thermal fatigue failure and damages the overflow return piping.

Therefore, in the conventional technology, when a reactor scram accompanied by a trip of the primary main circulation pump occurs, the operation of recovering the liquid level in the reactor vessel by the electromagnetic pump is stopped just before overflow, and the cooling system is operated to prevent thermal shock to various parts. After confirming that it would not occur, the overflow was performed again. For this reason, restarting the nuclear reactor after it has been shut down occurs only after overflow can occur, and it takes more than ten to several tens of hours.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to prevent the temperature drop of the overflow return pipe from occurring even if the overflow is interrupted, to prevent the occurrence of thermal shock when the overflow occurs again, and to restart the pipe promptly. The purpose is to provide an overflow system that makes it possible.

[Means for Solving the Problems] The present invention, which can achieve the above objects, provides a bypass pipe that branches from an overflow pumping pipe, passes inside the reactor vessel, and reaches an overflow return pipe in an overflow system of a fast breeder reactor. established,
It is constructed so that a portion of the liquid metal pumped up by the overflow pumping piping always flows into the overflow return piping.

One end of the bypass pipe may be directly connected to the side of the overflow return pipe, or may be installed so that liquid metal is poured from the upper end opening of the overflow return pipe. Further, a flow rate adjustment valve may be provided in the middle of the bypass pipe so that the optimum flow rate can be set.

[Operation] During normal operation, excess liquid metal above the overflow level flows through the overflow return piping. At the same time, a portion of the liquid metal pumped by the overflow pumping pipe flows into the overflow return pipe through the bypass pipe. Of course,
The remainder of the overflow pumped liquid metal flows into the reactor vessel. In this way, a circulation loop for the liquid metal can be created, and the liquid level in the reactor vessel is kept constant.

If the liquid metal level in the reactor vessel falls below the overflow level for some reason, the overflow is interrupted. However, in the overflow return line, a portion of the overflow pumped liquid metal continues to flow through the bypass line.
For this reason, unlike the conventional technology in which all the liquid metal flows out and becomes a complete gas space, a warm state is maintained by the liquid metal that continues to flow. Therefore, even if the overflow operation is restarted and high-temperature liquid metal flows, thermal shock can be prevented from occurring and the piping can be prevented from being damaged.

[Example] FIG. 1 is an explanatory diagram showing an example of a liquid sodium cooled fast reactor to which the present invention is applied. A reactor vessel 12 having a reactor core 10 inside has a sodium inlet pipe 14 connected to its bottom and a sodium outlet pipe 16 connected to its upper side, so that low-temperature liquid sodium flows into the reactor vessel 12 through the inlet pipe 14. It enters the reactor core, is heated by the reactor core 10, and flows out from the outlet pipe 16. The outside of the reactor vessel 12 is covered with a safety vessel 18. An overflow system is provided to keep the sodium liquid level within the reactor vessel 12 constant.

This overflow system includes a reactor vessel 12, an overflow return pipe 20 that is open at one end at an overflow level L inside the reactor vessel 12, and is drawn out through the reactor vessel 12 and the safety vessel 18.
Overflow tank 22 into which the other end is inserted
and the safety container 18 from the overflow tank 22.
and the reactor vessel 12 through the reactor vessel 12.
an overflow pumping piping 24 that opens into liquid sodium inside, an electromagnetic pump 26 attached to it, and the overflow pumping piping 24.
It is composed of a bypass pipe 28 and the like that branches from the reactor vessel 12 and reaches the overflow return pipe 20 through the inside of the reactor vessel 12.

A feature of the present invention is that the overflow system is provided with a bypass pipe 28 that branches from the overflow pumping pipe 24, passes through the inside of the reactor vessel 12, and reaches the overflow return pipe 20.

In this embodiment, as shown in FIGS. 2 and 3, the bypass pipe 28 branches from slightly above the discharge port 30 of the overflow pumping pipe 24 within the reactor vessel 12. The overflow return pipe 20 is arranged in liquid sodium so as to extend approximately 1/4 of the way around the wall.
It is connected slightly below the overflow port 32 of. The operation of the overflow system configured as described above is as follows. From the overflow tank 22 to the electromagnetic pump 26
The liquid sodium pumped up by the reactor vessel 12 passes through the overflow pumping pipe 24.
flows into the. When the sodium level rises as a result, an amount in excess of the overflow level L flows through the overflow return pipe 20 and returns to the overflow tank 22. At the same time, a portion of the pumped sodium from the overflow pumping pipe 24 flows through the bypass pipe 28 to the overflow return pipe 20. In this way, under normal conditions, a circulation loop for liquid sodium is formed, and the sodium level in the reactor vessel 12 is kept constant.

If a reactor scram occurs with a trip in the primary main circulation pump during reactor power operation, the sodium level in the reactor vessel 12 will drop rapidly due to the loss of the liquid level lifting force by the primary main circulation pump and thermal contraction of the sodium. decreases to For example, in the experimental fast reactor Joyo, the primary main circulation pump trip is approximately 30
cmSodium level decreases. Therefore, no overflow occurs. However, since a portion of the sodium pumped up from the overflow pumping pipe 24 continues to flow through the bypass pipe 28 to the overflow return pipe 20, a temperature drop is unlikely to occur. Therefore, even if the primary main circulation pump trips and overflow stops, the temperature drop in the overflow return pipe 20 is suppressed, and thermal shock at restart can be prevented.

Note that sodium may be injected from the bypass pipe 28 into the overflow return pipe 20 from above the overflow port 32, as shown in FIG.

In these configurations, the bypass flow rate is, in principle, fixed at the initial set value, but it is also possible to make it variable by installing a flow rate control valve in the bypass pipe, if necessary. In the case of the one shown in FIG. 1, the number of through holes formed in the reactor vessel is unchanged from the conventional one, and only the addition of a bypass pipe is required, so the structure does not become complicated.

FIGS. 5 and 6 are explanatory diagrams showing other embodiments of the present invention. Since the basic configuration is almost the same as that shown in FIG. 1, corresponding parts are denoted by the same reference numerals and a description thereof will be omitted.
In this embodiment, the branch point of the bypass pipe 28 from the overflow pumping pipe 24 is outside the safety container 18, and the flow rate control valve 34 is installed at a position of the bypass pipe 28 outside the safety container. In this way, the flow rate of sodium passing through the bypass pipe 28 can be easily adjusted.

[Effects of the Invention] As described above, the present invention provides a bypass pipe that branches from the overflow pumping pipe and passes through the inside of the reactor vessel to the overflow return pipe, so that a part of the overflow pumped liquid metal always flows to the overflow return pipe. Because of this structure, even if the overflow operation is interrupted, the liquid metal can continue to flow inside, which suppresses the temperature drop in the piping, and has the effect of preventing the occurrence of thermal shock even if the overflow operation is immediately resumed. be.

Therefore, while the conventional technology required more than ten hours to restart, it is now possible to restart within one hour, and since there is no need for time to raise the temperature of the overflow system, the time to reach the rated output is further reduced. 2～
This can reduce the time by about 3 hours, and has the effect of significantly improving the operating rate of the nuclear reactor.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an example of applying the present invention to a fast reactor overflow system, Fig. 2 is a plan view thereof, Fig. 3 is an enlarged view of the vicinity of the overflow port, and Fig. 4 is a bypass to the overflow return pipe. FIG. 5 is an explanatory diagram showing another embodiment of the present invention, and FIG. 6 is a plan view thereof. DESCRIPTION OF SYMBOLS 10... Reactor core, 12... Reactor vessel, 20... Overflow return piping, 22... Overflow tank, 2
4...Overflow pumping pipe, 26...Electromagnetic pump, 28...Bypass pipe, 30...Discharge port, 32...Overflow port.

Claims (1)

[Claims] 1. In an overflow system that communicates between the reactor vessel and overflow tank of a fast breeder reactor by an overflow return pipe and an overflow system pumping pipe, an overflow system branches from the overflow pumping pipe and passes through the inside of the reactor vessel to reach the overflow return pipe. An overflow system for a fast breeder reactor characterized by providing a bypass pipe and constantly supplying a portion of the liquid metal pumped up from the overflow to the overflow return pipe.