JPS63180890A - Fuel failure detector in fast breeder reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides an in-reactor system that enables accurate estimation of the scale of fuel pin damage in a fast breeder reactor (hereinafter referred to as FBR). This invention relates to a fuel failure detection device.

(Prior art) Nuclear reactors are operated with sufficient safety in mind and the usable time of the fuel pins loaded in the reactor core is limited, but in the unlikely event that the fuel pins break during operation. This may change the flow of coolant, disrupting the thermal balance of the nuclear fuel assembly and even the reactor core, and potentially interfering with reactor operation. Therefore, it is necessary to carefully monitor whether or not the fuel pin is damaged by any means.

By the way, in the FBH, if a fuel pin is damaged during operation, fission products (hereinafter abbreviated as FP) generated inside the fuel pin are released into liquid sodium, which is a coolant. Most of this FP is radioactive, and due to its nature, it contains rare gases FP (Kr, Xe, etc.), volatile FPs (Br, Rb, Te, I, etc.).

Cs, etc.) and non-volatile F P (Sr, Y, Zr, R
u, Ba, La.

Ce, etc.). By utilizing this phenomenon, the conventional F
To detect fuel pin damage in BH, the above-mentioned FP
Among them, the DN detection method detects volatile neutrons (hereinafter abbreviated as DN) emitted by some FP nuclides such as Br and I, and the rare gas FP that is released into the coolant and then transferred into the cover gas. Rh is detected by a gamma ray detector, and Rh is generated by radioactive decay of the rare gas FP.

A precipitator method, etc., which electrically collects ionic nuclides such as Cs and detects β rays emitted by the nuclides, has been adopted. When a fuel pin was found to be damaged using these detection methods, the FBH operation was stopped and the damaged fuel was replaced.

(Problems to be Solved by the Invention) However, the conventional detection method described above has the following problems. For example, in the DN method, a DN detector installed along the main piping of the secondary cooling system detects DN emitted from the FP existing in the piping, but a very large amount of neutrons are present in the reactor core. Therefore, it is difficult to shield the large amount of neutrons leaking from the reactor core. For this reason, the background due to leaked neutrons becomes extremely high and the DN signal is buried, resulting in a drawback that the background and the DN signal cannot be distinguished unless large-scale damage occurs.

In order to overcome this problem, attempts have been made to measure DN by bypassing a portion of the sodium in the secondary cooling system and transporting it to a location where it is less affected by neutrons leaking from the reactor core. However, in this method, since the bypass piping is thin, the DN discharge FP flowing through this bypass piping is
The detection sensitivity did not improve much because the absolute amount of DN-released FP was small, and all DN-released FPs were nuclides with a short half-life of less than one minute, so they decreased due to radioactive decay during sodium transport.

On the other hand, regarding the gamma ray detection method or precipitator method that uses the rare gas FP transferred to the cover gas, there are problems such as a low transfer rate of the rare gas FP from the sodium to the cover gas, a delay in the transfer time, and Since the cover gas was flowed while being diluted with argon gas, the detection sensitivity was still low. Furthermore, the argon gas used as the cover gas is activated by high-density neutron irradiation in the upper part of the reactor core. Therefore, Ar-4 emits strong γ-rays.
There was also a drawback that a large amount of 1 was generated in the cover gas, and the γ rays of the cover gas rare gas FP were interfered with by the radiation of Ar-41.

The present invention was developed based on these circumstances, and its purpose is to accurately estimate the scale of fuel pin damage without causing any hindrance to the operation of the nuclear reactor, and to improve the possibility of continuing the operation of the reactor. It is an object of the present invention to provide a fuel bottle damage detector device that can contribute to accurate judgment of the necessity of replacing damaged fuel, thereby contributing to economical operation of a nuclear reactor.

[Structure of the invention]

(Means for Solving the Problems) The method for detecting fuel damage in a nuclear reactor of the present invention includes providing a bypass flow path in the primary coolant circulation path of a nuclear reactor that uses liquid metal as the primary coolant, and A capture device for capturing FP is interposed in the bypass channel, and the liquid metal is caused to flow through the bypass channel for a predetermined period of time to cause the capture device to capture the FP in the liquid metal, and then the liquid metal is separated from the capture device. The present invention is characterized in that the scale of fuel pin damage is estimated by measuring the amount of radiation emitted from the FP captured in the capture device.

That is, the present invention focuses on the fact that FP released into a liquid metal is easily adsorbed to a specific member, and that the higher the sodium temperature and the faster the sodium flow rate, the faster the FP is deposited on the member. After capturing the FP in a capture device composed of the above-mentioned members and separating the liquid metal from the capture device, the FP captured by the capture device is
It is characterized by measuring radiation emitted from P.

(Function) According to the present invention, when the liquid metal is made to flow through the capture device for a predetermined period of time, a swirling flow of liquid sodium is applied to the capture device to increase the flow velocity, and the liquid sodium is filled into the capture device. F
Since the liquid metal is separated from the capture device after the FP deposition rate on the P adsorbent is increased to increase the amount of deposition, the FP can be concentrated and accumulated in the capture device.

For this reason, highly intense radiation is emitted from the FP captured by the capture device, and since the liquid metal is separated from the capture device, activation products of sodium, which is present in large quantities in the liquid metal, are emitted. It is possible to lower the background level of certain Na-24, etc., and to reduce the F captured by the capture device.
Radiation from P can be measured with high sensitivity, and the amount of FP in the liquid metal can be estimated with high accuracy. Furthermore, since the FP is captured in the bypass flow path, it is possible to measure the radiation dose at a location where there is less influence from the large amount of γ-rays and neutrons leaking from the reactor core.

(Example) An example of the present invention will be described below with reference to one drawing.

FIG. 1 is a diagram schematically showing an FBR plant applied to explain the first embodiment of the present invention, and the reference numeral 1 in FIG.
is the reactor vessel. A reactor core 2 is housed inside this reactor vessel 1 . This reactor core 2 is cooled by liquid sodium Q contained inside the reactor vessel 1. The upper and lower parts of the inside of the reactor vessel 1 are communicated by a primary cooling system piping 3, and liquid sodium Q is moved from the lower part of the reactor core 2 to the upper part of the reactor core 2 through this secondary cooling system piping 3. I have to. -The secondary cooling system piping 3 includes:
An intermediate heat exchanger 4 for releasing heat from the liquid sodium Q and a pump 5 for circulating the liquid sodium Q are provided. Note that R in the figure is a cover gas filled in the middle of the upper part of the liquid sodium Q.

A bypass path 6 is provided in the primary cooling system piping 3 on the outlet side from the reactor vessel 1 to the intermediate heat exchanger 4 . An FP detector 7 is installed in this bypass path 6.

Specifically, the FP detection system 7 is configured as, for example, a non-volatile FP detection system, as shown in FIG.

That is, the bypass channel 6 is provided with an F"P capture device 11. This FP capture device 11 is composed of a stainless steel container covered with a blocker such as lead. As shown in Figure 3, a roll-shaped metal filling S made of stainless steel or the like is filled.In the process of flowing liquid sodium Q into the interior, the non-volatile FP contained in the liquid sodium Q is captured by the FP. It is designed to be deposited on a roll-shaped metallic filling S made of stainless steel or the like inserted into the device 11.

Valves 12 and 13 are provided in the upstream side 6a and downstream side bypass channel 6b of the FP capture device 11, respectively. In addition, in the bypass flow path 6b on the downstream side where the valve 13 is provided, there is liquid sodium Q inside the FP capture device 11.
A pump 14 is provided for circulating the water.

There is a valve 15 on the L flow side of the FP capture device @11. It communicates with a tank 16 that stores liquid sodium through the tank 16.

On the other hand, on the downstream side of the FP capture device 11, a cover gas R such as argon gas or nitrogen gas is introduced into the interior via a valve 17.
The valve 1 communicates with the gas cylinder 18 filled with
It is connected to a vacuum pump 20 via 9.

In addition, in the vicinity of the FP capture device 11, there is a gamma ray for detecting gamma rays from radioactive substances deposited and remaining on the FP capture device 11 and the filling S inserted into the device, which is shielded from the outside with lead or the like. A detector 21 is installed.

Next, the capturing device 11 will be explained in detail. FIG. 3 is a longitudinal cross-sectional view of the capture device 11, and FIG. 4 is a cross-sectional view taken along the X-Y plane of FIG. 3. Several inflow pipes 6a for liquid sodium Q are provided on the side surface of the capture device 11. This inflow pipe becomes thinner toward the top. Of course, the number of inflow pipes 6a may be one. A roll-shaped filler S such as a thin metal plate such as stainless steel is placed inside the capture device 11 and supported by a spacer 8 . The liquid sodium Q that has passed through the capture device 11 flows out through the outlet 6b.

The operation of the nonvolatile FP detection system 7 configured in this way will be explained.

Currently, liquid sodium Q is in the reactor primary cooling system piping 3.
Assume that the is being circulated. If the fuel bottle (1) (not shown) housed inside the core 2 is damaged in this state, the above-mentioned rare gas FP and volatile FP will be present in the liquid sodium layer.
In addition to being released, if the damage becomes large-scale, non-volatile F
P is released.

These FPs circulate within the primary cooling system piping 3 as the liquid sodium Q moves.

When the valves 12 and 13 of the bypass channel 6 are opened and the pump 14 is operated, liquid sodium Q flows horizontally into the capture device 11 through the inflow pipe 6a. The liquid sodium Q flowing in the horizontal direction becomes a high-speed flow and rises along the rolled metal filling S as shown by an arrow 9. Since the pressure drop decreases as one goes upward, the cross-sectional area of the inflow pipe 6a is made smaller as one goes upward, so that the liquid sodium Q flows in at the same flow rate from all the inflow pipes.

The process by which FP in liquid sodium Q is deposited on a solid surface such as a channel wall surface is explained as FP reaching the solid surface and depositing due to diffusion within the boundary layer in liquid sodium Q.

The boundary layer refers to the laminar flow region between the zero flow velocity near the solid surface and the maximum velocity in the mainstream. The thinner this boundary layer is, and the faster the diffusion rate of FP is, the greater the amount of FP deposited on a unit solid surface per unit time. The faster the flow rate, the thinner the boundary layer becomes, and the higher the temperature, the faster the diffusion rate of FP, so the amount of deposition increases. The device of the present invention utilizes these phenomena, and gives a swirling flow within the capture device to increase the flow rate of liquid sodium Q and increase the FP deposition on the roll-shaped packing S. By maintaining the sodium temperature at a high temperature using a heat insulating material or a heating device not shown in , the amount of deposition on the filling material S was further increased. Furthermore, the amount of this deposit is proportional to the amount of nonvolatile FP present in the liquid sodium Q, that is, the amount is proportional to the scale of the fuel pin damage. In this way, the FP is placed on the filling S.
Increasing the amount of deposition increases the intensity of the gamma rays to be detected, which has the great advantage of increasing the sensitivity for detecting fuel damage.

After a predetermined period of time has elapsed, the valves 12 and 13 are closed, the valves 15 and 17 are opened, and the cover gas R filled in the gas cylinder 18 is supplied into the FP capture device 11. As a result,
The liquid 1-relum Q contained in the FP capture device 11 is discharged into the tank 16. Therefore, FP capture device 1
As described above, only non-volatile FP remains inside 1. Therefore, the gamma rays emitted from this remaining non-volatile FP are measured by the gamma ray detector 21.

When the measurement is completed, valves 15 and 17 are closed, valve 19 is opened, and vacuum pump 20 is operated. As a result, the cover gas R filled inside the FP capture device 11 is evacuated and guided to an exhaust gas treatment system (not shown). After this evacuation is completed, the valve 19 is closed, and the valves 12 and 13 are opened again to introduce liquid sodium Q into the FP capture device 11.

As described above, the special shape of the FP capture device 11 allows nonvolatile FPs in the liquid sodium Q, especially nuclides such as Sr, Y, Ba, and La, to be quickly and efficiently transferred onto the filling material S in large quantities. Deposited and cooled down
Since the sodium containing the radioactivity of -24 is discharged into the tank 16, only non-volatile FP, which accurately indicates the scale of damage to the fuel pin, is deposited and accumulated in the roll-shaped filling S. Emitted gamma rays can be measured with very high sensitivity.

In addition, as a method for detecting this γ-ray, 1428.3Ke
The best method is to detect Sr-9'L, which emits V gamma rays. In this case, the energy of the γ-rays emitted from 5r-94 is higher than that of Mn-54 and Co-60, which are radioactive corrosion products (CP) contained in sodium and deposited on the stainless steel surface. Since this value is sufficiently high, extremely high sensitivity measurement is possible without being interfered with by Compton scattering of CP gamma rays. In addition to the above Sr-94, 9+ 5r-91, 5r-92, Sr 93. Y-t4
．． Tc-101, Ba-141, Ba-143, La
γ-rays such as -140 and La-142 may also be detected.

As another embodiment of the invention, the roll-shaped filler S made of stainless steel may be made of nickel. Further, any metal may be used as long as it is resistant to corrosion by liquid sodium.

In the above embodiment, the liquid sodium Q was flowed into the capture device 11 from below, but the present invention is not limited to this.

For example, it may be introduced from above.

Furthermore, if the roll-shaped packing S is used in the form of a roll of corrugated plates as shown in Figure 5 instead of a flat plate, the flow will become more complicated, and the boundary layer in the liquid sodium will become thinner and more uniform. Since the separation of the boundary layer occurs in this region, the amount of FP deposited increases accordingly, and the surface area for deposition also increases, which is further advantageous. On the other hand, as the filler S, a flat plate with protrusions may be used, or a metal mesh may be used.

It is also possible to provide a cover gas section in the upper part and make the upper part a flange structure so that the filling material S can be replaced easily.

Furthermore, if several of the above-mentioned capture devices 11 are installed in parallel, FP in the liquid sodium can be semi-continuously monitored by sequentially conducting liquid sodium flow, draining, and gamma ray measurement through the capture devices 11. becomes possible.

〔Effect of the invention〕

According to the present invention, Sr + Tc + B is released into the coolant only in the event of a fuel failure on a scale that makes it difficult to continue operation of the nuclear reactor.
Since gamma rays from non-volatile FPs such as a and La can be detected with high sensitivity, the scale of fuel damage can be accurately detected without affecting the operation of the reactor. The number of times the reactor is scrammed can be reduced, ultimately increasing the economic efficiency of reactor operation.

[Brief explanation of the drawing]

FIG. 1 shows an FBR for explaining the first embodiment of the present invention.
Fig. 2 is a flow sheet diagram showing the FP detection system in the plant shown in Fig. 1; Fig. 3 is a vertical cross-sectional view showing the FP capture device in Fig. 1; 4 is a cross-sectional view of a trapping device according to the present invention, and FIG. 5 is a cross-sectional view of a trapping device according to a second embodiment of the present invention. 1...Reactor vessel, 2...Reactor core, 3...-
Secondary cooling system piping, 4...Intermediate heat exchanger, 6...Bypass piping, 7...Nonvolatile FP detection system, 11.
...FP capture device, Q...liquid 1~lium. Agent Patent Attorney Nori Ken Chika Yudo Hirofumi Futamata − ′″′ Figure 3 b Figure 4

Claims (1)

[Claims] A bypass flow path is provided in the primary coolant circulation reactor of a nuclear reactor that uses liquid metal as the primary coolant, and a tank filled with stainless steel roll-shaped filler is provided in this bypass system, and the primary coolant is supplied to the tank through bypass piping. In the method of detecting fuel damage by draining the coolant in the tank and measuring the radioactivity of the radioactive fission products deposited in the filling, the coolant is injected from the side of the tank and the coolant is injected into the tank. A fuel failure detection device in a fast breeder reactor, which is characterized by generating a high-speed swirling flow and greatly increasing the amount of fission products deposited on a roll-shaped packing.