JPS6111398B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cooling pot (transfer pot) used when removing spent nuclear fuel from a reactor.

When taking spent fuel out of the reactor, the fuel assembly is first stored in a transfer pot by the in-core relay mechanism, then transferred as is to the out-of-core relay container by the in-core cask car, and then transferred to the storage rack by the out-of-core relay mechanism. It can be paid to During this process, the decay heat of the fuel assembly continues to be removed by the sodium in the transfer pot. Because the fuel assembly is placed in the cooling pot by remote control and because the atmosphere is an inert gas, the top of the cooling pot remains open.

Conventional pots with such functions are
As shown in the figure, there is a cooling fan 2 on the outer periphery of the vertically placed pot body 1, and the inside is filled with sodium 3, and the spent nuclear fuel assembly 4 is immersed in the sodium 3 while remaining in the wrapper tube 5. The structure is such that the wrapper tube 5 is supported by the support mechanism 7 so as to ensure a gap for the sodium 3 to flow easily at the lower part of the entrance nozzle 6. The sodium heated by the decay heat of the fuel assembly 4 rises and radiates heat through the cooling fan 2, and is naturally circulated as shown by the arrows.

By the way, there is a good chance that some of the fuel pins may have broken cladding. If the cladding is damaged, fission products are released through it. Among the fission products, ¹³⁴ and ¹³⁷ Cs in particular have a long half-life, are easily soluble in sodium, and are more volatile than sodium, so they evaporate from the sodium liquid surface and dissipate to the outside through the opening of the pot body 1. This would lead to unnecessary radioactive contamination. It is extremely beneficial to prevent this radioactive contamination and at the same time to reduce the degree of radioactive contamination of the fuel assembly 4 including the wrapper tube 5 as much as possible in improving the workability of the subsequent fuel treatment process. In addition, the pot body 1
It is also desired to reduce the degree of radioactive contamination of sodium 3 and sodium 3. This sodium is not replaced for each fuel assembly, but is utilized for as many fuel assemblies as possible.
When disposing of sodium, the level of radioactive contamination affects the disposal method and work process.

The present invention was made to solve these problems in the conventional technology, and its purpose is to prevent the spread of contamination outside the cooling pot by nuclear fission products, especially the problematic ¹³⁴ and ¹³⁷ Cs. To provide a spent fuel transfer pot that simultaneously reduces contamination of the fuel assembly, fuel pipe, sodium, and pot body, thereby improving safety, maintainability, and efficiency in handling the fuel assembly. There is a particular thing.

In order to achieve this object, in the present invention, a cesium collector is installed in the cover gas space above the sodium liquid level of the cooling pot.

Hereinafter, the present invention will be explained in detail based on the drawings.
FIG. 2 is a longitudinal sectional view showing one embodiment of the present invention, and FIG. 3 is a plan view thereof. There are cooling fins 2 on the outer periphery of the pot body 1, and the inside is filled with sodium 3. The spent nuclear fuel assembly 4 is inserted into the wrapper tube 5 so as to be immersed in the sodium 3. The support mechanism 7 is attached to the support tube 5 so that a gap can be secured at the bottom of the entrance nozzle 6 to allow the sodium 3 to flow easily.
Supported by The configuration up to this point is the same as the conventional transfer pot. A feature of the present invention is that a collection device for volatile radioactive substances, particularly fission product cesium, is provided in the cover gas space above the sodium liquid level.

The collection device 10 is a carbon member (granular material such as graphite, activated carbon, glassy graphite, etc.)
A cesium collector 11 made of cesium, etc. is wrapped in a package 12 made of stainless steel, nickel wire mesh, etc., and a plurality of approximately fan-shaped elements each having a stainless steel floating bag 13 sealed with argon gas fixed to the bottom are arranged closely together. This is designed to cover almost the entire surface of the sodium 3 liquid filled in the pot body 1. Each fan-shaped element independently has a support 14 protruding outward from its arc portion, and by pivoting the tip of the support 14 to the inner wall of the pot body 1, it can be rotated in the vertical direction. Supported by
Normally, it is left still on the sodium 3 surface by the action of the floating bladder 13.

When the fuel assembly 4 and the tube 5 are inserted into the sodium 3 in the pot, each fan-shaped element of the cesium collector 10 is rotated downward and pushed into the sodium 3, but the upper part of the tube 5 is submerged. Then, due to the action of the swim bladder 13, it returns to its original position,
Cover the sodium liquid level. The sodium vapor and cesium vapor evaporated from the liquid surface of the sodium 3 are convected or diffused out of the pot body 1 through the gap in the cesium collector 11 made of a carbon member. In this process, cesium ( ¹³⁴ , ¹³⁷ Cs) is effectively collected by the cesium collector 11.

In addition to the carbon material, zeolite, magnesium oxide, aluminum oxide, etc. may also be used as the cesium collector.

It has already been demonstrated that carbon members can effectively capture cesium in sodium, not only in the sodium but also in the sodium cover gas phase. For example, when vacuum distillation was performed at 500°C with activated carbon placed in a cover gas phase of sodium containing dissolved ¹³⁷ Cs to cover the sodium surface, approximately 90% of the ¹³⁷ Cs in the sodium was captured by the activated carbon. Furthermore, the collected ¹³⁷ Cs was stable even in air at 500°C. Therefore, in the present invention, ¹³⁴ and ¹³⁷ Cs evaporated from sodium whose temperature has increased due to the decay heat of the fuel assembly 4 is collected while passing through the cesium collector 11. Since the cesium concentration in the gas in this part decreases, ¹³⁴ and ¹³⁷ Cs are further evaporated from the sodium and collected in the cesium collector 11.

In this way, the concentration of ¹³⁴ and ¹³⁷ Cs in sodium is rapidly reduced. In addition, it is known that ¹³⁴ and ¹³⁷ Cs in sodium are physically adsorbed on metal surfaces;
¹³⁴ , ¹³⁷ When the Cs concentration decreases, ¹³⁴ , 137 Cs is adsorbed on the walls of the fuel assembly 4, wrapper tube 5, pot body 1, etc. in order to maintain equilibrium with the concentration.
¹³⁷ Cs is desorbed. It is thought that such a phenomenon continues until the cesium trapping ability of the cesium collector 11 disappears. ¹³⁴ collected in a cesium collector,
¹³⁷ Cs does not release even when released into the air, making its disposal easy.

Since the present invention is configured as described above, it is possible to prevent the spread of ¹³⁴ , ¹³⁷ Cs contamination outside the pot, and to significantly reduce the level of ¹³⁴ , ¹³⁷ Cs contamination of the fuel assembly, wrapper tube, sodium, pot body, etc. Therefore, it has excellent effects such as improved safety, improved work efficiency, and increased economic profit regarding the subsequent handling of the fuel body.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the prior art, FIG. 2 is a vertical sectional view of an embodiment of the present invention, FIG. 3 is a plan view thereof, and FIG.
The figure is a sectional view showing the main part. 1... Pot body, 2... Cooling fin, 3...
... Sodium, 4 ... Fuel assembly, 5 ... Rapper tube, 10 ... Cesium collection device, 11 ... Cesium collector, 12 ... Packaging material, 13 ... Swimming bladder,
14...Support.

Claims (1)

[Claims] 1. In a vertical, sodium-accommodating cooling pot for transferring spent nuclear fuel for a nuclear reactor from inside the reactor to outside the reactor, the volatile fission product cesium is stored in the sodium cover gas space inside the cooling pot. The cesium collector is wrapped in a packaging material, and a plurality of fan-shaped elements each having a floating bladder at the bottom are placed close together to cover the sodium surface inside the cooling pot. A spent fuel transfer pot characterized in that each sector-shaped element is independently pivoted to a wall of the cooling pot so as to be rotatable in the vertical direction.