JPS62129795A - Method of washing spent fuel 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 [Industrial Application Field] The present invention relates to a method for cleaning spent fuel in a fast breeder reactor by reacting the sodium adhering to the spent fuel with a less corrosive compound.

[Conventional technology]

When replacing a fast breeder reactor that uses sodium as a coolant, #'fJ fuel taken out of the reactor still has a high PJ heat level, so it is temporarily stored in an underwater storage tank outside the reactor. After the radioactivity is sufficiently reduced, it is sent to processes such as reprocessing. At this time, since sodium adheres to fuel cladding materials, etc., sodium is washed and removed as a pretreatment before storing them in an underwater storage tank.

Conventionally, the spent fuel cleaning method involves first receiving the spent fuel in a cleaning tank, then forcedly circulating a dry inert gas such as argon or nitrogen gas to clean the fuel at a high temperature of about 400°C. Cool to around °C. Then, when a wet gas, which is a mixture of inert gas and water vapor, is supplied to the cleaning tank, the laminate adhering to the fuel cladding material reacts with the water vapor to oxidize it. The reaction formula in this case is 2 Na+2820=2 NaOH10Hz. In this reaction, in order to prevent an unexpectedly excessive reaction, operation is carried out by keeping the water vapor concentration low at the initial stage of supply, and gradually increasing the water vapor concentration while checking the reaction state. The highly concentrated sodium hydroxide adhering to the fuel cladding material is then washed away with pure water, and the fuel is then stored in an underwater storage tank and sent to processes such as reprocessing.

[Problem that the invention seeks to solve]

By the way, in this cleaning method using wet gas, a high concentration of hydroxide (hydroxide) rim is generated on the surface of the fuel cladding material due to the reaction between sodium and water vapor, but this compound is highly corrosive and can be used as a component of the fuel cladding material. It is generally said that stress corrosion cracking may occur in stainless steel materials under high temperature conditions of 120°C or higher.

Corrosion of this coating material increases the amount of radioactive corrosion products generated, and stress corrosion cracking can also cause fuel failure.
Neither is preferable.

Therefore, when cleaning spent fuel in the past, the temperature of the cladding material was sufficiently lowered before entering the wet gas cleaning process as described above, but when the decay heat level of the spent fuel was high, A large amount of cooling gas is required for this cooling, and a large amount of time is required for cooling.

The present invention was devised in view of the above-mentioned problems, and provides a cleaning method in which natrihum adhering to a coating material is reacted and generated into a less corrosive compound.

[Means for solving problems]

The cleaning method according to the present invention includes storing spent fuel in a cleaning tank in a fast breeder reactor, cleaning the spent fuel in the cleaning tank with a wet gas mixed with water vapor and carbon dioxide,
It is characterized in that the sodium adhering to the fuel is reacted with a wet gas to generate sodium carbonate.

[For production]

In this cleaning method, sodium (N n) reacts with water vapor (H2O) in the humid gas, forming 2Na+HzO=
It becomes Na20+H2 and produces sodium oxide (Na20).

Next, this Nn20 immediately reacts with carbon dioxide gas (CO2) and H2O to form Na 20 + CO2= N n 2 CO3N
a20 + H20 = 2 N ao H, and the sodium hydroxide (N (108
) continues to react with Co2 to form N n OH+ CO2= Na HCOs,
In addition, a part of the above sodium carbonate (N n 2 CO) is hydrolyzed to form Na 2 CO* + H20= Na HCO3
+N a OH. In this way, sodium carbonate (Na2CO0) and sodium bicarbonate (Na
Generates sodium carbonate such as HCO3).

〔Example〕

An embodiment of the cleaning method of the present invention will be described with reference to a block connection diagram of a cleaning device shown in FIG.

In the figure, 1 is the cleaning tank, and the spent fuel 2 is the upper valve 1.
It is designed to be stored and taken out through 4. Inert gas is supplied near the top of this cleaning tank 1! ? 3,
A desalinated water supply pipe 4 and a wet gas supply pipe 5 for cleaning are connected, and a waste gas pipe 6 for extracting waste gas generated in the cleaning tank 1 is also connected. The cleaning Jfe1 includes a cooler 7, a steam/water separator 8, a circulation block for inert gas,
and a heater 10 are connected in series, and a cooling circuit 11 is provided, the inlet side of which is connected to the vicinity of the corner of the cleaning tank 1, and the outlet side connected to the humid gas supply pipe 5, respectively. 12 is a washing water circulation pump, which transports washing water to washing tank 1.
A branch pipe is connected to the waste liquid tank 13 in such a way that the liquid is extracted from the lower part and recirculated to the upper part.

The structure is as described above, and the spent fuel 2 is transferred to the cleaning tank 1.
After closing the upper valve 14 of the cleaning tank, the circulation blower 9 is started to circulate the inert gas contained in the cooling circuit 11, the cleaning tank 1, etc. to cool the spent fuel. .

Next, the wet gas supplied from the wet gas supply pipe 5 is mixed with this inert gas and the mixture is supplied to the cleaning tank 1. The wet gas is a mixture of water vapor and carbon dioxide gas, and this wet gas reacts with sodium adhering to the spent fuel 2 to generate sodium carbonates such as sodium carbonate and sodium hydrogen carbonate. Here, the inert gas, excess water vapor, and carbon dioxide gas return to the cooling circuit 11, are cooled in the cooler 7, and after moisture is removed in the steam separator 8,
The supply temperature is adjusted by the heater 10 through the circulation blower 9, and the gas is mixed with the wet gas again and supplied to the cleaning tank 1.

Hydrogen gas and the like generated in the cleaning tank 1 during this reaction are diluted by the inert gas supplied from the supply W3 and extracted from the waste gas pipe 6 at an appropriate time.

While this reaction is in progress, the hydrogen concentration in the inert gas is constantly monitored, and when the amount of hydrogen produced as a reaction product decreases, the reaction is considered to be complete, and then desalinated water is supplied to W.
4 supplies desalinated water to the cleaning tank 1, and the circulation pump 12 is operated to circulate the cleaning water into the cleaning tank 1. Sodium carbonate, sodium hydrogen carbonate, and a small amount of sodium hydroxide thus produced are dissolved, and these solutions are stored in the waste liquid tank 13, completing the cleaning of the spent fuel 2.

〔Effect of the invention〕

As explained above, the present invention cleans sodium adhering to spent fuel with a wet gas mixed with water vapor and carbon dioxide, and after cleaning, sodium carbonates such as sodium carbonate and sodium hydrogen carbonate are produced. This product is significantly less corrosive than conventional sodium hydroxide, so it can prevent corrosion and stress corrosion cracking of fuel cladding materials, and it can also be used in high-temperature conditions of 120"C or higher. Since cleaning treatment is also possible, it is possible to downsize the equipment for cooling the fuel in the fuel cleaning equipment, reducing equipment costs.Furthermore, the time required to cool the cladding material can be shortened, and the use The spent fuel can be washed quickly, contributing to a reduction in operating costs.Therefore, the cleaning method according to the present invention is particularly effective when cleaning spent fuel with a high level of decay heat.

[Brief explanation of drawings]

Figure f51 is a block connection diagram of a cleaning device for explaining the cleaning method of the present invention. 1...Cleaning tank, 2...Spent fuel, 3...Inert gas supply pipe, 4...
... Desalinated water supply pipe, 5 ... Moist gas supply pipe, 11 ... Cooling circuit, 12 ...
... Circulation pump, 13 ... Waste liquid tank

Claims (1)

[Claims] The spent fuel is stored in a cleaning tank, and in the cleaning tank, the spent fuel is cleaned with a wet gas that is a mixture of water vapor and carbon dioxide, and the sodium adhering to the fuel is reacted with the wet gas to remove sodium. A method for cleaning spent fuel in a fast breeder reactor, characterized by generating carbonate.