RU2296381C2 - Method for opening fuel element can - Google Patents

Abstract

FIELD: atomic industry.

SUBSTANCE: proposed method includes opening of fuel element can using liquid metal for the purpose. Fuel element can is opened in pure molten zinc at temperature of 600-850 °C; this process takes 0.25-10 h at ratio of mass of fuel element can to that of liquid metal being 1:10 - 1:40, followed by zinc distillation.

EFFECT: facilitated procedure.

2 cl, 1 tbl

Description

The invention relates to the field of the nuclear industry, in particular to the nuclear fuel cycle and the processing of radioactive materials from reactor fuel cells.

The prior art known "Method of dissolving the shells of fuel cells made of zirconium alloy" described in the patent of the Russian Federation No. 2035773, CL. G 21 C 19/38, publ. 05/20/1995. In this method, for the selective dissolution of zirconium-niobium shells, a mixture of 5-10 g / l of a solution of nitric or acetic acid and 5-10 g / l of a solution of a fluorine-containing reagent is used, the process is carried out with stirring at a temperature of 25-80 ° C.

However, the known method has the following disadvantages:

1. For fuel elements with nitride fuel, this will lead to the dissolution of both the shell and the nitride fuel of spent nuclear fuel (SNF).

2. The use of aqueous solutions capable of undergoing radiolysis is unacceptable in powerful radiation fields.

3. This method inevitably leads to the appearance of a large number of highly radioactive solutions and, accordingly, to the necessity of processing significant volumes of aqueous solutions.

A known method of electrolytic opening of the cladding of fuel rods made of stainless steel (RF patent No. 212752, MKI G 21 C 19/38, publ. 11/27/98, "Method and device for opening the cladding of the cladding of stainless steel").

Opening the shell is carried out by electrolysis in a sulfuric acid electrolyte with alternating current of industrial frequency.

The known method has the same disadvantages as the above analogue and is additionally more complex and time-consuming, requiring sophisticated equipment for its implementation, since the dissolution is carried out during electrolysis.

The well-known "Method of opening the irradiated nuclear fuel elements" (patent GB 1274357, MKI G 21 C 19/34, publ. 17.05.72,). This method consists in mechanically removing the stainless steel fuel cladding by local induction heating above the melting point of the cladding.

However, the known method also cannot be used to remove the cladding of a stainless steel fuel rod with nitride fuel, since at temperatures above 1300 ° C, the cladding is fused with nitride fuel, which will lead to significant loss of fuel cells.

Closest to the proposed method (prototype) is the "Method of opening the cladding of a fuel rod" (US patent 3666425, MKI C 22 B 61/04, publ. 30.05.72,).

The known method consists in the selective dissolution of the stainless steel fuel cladding in alloys of the copper-antimony system (with the addition of manganese). The process is carried out at a temperature of 700-1000 ° C for 1-1.5 hours.

The main disadvantages of the prototype are:

1. The impossibility of its use for separating the cladding of a fuel rod from nitride fuel due to the chemical interaction of nitrides with alloy components with the formation of intermetallic fuel cells and nitrogen.

2. The practical impossibility of regeneration of the solvent (antimony-copper alloy), which leads to significant volumes of highly active waste (HLW).

3. The use of antimony significantly narrows the choice of resistant structural materials for the manufacture of dissolution apparatuses.

The objective of the present invention is to increase the productivity, manufacturability of a method for processing radioactive materials, providing the opening of the cladding of a fuel rod with both nitride and oxide fuel and distillation of the solvent.

To solve the problem, the opening of the shell of the fuel element is carried out in a liquid metal solvent, namely in zinc. The process is carried out at a temperature of 600-850 ° C for 0.25-10 hours with a ratio of the shell mass to the mass of liquid metal 1: 10-1: 40.

At a lower temperature (below 600 ° C), the solubility of the components of the shell material in liquid zinc decreases, and the dissolution rate also decreases, which leads to the need for an inappropriate increase in the amount of solvent — zinc (more than 40: 1) and the process time (more than 10 hours) .

At a higher temperature (above 850 ° C), the volatility of zinc increases significantly (boiling point 907 ° C), which requires additional measures to seal the reactor and, therefore, increases the cost of the method.

The time interval is due to the need to warm the fuel rod to the process temperature (at least 0.25 hours), on the one hand, and to optimize and synchronize the fuel residence time at this redistribution (not more than 10 hours), on the other. It is clear that when the temperature decreases (in the indicated interval), the dissolution time of the fuel cladding is also determined by the solubility of the cladding material in zinc and its dissolution rate.

Deviation from the indicated limits towards smaller amounts of zinc (less than 10 mass parts of zinc per part of the cladding material of a fuel rod) leads to the formation of saturated solutions on the cladding surface and termination of the dissolution process. Large amounts of zinc (more than 40 parts by weight of zinc per part of the fuel cladding material) lead to unjustified additional costs of time and energy for its transportation and regeneration.

The amount of solvent is selected from the interval in relation to the mass of the shell to the mass of liquid metal 1: 10-1: 40 depending on the temperature of the process.

New is:

1) the use of molten zinc as a solvent;

2) the ratio of the mass of the shell to the mass of liquid metal as 1: 10-1: 40;

3) the process time is 0.25-10 hours;

4) the temperature of the process 600-850 ° C.

The advantages of the invention allow:

- open the cladding of a fuel rod with virtually no loss of fuel of both nitride and oxide fuels;

- open the cladding of a fuel rod without separating the structural elements of the fuel assembly (FA);

- increase the speed of the process by 1-2 orders of magnitude and, accordingly, significantly reduce the volume of media used, due to the simultaneous distillation of the solvent, which is important both for the organization of the processing process and for creating conditions for the disposal and storage of waste.

The temperature in the lower part of the fuel assembly (FA) is higher than the temperature of the upper end of the fuel assembly (fuel assembly) (gradient 1-4 ° C / 1 cm) to accelerate the dissolution of the cladding in the lower part of the fuel assembly, thereby preventing possible spillage of fuel from the insoluble part of the cladding of the fuel rods and grids.

The cladding of not only one or several fuel rods, but also a fuel assembly (FA), including several hundred fuel rods, spacer grids, guide tubes, etc., can be dissolved, which significantly increases the productivity and manufacturability of the method.

After a cycle of dissolution of a fuel element in order to reduce the volume of radioactive materials and return zinc to a further dissolution process, zinc is distilled off with a gradual increase in temperature from 800-1100 ° С at a pressure of 0.1-0.001 MPa.

The bottom residue after the accumulation of a certain amount (zinc content of not more than 2-10%) is sent for disposal.

As structural materials selected products from carbon-carbon materials or special steels. In case of operation at elevated temperatures, protective coatings are used. All materials and solvent used are resistant to powerful radiation fields.

Examples of specific performance (table).

Removal (opening) of the cladding of a fuel rod is carried out on a model assembly of two to six fuel rods, including an element of a thicker-walled tube (part of the tube) of a fuel rod. Graphite rods of the corresponding diameter were used as an analog of nitride fuel (assembly sizes are given in the table). At the initial stage of the experiment, the model assembly is placed in the upper cold part of the retort, at the bottom of which there is a given amount of zinc. The ratio of the mass of zinc to the mass of the metal part of the assembly varies in various experiments in the range from 10: 1 to 40: 1. After evacuation and filling the retort with an inert gas (argon), zinc is heated to operating temperature. The operating temperature varies in different experiments ranging from 600 to 850 ° C.

In a number of experiments, a predetermined temperature gradient was established in the zinc melt (from 1 to 5 degrees / cm) - a higher temperature in the lower part of zinc. After establishing a predetermined temperature regime, the model assembly is immersed in molten zinc to a predetermined depth. After a specified time, from 0.25 hours or more, the assembly is removed into the cold zone and, after cooling, the retort is disassembled and the results of the experiment are analyzed, after which the temperature in the retort is raised to 800-1100 ° C and zinc is distilled off.

From the foregoing it can be seen that the problem is solved by our invention, namely when using this method, both productivity and manufacturability of processing of radioactive materials are increased.

Table The results of experiments on the dissolution of the model of "assembly" of fuel assemblies in liquid zinc. Experience number Zinc Weight, g Temperature *, ° С Temperature gradient * ° C / cm Time h The weight of the "assembly" (without graphite), g The length of the "assembly", mm Number of tube plates, pcs. Ratio of shell mass / Zn Degree of dissolution Note one. 2. 3. four. 5. 6. 7. 8. 9. 10. eleven. 1 steel 1050 600 9 10 24 one hundred 1-1 1:43 In the lower part there are undissolved sections of the tube. The plate did not completely dissolve. Areas with sponge Fe-Zn intermetallic compounds 2 steel 1050Zn + 24 steel 750 0 four 25 one hundred 6-1 1:21 In the lower part there are undissolved sections of the tube. The plate at the bottom has completely dissolved. - ″ - 3 steel 1000 750-760 one 2 72 110 4-1 1:13 In the lower part there are undissolved sections of the tube. The plate has completely dissolved. - ″ - 4 steel 600 800-810 3 2 35 fifty 4-1 1:17 Both the tubes and the plate completely dissolved. Sublimates of zinc 5 zirconium 500 850-860 four 0.25 46 70 4-1 1:10 Both the tubes and the plate completely dissolved. - ″ - * - a higher temperature value corresponds to the lower part of the "assembly" immersed in the melt.

Claims (2)

1. The method of opening the shell of the fuel element by using liquid metal, characterized in that the process of opening the shell of the fuel element is carried out in molten zinc at a temperature of 600-850 ° C for 0.25-10 hours at a ratio of the mass of the shell of the fuel element to the mass of liquid metal as 1: 10-1: 40 followed by distillation of zinc. 2. The method according to claim 1, characterized in that the distillation of zinc is carried out at a temperature of 800-1100 ° C and a pressure of 0.1-0.001 MPa.