RU2731015C1 - Method of processing liquid radioactive wastes - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to nuclear engineering, particularly to radiochemical processing of irradiated nuclear fuel. Method of processing liquid radioactive wastes containing nitrates of mono- and polyvalent cations involves evaporation of liquid radioactive wastes to salt melt, adding a reagent to the still residue, which is a nitrate ion reducer, and treating the still residue with subsequent calcination. Reagent reducing agent used is dicyandiamide (cyanoguanidine).

EFFECT: invention increases safety of the process by eliminating ammonia release during solidification of liquid radioactive wastes, reducing the specific consumption of the reducing reagent with respect to the weight of sodium nitrate in the processing solution.

3 cl, 1 ex

Description

The invention relates to nuclear technology, in particular to the radiochemical processing of irradiated nuclear fuel, and can be used in technological schemes for the processing of liquid radioactive waste (LRW) containing nitrate ion.

Radiochemical reprocessing of irradiated fuel elements of nuclear reactors in order to extract recyclable fuel components generates a large amount of aqueous solutions containing radioactive waste. The specified wastes, being corrosive and dangerous, create problems when handling them. With prolonged storage, the decomposition of highly active fission products occurs. Waste contained in aqueous solutions, by evaporation, calcination, followed by sintering or vitrification, is converted into a solid form, which allows reducing the volume of the product in relation to the volume of the solution, and minimizes problems during long-term storage. Radioactive solutions contain significant amounts of nitrate and nitrite ions, mainly in the form of sodium salts. The preparation of a solid matrix for storing waste through the above procedures is associated with the formation of a large amount of toxic nitrogen oxides (NO _x ), especially nitrogen dioxide.

A known method of processing LRW containing sodium nitrate, by converting them into a solid state by drying, followed by decomposition of dried salts to form a mixture of oxides (Nikiforov A.S. et al. Neutralization of liquid radioactive waste. - M .: Energoatomizdat, 1985, p. 143-148).

The disadvantage of this solution is the significant release of toxic gases, the lack of compactness of the final product, a high degree of leaching of radionuclides.

A known method of solidification of LRW for subsequent long-term storage in the solid phase, containing significant amounts of sodium compounds by calcining in a kiln with a fluidized bed. The formation of sodium nitrate, which causes agglomeration of fluidized bed particles, is slowed down by the addition of aluminum and fluoride to the waste to produce cryolite during calcination. The off-gas of the kiln is purified with a solution containing aluminum to bind into a complex compound of any fluoride that can be released during the subsequent dissolution of cryolite and to prevent corrosion in the off-gas purification system (US patent 3943062, IPC G21F 9/16; publ. 09.03. 1976).

The disadvantage of this method is the release of toxic gases, the possibility of the formation of deposits of fluoride salts in communications, corrosive effects on the structural materials of equipment.

There is a known method for LRW processing, according to which the said waste containing nitrates of mono- and polyvalent cations is evaporated to a salt melt, a reagent-reducing agent of nitrate ions is added to the bottom residue and heat treatment of the bottom residue is carried out followed by calcination; urea is used as a reducing agent (patent US 3 962 114, IPC G21F 9/08; publ. 08.06.1976).

The decomposition of sodium nitrate proceeds according to the following scheme

NaNO ₃ + 3CO (NH ₂ ) ₂ + O ₂ = NaOH + 2N ₂ + 3CO ₂ + 3NH ₃ + H ₂ O

This method of processing liquid radioactive waste containing sodium nitrate was chosen by us as a prototype.

The main disadvantages of this method are:

- low efficiency of nitrate ion reduction due to the decomposition of carbamide at a temperature of 132 ° C and higher;

- the release of ammonia (3 moles of gas per 1 mole of sodium nitrate) as a reaction product and, as a consequence, the occurrence of fire hazardous conditions during the process with the supply of oxygen (reagent gas) to the reaction zone;

- high specific consumption of carbamide - 68% of the mass. urea by 32% of the mass. sodium nitrate.

The objective of the invention is to improve the safety of the process by eliminating the release of ammonia in the process of solidification of liquid radioactive waste, to reduce the specific consumption of the reducing agent in relation to the mass of sodium nitrate in the solution being processed.

The problem is solved by the fact that in the method of processing liquid radioactive waste containing nitrates of mono- and polyvalent cations, including the evaporation of liquid radioactive waste to a salt melt, the addition of a reducing agent-reducing agent of nitrate ions to the bottom residue and heat treatment of the bottom residue followed by calcination, as a reagent the reducing agent is used dicyandiamide (cyanoguanidine) in the form of a saturated solution.

The decomposition of sodium nitrate with the participation of dicyandiamide (DCDA) in the reaction proceeds according to the following scheme:

5C ₂ H ₄ N ₄ + 12NaNO ₃ = 4CO ₂ + 10H ₂ O + 16N ₂ + 6Na ₂ CO ₃

The specific consumption of dicyandiamide is 29% of the mass. by 71% of the mass. sodium nitrate.

Execution example.

A simulator of liquid radioactive waste was prepared similarly to the prototype. The reducing agent (dicyandiamide) was introduced after evaporation of the simulator to the state of salt melt at a mass of dicyandiamide: nitrate ion equal to 1: 1.9 in the form of a previously prepared solution saturated at 80 ° C (DCDA concentration - 30%). The resulting mixture was evaporated and heated further to the stage of intense gas evolution. Exhaust gas analysis showed the expected absence of ammonia.

The gassing process was completely completed at a temperature of 350 ° C. Then the reacted mixture, mainly sodium carbonate, was cooled. Then the final product was analyzed for the presence of nitrate ion. No nitrate ion was detected. The mass consumption of the reducing agent is reduced by 5 times compared with the prototype method.

Claims (3)

1. A method for processing liquid radioactive waste containing nitrates of mono- and polyvalent cations, including the evaporation of liquid radioactive waste to a salt melt, the addition of a nitrate-ion reducing reagent to the bottom residue and heat treatment of the bottom residue followed by calcination, characterized in that the reagent is the reducing agent is used dicyandiamide (cyanoguanidine) in the form of a saturated solution. 2. The method according to claim 1, characterized in that the dicyandiamide is mixed with the distillation residue at a mass ratio of dicyandiamide: nitrate ion of at least 1: 1.8. 3. The method according to claim 1, 2, characterized in that the dicyandiamide is administered in the form of a solution saturated at a temperature of at least 60 ° C.