RU2408101C2 - Waste nuclear fuel processing method - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: fission products are concentrated by means of evaporation of solutions of nitrates from extraction processing with evaporation ratio till concentration of nitric acid of up to 8 mol/l is obtained in stillage solution with further output of stillage residue to dilute waste nuclear fuel.

EFFECT: simplifying the process and its instrumentation, reducing corrosive power of process medium from the point of view of corrosion effect on structural materials of the equipment, reducing the concentration of corrosion products in BAO, and excluding double evaporation processes, which increases efficiency of the process.

Description

The invention relates to the field of nuclear energy, in particular to the field of spent nuclear fuel (SNF) processing, and can be used in technological schemes for processing SNF.

Known methods for processing spent nuclear fuel include dissolving spent nuclear fuel in nitric acid and further processing the resulting solutions by extraction methods, in which a reduction in the volume of liquid radioactive waste is achieved by obtaining solutions that are supersaturated with uranyl nitrate and subsequent crystallization of uranyl nitrate from them (see Takeshi Takata et al. “Conceptual Design Study on Advanced Aqueous Reprocessing System for Fast Reactor Fuel Cycle”, Journal of nuclear science and technology, volume 41, number 3, p.307-314; E. Henrich et al. “A new concept for product refining in the Purex-process ”, Ato mkernenergie-Kerntechnik, 1986, vol. 48, No. 4, p. 241-245). In the known methods, the necessary degree of reduction in the volume of liquid radioactive waste is not achieved due to the limited concentration of uranyl nitrate in supersaturated solutions. The use of the process of crystallization of uranyl nitrate instead of the second extraction cycle, despite the possible simplification of the technological scheme of uranium refining, does not solve the problem of concentration of radionuclides from liquid radioactive waste of the first extraction cycle.

A known method of processing spent nuclear fuel (SNF), including the dissolution of spent nuclear fuel in nitric acid and further processing of the obtained solutions by extraction methods, characterized in that the spent fuel is dissolved in the raffinate from the extraction processing of the solutions obtained during dissolution using an extractant with nitrogen as a solvate acid (see Bondin V.V., Bychkov S.I. Efremov I.G., Kudinov K.G., Lapshin B.M. Method for processing irradiated nuclear fuel. RF Patent No. 2225719, publ. 27.05.2008, bull. . 15).

The known method involves contacting the extractant with highly concentrated solutions of nitric acid and radionuclides, which can lead to nitration of the components of the extractant and, as a consequence, to unpredictable emergency situations. In addition, the use of the known method will require a substantial alteration of existing extraction technological schemes with the inclusion of a chain of extractors to obtain a solvate of the extractant with nitric acid.

A known method (prototype) of spent nuclear fuel reprocessing, including the dissolution of spent nuclear fuel in nitric acid, clarification of the initial fuel solution by filtration, further processing of the resulting solutions by extraction methods, concentration of fission products, solidification of highly active technological waste. The concentration of fission products in liquid radioactive waste in the known method is carried out by deep evaporation of nitric acid solutions with the regeneration of nitric acid (Kopyrin A.A., Karelin A.I., Karelin V.A. Production technology and radiochemical processing of nuclear fuel. M .: ZAO " Atomenergoizdat Publishing House, 2006, pp. 317-325). Evaporation processes with the organization of nitric acid regeneration make it possible to obtain concentrated nitric acid in the form of reflux, which can be used to dissolve SNF. Thus, in the known method, the problem is solved not only to reduce the volume of highly active waste, but also the rational use of nitric acid, as well as reduce the volume of radioactive waste. However, the reprocessing of irradiated nuclear fuel by a known method is expensive and require the use of expensive equipment made of special steels. In addition, the bottom solution from evaporation of the raffinate contains a high concentration of nitric acid, which also leads to the complication of the technological scheme of the curing process with a simultaneous increase in the cost of the curing process.

The aim of the present invention is to eliminate the process of deep evaporation of nitric acid solutions with the organization of the regeneration of nitric acid and a significant decrease in the concentration of nitric acid in the highly active process waste fed to the cure.

This goal is achieved by the fact that in the known method of SNF processing, including SNF dissolution, clarification of the initial fuel solution by filtration, extraction processing of the resulting solutions, concentration of fission products by deep evaporation of the raffinates of the first extraction cycle with the regeneration of nitric acid for its subsequent use in dissolution processes fuels, fractionation of fission products, solidification of highly active technological waste, concentration of fission products the wiring by evaporating the nitric acid solutions from the extraction processing of a multiplicity of evaporation to give a nitric acid concentration of the bottoms solution to 8 mol / l with the subsequent delivery bottoms dissolution of spent nuclear fuel. Evaporation of the raffinate is carried out without regeneration of nitric acid. The recycling of fission products with raffinates from extraction processing through evaporation and subsequent dissolution in a still solution of spent nuclear fuel is carried out to create a concentration of radionuclides that correspond to their technologically necessary accumulation. With the accumulation of radionuclides, the raffinate from extraction processing is removed from the recycle and after the fractionation operation is sent for curing.

The elimination of the process of nitric acid regeneration during evaporation of raffinates at a low evaporation rate until an azeotropic content of nitric acid in the bottoms solution is created significantly simplifies the process and its hardware design, reduces the aggressiveness of the technological environment from the point of view of corrosive effects on structural materials of equipment, and reduces the concentration of corrosion products in HLW , and also eliminates the double evaporation process, which increases the efficiency of the process.

The direction of the raffinate from the spent nuclear fuel extraction processing to the subsequent reprocessing with the recycling of fission products with the technologically necessary content of fission products in the raffinate simplifies the technological scheme of fractionation of fission products, since the solution receives a more concentrated fission product for fractionation. A lower concentration of nitric acid in the solution simplifies the subsequent curing of HLW.

The method is as follows. The raffinate from the first extraction cycle is directed to evaporation with a multiplicity of evaporation, ensuring the creation of a concentration of nitric acid in a still solution of an azeotropic mixture (8 mol / l). Evaporation is carried out in a tray evaporator in order to flush the gas phase from entrainment to obtain cleaner condensate. The bottom solution after clarification by filtration is sent together with other highly concentrated solutions of nitric acid obtained from the concentration of water-tail solutions from refining processes to a solvent reactor. The resulting fuel solution with an optimal concentration of uranium (350-400 g / l) and nitric acid after clarification by filtration is sent to the extraction processing according to known technological schemes. Recycling of fission products by returning the bottom solution from evaporation of the raffinate of the first extraction cycle is carried out to create technologically necessary concentrations.

EXAMPLE

2.4 kg (in terms of uranium) of the irradiated fuel was divided into fractions (40 fractions, each fraction of 60 grams). The first fraction was dissolved in a pure solution of nitric acid, and all subsequent ones in a still solution from evaporation of the raffinate of the first extraction cycle after clarification by filtration with the addition of highly concentrated solutions of nitric acid (8 mol / L) from the concentration of water-tail solutions of refining operations to obtain the initial solution with a uranium concentration of 350 g / l and nitric acid 2.5 mol / l. The resulting solutions after clarification by filtration were sent for extraction processing according to the known technological scheme with obtaining a raffinate with a concentration of nitric acid of 2.5 mol / L. Upon receipt of the last portion of the initial solution, the raffinate was obtained in the form of a fission product concentrate. The composition of the highly active raffinate, directed to fractionation and subsequent vitrification: uranium - <0.01 g / l, plutonium - <10 μg / l, nitric acid - 2.5 mol / l. The volume of the output of highly active raffinate, directed to fractionation and subsequent vitrification, amounted to 283 ml (180 l / t of uranium). Such a volume is close to theoretically possible, since upon dissolution of uranium dioxide in nitric acid, mainly water is formed. In another evaporator, nitric acid solutions were evaporated from the refining of uranium and plutonium also with the multiplicity necessary to achieve a nitric acid concentration of 8 mol / L in the bottom solution. VAT solution was also used in the operation of dissolving the fuel. The coefficient of purification of uranium and plutonium from fission products at the extraction stage corresponded to the parameters obtained by the prototype.

Compared with the prototype in the proposed method, a high reduction in the volume of highly active raffinate was obtained without using the deep evaporation of the raffinate with the regeneration of nitric acid, to obtain it in the form of a solution prepared for vitrification.

Claims (3)

1. A method of processing spent nuclear fuel (SNF), including the dissolution of spent fuel, clarification of the initial fuel solution by filtration, extraction processing of the resulting solutions, concentration of fission products by evaporation of the raffinates of the first extraction cycle with the regeneration of nitric acid for its subsequent use in fuel dissolution processes, fractionation of fission products, solidification of highly active technological waste (HLW), characterized in that the concentration of products Ia is carried out by evaporating the nitric acid solutions from the extraction processing of a multiplicity of evaporation to obtain a bottoms concentration of nitric acid solution to 8 mol / l with the subsequent delivery bottoms dissolution of spent nuclear fuel. 2. The method according to claim 1, characterized in that the use of the bottom solution for the operation of dissolving the spent nuclear fuel (recycling of nitric acid and fission products) is carried out before the technologically necessary concentration of fission products is accumulated in the raffinate of the first extraction cycle. 3. The method according to claim 1, characterized in that the fractionation of fission products is carried out from a raffinate with a technologically necessary concentration of fission products, followed by curing of the HLW.