An Analysis of Modern Methods for Processing Vat Residues at NPPs

Abstract

To date, more than 600 thousand m³ of liquid radioactive waste has been accumulated at Russian nuclear power plants (NPPs). In addition to radioactive components, they contain a large amount of mineral salts, organic matter, oil products, and suspensions. The problem of overfilling the storage facilities for liquid waste at some NPPs is critical and has to be solved urgently.

The article presents modern methods for processing vat residues and the existing prerequisites for modernizing ion-selective purification plants, which are based on the results of research works carried out by various teams. A technological analysis of the methods for evaporation and ion-selective purification of vat residues with identified shortcomings of individual stages of the technologies is carried out. Decontamination of secondary products of liquid radioactive waste processing implies the possibility of subsequently separating valuable components for reusing them. Their decontamination is necessary, since these wastes are essentially high-salt solutions with a specific activity of 10⁶-10⁷ Bq/l contaminated with fission products, radionuclides of corrosion origin, and various substances used to decontaminate the equipment and maintain the water chemistry. It is proposed to apply the electrodialysis method for separating a multicomponent solution of the decontaminated NPP vat residue and to use the obtained products at the plant. Owing to the partial extraction of valuable components from decontaminated vat residues, it becomes possible to reduce the amount of waste and return the separated substances into the main process cycle, thereby reducing the expenditures for initial reagents. To implement the developed technology for component-wise separation of the initial solution, a laboratory electrodialysis facility was manufactured. The project implementation feature was that in dealing with boric acid salts, it is necessary to take into account the danger of their precipitation at certain pH values. It was found that to minimize the escape of boric components, the process should be conducted at low pH values, i.e., the solution should be acidified with the nitric acid obtained in the process. The obtained study results can be used to modernize the cementation plants.