ИСТИНА

The greatest danger of contamination of the World Ocean by radioactive substances is the testing of nuclear weapons. Other major sources of radioactive contamination of the World Ocean are global fallout of the products of atmospheric nuclear tests and technogenic radionuclides, which move through rivers into the sea from radiochemical plants for the reprocessing of spent nuclear fuel (SNF). Scientists of the FEFU and the CI FEB RAS have been tasked with developing modern technologies that ensure the radiation safety of Primorye and Vladivostok: to remove finally the accumulated liquid radioactive waste (LRW), to clean and rehabilitate the contaminated natural objects from radionuclides and toxic elements. An important task was to discover the promising sorption materials for extraction, concentration, separation from LRW on the basis of sea water of the most dangerous radionuclides: 137Cs and 90Sr. One of the promising classes of ion-exchange materials for the purification of aqueous media from cations of radioactive elements is sulfonic cation exchangers, based on a copolymer of styrene with divinylbenzene – type KU-2*8. In some technological operations, such a cation exchanger may be in contact with nitric acid solutions, which may be accompanied by the evolution of heat and gaseous products of interaction. To exclude the possibility of a thermal explosion with such a contact, it is necessary to establish the limits of safe operation and maintenance, which requires information on the thermal stability of such systems. In this work, we studied the thermal stability of KU-2*8 cation exchanger mixtures with 4 and 12 mol·L-1 nitric acid under isochoric conditions using the DSC method. It was determined, that the heat of interaction of the cation exchanger KU-2*8 with 4 and 12 mol·L-1 nitric acid in the temperature range less than 200 °C has a value of 310-320 and 1730-1890 kJ/kg, respectively. A two-stage heat generation process was noted for mixtures of cation exchanger KU-2*8 with nitric acid in the studied temperature range. Irradiation decreases the starting temperatures of exothermic processes, compared to unirradiated samples under similar conditions. For the sample of cation exchanger KU-2*8, irradiated up to the absorbed dose of 2 MGy, which is in contact with 4 mol·L-1 nitric acid under isochoric conditions, the temperature range of exothermic processes was 60-180 °C. The value of the thermal effect also increased to 430 kJ/kg. The results of the study showed, that in order to ensure the safety of sorption processes with sulfonic cation exchangers, it is necessary to limit the process temperature and the concentration of oxidizer, depending on the dose of ionizing radiation absorbed by the sorbent, and to ensure the removal of the resulting oxidation products. The value of safe temperature should be determined from the thermophysical calculation of the sorption equipment, selected for the technological process, considering the kinetics of the proceeding oxidation processes. Acknowledgements The study was conducted at the expense of the Program of the RAS Presidium I.34, project 0137-2018-0051.