ESTIMATION OF THE INFLUENCE OF “IMPURITY” RADIONUCLIDES ON THE CLASS OF RADIOACTIVE WASTE OF SODIUM FROM FAST REACTORS AFTER THE FINAL SHUTDOWN OF THE REACTOR
One of the tasks in the decommissioning of a fast reactor is the disposal of radioactive sodium. The cost of sodium disposal depends on its radioactive waste class, which is determined by the activity of radionuclides, presenting in sodium after the reactor final shutdown. The main sources of radionuclides affecting on sodium activity are: - radionuclides from fuel; - radionuclides washed off by sodium from the surface of the primary circuit steels; - radionuclides formed from impurities of sodium itself. This article presents the research results of the only «impurity» radionuclides influence on the sodium radioactive waste class of a BN-600 reactor after its final shutdown. This work considers radionuclides that are formed in sodium as a result of neutron irradiation of impurities in sodium itself during the reactor operation. In the calculations, the maximum values of the impurity content were used from the regulatory documents. In addition, the decrease in the value of the «impurity» radionuclides activity in sodium due to the precipitation of radionuclides in cold traps and on the surfaces of the primary circuit was not taken into account, therefore the obtained estimates of the «impurity» radionuclides effect on the class of sodium radioactive waste can be considered conservative. Analysis of the results of calculating the «impurity» radionuclides activity values and their class of radioactive waste showed that only eight (Mn-54, Co-60, Zn-65, Ag-108m, Ag-110m, Sn-121m, Ba-133, Po-210) of all considered «impurity» radionuclides have the activity within the limits the boundaries of the 4th class of radioactive waste. The analysis also showed that in the regulatory document of requirements for reactor purity sodium (OST 95 10582-2003) there are no standards for the content of some impurities - Mn, Co, Zn, Ag, which, as the results of calculations have shown, must be taken into account.