The results of the transmutation of fission fragments in the spectrum of neutrons of thermal and fast reactors
The radioactivity of discharged spent nuclear fuel (SNF) from the reactor during the first hundred years is determined by fission fragments (FF), subsequently actinides contribute the main to the activity of SNF. Actual scenarios for SNF management are based on the transmutation of minor actinides (MA) into fission fragments in fast reactors. The scenarios of transmutation of fission fragments in a thermal neutron spectrum and fast neutron spectrum and the radiation characteristics as a function of time are considered. The nuclide composition of the fission fragments is based on results of modeling the burnup of the assembly 439GT (TVSA type) for VVER-1000 over three years on the MCU-5 complex. The data obtained was used to determine the initial composition of nuclides at different exposures before the initiation of transmutation (irradiation in neutron fluxes) for the ORIGEN2 program.
Three possible ways of irradiation of fission fragments are given: transmutation without cooling, cooling of fission fragments for four years before irradiation, cooling for 30 years before irradiation. The duration of irradiation was chosen equal to 3 and 15 years. Transmutation efficiency is determined by a time-dependent «coefficient of transmutation» equal to the ratio of radioactivity of nuclides in the process of transmutation and after its termination to their radioactivity without transmutation. The coefficients of transmutation were noticeable only during irradiation in the reactor: their values reached 5 – 10 and depended on the duration of the fission fragments cooling before the beginning of transmutation. After the extraction of fission fragments from the neutron flux, the coefficient of transmutation was reduced to one within several years. A hundred years after irradiation in the thermal neutron spectrum, the coefficient of transmutation is reduced to 0.8 – 0.5, depending on the duration of the transmutation process. After irradiation in the fast neutron spectrum in the interval 200 – 1000 years, a slight increase in the transmutation coefficient to values 1.2 – 1.8 is observed and then after a thousand years decrease to values of 0.9 – 0.7.
The main conclusion is a special burning of fission fragments does not make sense, because non-significant gain in radioactivity (a little less than double) comes after the thousand years.
The indifference of fission fragments to transmutation can be explained by the fraction of stable nuclides, which increases with the duration of storage of fission fragments. After the end of the cycle of fuel use, there are approximately 15% of stable nuclides among the fission fragments, and after thirty years of aging, the amount of stable isotopes reaches 85%.
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