CONVERSION OF RBMK-1000 REACTORS TO REPROCESSED FUEL WITH AN INCREASED CONTENT OF EVEN URANIUM ISOTOPES

Abstract

The structure of the Russian nuclear power industry includes reactors with different designed fuel enrichment. It is possible to mix, in certain proportions, nuclear fuel (NF) from various reactors, thus closing the nuclear fuel cycle. Reprocessed uranium is a product of radiochemical reprocessing of spent nuclear fuel (SNF) from NF with a high initial enrichment. Use of uranium-erbium fuel based on reprocessed uranium is planned for the RBMK-1000 reactor. Along with 235U and 238U, SNF contains non-fissionable ballast isotopes of uranium (232, 234, 236 U). The 232,234U isotopes have a relatively high radioactivity and the presence of these leads to an increased dose rate of ionizing radiation but, due to their small content in fuel, does not affect the neutron balance, the neutron multiplication factor, and the reactivity margin. A large presence of 236U requires additional enrichment with 235U due a greater probability of inefficient neutron absorption by the 236U nuclei. This absorption with no fission leads to a reduced neutron multiplication factor, a reduced reactivity margin in fresh fuel, and a smaller burn-up of unloaded fuel. Analyzing the effects the increased content of even uranium isotopes (IEI) has on the reactor’s neutronic performance and fuel burn-up makes it possible to determine the amount of additional 235U fuel enrichment to make up for the negative effects of 236U on the RBMK-1000 neutronic performance.