Sistema híbrido fusão-fissão: avaliação neutrônica de refrigerantes e da camada produtora de trítio

Abstract

The Fusion-Fission Hybrid System based on Tokamak has been studied with the transmutation purpose of transuranic nuclides, which have long half-life and high radiotoxicity. The system consists in a nuclear fusion reactor coupled to a subcritical fission blanket - called transmutation layer. Previous studies indicated the best location to place the transmutation layer, which should have a harden neutron spectrum allowing to increase the probability of fission reactions over the transuranic nuclides. The material used for cooling the transmutation layer in the system was a lithium-lead alloy, which has 20% lithium-6 enrichment on its composition for tritium generation purpose, element necessary to supply the nuclear fusion reactions. Based on the previous studies, this work analyzed the use of different coolant materials for the transmutation layer in the hybrid system with the purpose to increase the capacity in the transuranic transmutation through fission reactions. Liquid metals are proposed to be used as coolant materials for fast reactors. The analyzed material for cooling the transmutation layer was lead, sodium, and sodium-potassium, lead-bismuth, leadmagnesium and, of course, lithium-lead alloys for comparison purpose. It was evaluated neutronic parameters, such as the effective multiplication factor, neutron flux, and nuclide composition after a fuel burnup. Among them, the lead-bismuth eutectic alloy presented the highest transmutation values. It was analyzed the reaction rates for tritium production, as well as, the neutron flux and nuclide composition for each breeder location and material. However, the use of the lead-bismuth as coolant decreases the reaction rates for tritium production due to the lack of lithium in the system, decreasing the tritium amount required to supply the fusion reactions. Therefore, it was evaluated the insertion of a tritium breeder layer at distinct locations in the system for different breeder materials with the purpose to achieve high tritium production in the system using the chosen coolant. In addition, the evaluation consists in finding the best location to place the tritium breeder layer and also the most suitable material to be used as a tritium breeder in the layer that produces a significant amount of tritium without significantly affecting transuranic transmutation. Based on the results obtained, the tritium breeder layer was placed before the transmutation layer and was composed by lithium with 20% lithium-6 enrichment on its composition. vi All analyzes were performed using the MONTEBURNS code, which links the MCNP and ORIGEN codes. The neutronic parameters were obtained from the MCNP code and the fuel burnup data, which includes the buildup, decay and processing of radioactive materials were obtained from ORIGEN