Avaliação da camada de fissão e estudo comparativo de combustíveis em um sistema híbrido de fusão-fissão baseado no reator ARC

Abstract

Hybrid fusion–fission systems (HFFRs) emerge as a promising alternative for sustainable nuclear energy generation and the reduction of long-lived radioactive waste. In this work, a hybrid system based on the Affordable Robust Compact Reactor (ARC) fusion reactor was developed with the objective of investigating neutron flux behavior, assessing the insertion of a fission transmutation blanket, and analyzing the efficiency of different reprocessed fuels. The methodology was structured in successive stages. Initially, a simplified model of the ARC reactor was developed using the MCNP5 code in order to analyze the neutron spectrum generated by fusion and to identify the most suitable position for the insertion of the transmutation blanket. Subsequently, five distinct hybrid models (HF, H4, H6, H9, and H12) were proposed, considering different geometric configurations of the fission blanket and the ratio between coolant and fuel volumes. These models were first evaluated with reprocessed dioxide-based fuel (TRU, Th)O₂, under steady-state conditions and throughout the burnup process, by coupling the MCNP5 and MONTEBURNS/ORIGEN2.1 codes. Later, the analysis was expanded to include nitride-based fuel (TRU, Th)N, applied to the H4 and H6 models, in order to compare its performance against the dioxide-based reprocessed fuel. The results indicated that the strategic placement of the transmutation layer downstream of the beryllium blanket hardened the neutron spectrum and made it possible to achieve a tritium breeding ratio (TBR) of approximately 1.85, ensuring tritium self-sufficiency. The analysis of the models using (TRU,Th)O₂ showed that H4 exhibited the highest effective multiplication factor (keff) and transmutation rates, followed by H6. In contrast, for the (TRU,Th)N systems, H6 achieved the best transmutation rate despite having a lower keff than H4, while more highly discretized models favored neutron capture and the accumulation of ²⁴¹Am, implying additional challenges for fuel-cycle management. The comparative assessment between the fuels further demonstrated that, under the same conditions, the choice of transmutation-layer configuration and fuel type has a decisive influence on burnup efficiency and on transmutation dynamics.