Neutronic assessment of the Mizton multipurpose nuclear microreactor: Design alternatives and performance

Mizton is a multipurpose nuclear microreactor developed by a research group of the School of Engineering of the National Autonomous University of Mexico, currently in the conceptual design stage. The reference microreactor design has a thermal power of 15 MW, using heat pipes with sodium as a coolant, 19.75 % ²³⁵U enriched uranium nitride (UN) TRISO particles as fuel, a monolith of SiC and a secondary reflector of Zr₃Si₂. This research proposes an alternative design with a monolith of graphite, a secondary reflector of ZrC, and a fuel of uranium–plutonium nitride; four models were analyzed in total. The Monte Carlo code Serpent, version 2.1.32 and the JEFF-3.1 cross-section library were used for neutronic simulations. For each of the models, the behavior of the effective neutron multiplication factor and the effect on the reactivity of the variation of the density of the monolith were analyzed. Furthermore, the primary safety parameters such as the Doppler coefficient, the control rods’ worth, the delayed neutron fraction and the neutron generation time were also calculated. In addition, the fuel evolution over a given period at full power was analyzed for each of the models studied. According to the results, the alternative design achieved higher effective neutron multiplication factor values than the reference design. For all the models, the control rods inserted enough reactivity for the safe shutdown, the Doppler coefficient was negative, and the effect on the reactivity of the variation of the monolith density was negligible. The alternative design with enriched UN fuel achieved a longer operating cycle of approximately 9.9 years and reached a burnup of 19,224 MWd/tU.