The Molten Salt Fast Reactor (MSFR) has garneredmuch interest for its inherent safety and sustainbility features.The MSFR can adopt a closed thorium fuel cycle for sustainableoperation through the breeding of ~(233)U from ~(232)Th.The fuel composition changes significantly over the courseof its lifespan. In this study, we investigated the steady stateand transient behavior of the MSFR using Moltres, a coupledneutronics/thermal-hydraulics code developed within the MultiphysicsObject Oriented Simulation Environment (MOOSE)framework. Three di erent fuel compositions, start-up, earlylife,and equilibrium, were examined for potentially dangerouscore temperature excursions during a unprotected loss of heatsink (ULOHS) accident. The six-group and total neutron fluxdistributions showed good agreement with SERPENT and publishedMSFR results, while the temperature distribution andtotal power showed discrepancies which can be attributed toknown sources of error. For the transient behavior under theULOHS scenario, while the transition time towards the newsteady state core temperature is also in good agreement withexisting MSFR simulations by Fiorina et al., Moltres underestimatedthe temperature rise by a factor of ten, due to thesame sources of error a ecting the steady state results. Whilean MSFR loaded with start-up fuel composition operates at ahigher temperature than with the other two fuel compositions,all three cases were shown to be inherently safe due to thestrong negative temperature feedback.
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