Neutronics–Mechanics Coupling for Fast Transient Simulation in Molten Salt Reactors

Abstract Prompt supercritical transients initiated by high and rapid reactivity insertions are an important safety concern for fluid-fuel nuclear systems like molten salt reactors. Because of their short time scales, their study must involve a compressible treatment of the fluid fuel with finite velocity of mechanical waves. Otherwise, the delay in the negative reactivity feedback due to fuel expansion and the deformation of the vessel resulting from strong propagating pressure peaks would be overlooked. For a liquid fuel, this is especially true if it contains gas bubbles. Accordingly, and owing to the highly multiphysical nature of such excursions, a new neutronics/compressible-thermal-hydraulics numerical coupling tool based on the APOLLO3 and EUROPLEXUS codes is presented. An analytical solution to the coupled problem close to the one of the Nordheim-Fuchs model is also given. Excellent agreement between simulations and theory is obtained and demonstrates the successful implementation of the numerical tool.