Multiphysics modelling and preliminary analysis on the freeze valve behaviour in a Molten Salt Fast Reactor

In the Molten Salt Fast Reactor (MSFR), the safety function related to criticality and cooling is also delivered via draining tanks developed to accommodate liquid fuel discharged by gravity during normal maintenance or accidental events. In particular, a safety barrier that is under investigation to manage accidental scenarios consists of a salt-frozen plug designed to melt in case of loss of power or overheating. Specifically, the salt plug should melt as a consequence of an unintended temperature increase, allowing the salt to drain into a safety tank. The mechanism based on the state of the plug embraces the passive safety concept, highlighting the importance of delving into the steady state of the freeze valve and the transient behaviour. This paper proposes a preliminary analysis of the behaviour of the freeze valve within a domain that represents a symmetric portion of the MSFR primary loop. The model, developed in OpenFOAM, couples melting and solidification phenomena to a multiphysics solver, coupling neutronics and thermal-hydraulics. The 3D domain represents 1/16th of the MSFR fuel loop and is equipped with a cylindrical region that mimics the presence of the freeze valve. Two scenarios are considered to assess the impact on the freeze plug behaviour of initial and boundary conditions meant to represent different cooling strategies. When the plug is coupled with the MSFR fuel loop, the selection of boundary and initial conditions strongly affects the plug melting time. This highlights the need for design specifications for the geometry and the cooling mechanisms to correctly operate the freeze valves.