Simplified 0-D semi-analytical model for fuel draining in molten salt reactors

A key feature of molten salt reactors is the possibility to reconfigure the fuel geometry (actively or passively driven by gravitational forces) in case of accidents. In this regard, the design of reference molten salt reactor of Generation IV International Forum, the MSFR, foresees the Emergency core Draining System (EDS). Therefore, the research and development of MSFRs move in the direction to study and investigate the dynamics of the fuel salt when it is drained in case of accidental situations. In case of emergency, the salt could be drained out from the core, actively or passively triggered by melting of salt plugs, and stored into a draining tank underneath the core. During the draining transient, it is relevant from a safety point of view that thermal and mechanical damages to core internal surfaces and to EDS structure - caused by the temperature increase due to the decay heat - are avoided. In addition, the subcriticality of the fuel salt should be granted during all the draining transients. A simplified zero-dimensional semi-analytical model is developed in this paper to capture the multiphysics interactions, to separate and analyse the different physical phenomena involved and to focus on time evolutions of temperature and system reactivity. Results demonstrate that the fuel draining occurs in safe conditions, both from the thermal (temperature-related internal surface damages) and neutronic (sub-critical states dominate the transient) view points and show which are the main characteristics of the fuel salt draining transient.