Dynamics of Natural Circulation in Presence of Distributed Heating: Results of the SAMOFAR CFD Simulation Campaign

In molten salt fuelled nuclear reactors (MSRs), it is paramount to analyse the natural circulation capacity of the coolant salt, as it affects both the reactor design (i.e., sizing of primary pumps and passive decay heat removal systems) and its dynamics during accidents (i.e., loss of flow scenarios). MSRs are characterized by the presence of a distributed volumetric heat source in the coolant stemming from fission and decay heat. This source alters the temperature distribution in the system and consequently the buoyancy forces, possibly modifying the natural circulation capabilities with respect to localized sources and sinks. The main purpose of this work is to present the results of a numerical simulation campaign performed during the SAMOFAR H2020 project to preliminarily study the stability of salt natural circulation in the DYNASTY (DYnamics of NAtural circulation for molten SalT internallY heated) loop. The facility is a natural circulation loop – built at Politecnico di Milano – that provides homogeneously distributed heat by means of electric strips, in order to reproduce the fission and decay heat source in MSRs. During the project, different CFD models have been developed by PoliMi, EDF, and TUDelft with different simulation tools, namely OpenFOAM, Code_Saturne, and DGFlows respectively. The predicted behaviour of DYNASTY in different operating conditions is carefully analysed and independently compared considering the equilibrium stability of DYNASTY on a finite-time horizon. The outcomes provide useful insights of the impact of numerical approximations and assumptions on salt natural circulation modelling.