NS1 – a natural circulation loop facility for MSR research

A new natural circulation molten salt loop, NS1, is built at Seaborg Technologies (Denmark) to investigate the dynamics of molten solar salts, intended for application in the molten salt reactor (MSR) design. This includes the verification and validation of Computational Fluid Dynamics (CFD) modeling of the facility. Major aspects of the NS1 experimental campaign are successful salt handling at high temperature and near-atmospheric pressure, and the investigation of freeze valve operation. In the MSR concept, the freeze valve, made of a frozen fuel salt plug, constitutes a key safety component; it is designed to melt and drain the reactor core under the influence of gravity in case of an unexpected temperature increase. To investigate this concept, NS1 is equipped with a freeze valve that isolates the circulating salt from the loop’s drain tank. In this paper, we describe the NS1 facility including its monitoring equipment, operating procedures, and salt loading and draining. The thermal hydraulics of the loop including the freeze plug is modeled through simulations using the CFD software STAR-CCM+, and compared to the experimental data, validating the results. The freeze valve is analyzed under prescribed operating conditions of the loop to investigate the performance of this safety component. Although performing satisfactorily when actively heated and cooled, the freeze valve in its current implementation could not be relied on as a passive safety component.