Stability analysis on two single-phase coupled natural circulation loops

In designing Molten Salt Reactors, implementing passive safety systems that rely on natural circulation phenomena can be an attractive proposal to remove decay heat from the core. In this type of reactor, when the active cooling system fails, the fuel-coolant salt flows by natural circulation in the presence of Internal Heat Generation due to the fuel dissolved and mixed within the coolant salt. Adopting a Natural Circulation Loop for the dissipation of the heat generated can be an attractive safety system, but it is affected by some undesired phenomena regarding flow mass rate oscillation, which influences the heat transfer efficiency and local temperature. In particular, it is necessary to understand the influence of the Natural Circulation loop on the internal stability of the reactor. The purpose of the DYNASTY-eDYNASTY facility (DYnamics of NAtural circulation for molten SalT internallY heated), built at Politecnico di Milano, is to investigate the dynamical effects manifesting in a Coupled Natural Circulation Loops and to get insights about the phenomenology which can result relevant for the study of accidental scenarios in Molten Salt Reactors, simulating the natural circulation in the reactor and the Natural Circulation Loop of the passive safety system. This paper, starting from the one-dimensional model of the DYNASTY-eDYNASTY facility, develops a in-house code for the computation of the steady states and a Stability Map for the stability analysis of the Coupled Natural Circulation Loops. Verification of the results has been carried out by comparing the outcomes derived from the MODELICA model of the DYNASTY-eDYNASTY facility adopting the DYMOLA® environment. The steady-state model can predict the results of the DYMOLA® simulations with acceptable accordance, and the verification of the Stability Map with DYMOLA® simulations highlighted a good prediction of the model developed for the stability analysis.