Modelling of heat transfer in a molten salt loop using computational fluid dynamics

Abstract

Compared to the traditional water-based reactor fleet, molten salt reactors (MSRs) operate at significantly higher temperatures. This necessitates the development of experimental as well as modelling and simulation tools that can be used to optimise the design to ensure reactor safety and concept feasibility. Canadian Nuclear Laboratories (CNL) is currently at the stage of conceptually designing a lab-scale MSR loop to provide experimental data to benchmark code predictions. Computational fluid dynamics (CFD) modelling is undertaken within this investigation to understand the natural circulation of fuel salt in a conceptual D-shaped loop with two arms, a riser and a downcomer. The fuel salt used within ORNL MSRE was used within the 3D CFD modelling to execute steady-state analyses, with conjugate heat transfer between moderator and fuel accounted for. An attempt is made to follow CFD best practice guidelines to ensure the numerical accuracy of the CFD results. The effects of internal heat generation on the Rayleigh number, flow and temperature distributions were studied qualitatively within this modelling. Stronger mixing in the downcomer, along with prominent recirculation zones were observed at the connecting regions (arms) of the riser and downcomer, which intensified with the increase in the fuel salt power. Prior to attempting the execution of a detailed experimental test matrix, prediction of the natural circulation within the reactor geometry and the ability to understand the flow patterns as a result of varying a range of numerical parameters is required. It is expected that this initial modelling study to adopt the paradigm of CFD-informed experimentation will aid in developing an MSR flow loop at CNL by reducing the number of design iterations, thus shortening the loop design and commission time.