Numerical methodology for design and evaluation of natural circulation systems for MSR applications

This work focuses on the development of a comprehensive numerical methodology for the study of natural circulation systems using molten salts as working fluid, in particular for its application in the design of passive safety systems for Molten Salt Reactors (MSRs). The goal of the methodology is to evaluate the performance of possible 3D natural circulation configurations through the description of their dynamic behavior, with a focus in the identification of bifurcation phenomena and performing a stability analysis. The numerical tool implementing this methodology is based on a CFD-based MATLAB-OpenFOAM coupling and has been evaluated against a numerical benchmark of a differentially heated cavity, showing good agreement with literature results. For a first assessment of the performance of the tool a simple 2D case of a simple natural circulation configuration known as Rayleigh-Bénard convection has been used. This configuration results from a 2D enclosure heated and cooled from the bottom and top surfaces respectively. The dynamic states of this system have been studied for a large range of both geometric and physical parameters, showing both steady state and oscillating solutions. A bifurcation diagram is produced, showing the transition between the different solutions. In the vicinity of certain bifurcation points hysteresis phenomenon has been observed. The numerical tool developed for the evaluation of natural circulation systems has shown a good performance both for the description of the thermal-hydraulic system as well as for the stability analysis. The complexity and variety of the possible solutions for this first assessment illustrates the need of a robust and systematic methodology for the considered applications which in practice will involve more complex geometries and additional underlying phenomena.