Large eddy simulations for the modelling of the dynamic behaviour of the DYNASTY natural circulation loop

In Generation 3 and 4 nuclear reactors, passive safety systems often rely on natural circulation to remove decay heat from the fuel during accidental scenarios. The study of the dynamic stability of natural circulation systems is fundamental to ensuring efficient passive heat removal. For this purpose, DYNASTY, a natural circulation loop featuring distributed heating, has been built at Politecnico di Milano. This work consists of a CFD campaign with large eddy simulation (LES) of turbulent flow and solid wall thermal coupling, to study the dynamic response of DYNASTY during typical transients with molten salt as the working fluid. LES is chosen for its potential to accurately represent the complexity of the flows predicted in natural circulation systems, in contrast to the more popular but less accurate Reynolds-averaged Navier-Stokes models.

Due to the documented sensitivity of LES to simulation parameters such as mesh refinement, boundary and initial conditions, a low Reynolds pipe flow has been investigated first. This preliminary analysis outlines the mesh requirements to be adopted in the model of DYNASTY, to achieve sufficient confidence without resorting to a computationally expensive sensitivity analysis on the actual facility’s model. The following simulations of DYNASTY, carried out in OpenFOAM, show the possibility for LES to represent stable and unstable transients in the facility. Due to the details on the flow provided by LES, new insights on the features of flow reversals during unstable transients are also reported.