A new thermal-hydraulics/thermomechanics coupling methodology for the modeling of the behavior of sodium-cooled fast reactors fuel subassemblies under irradiation

The fuel pin bundles of Sodium-cooled Fast Reactors (SFR) undergo significant geometrical changes during their irradiation, which affect the flow and temperature distributions of the coolant in the fuel subassembly, the knowledge of which is necessary for safety assessments. Since the phenomena responsible for the deformation of the fuel bundles, namely the swelling and creep of the cladding, strongly depend on temperature, a coupling between the thermal-hydraulic and thermomechanical evolutions of the fuel subassemblies exists. This paper introduces a novel methodology for the numerical simulation of SFR subassemblies, based on the coupling between the industrial Computational Fluid Dynamics (CFD) code Star-CCM + and specialized numerical tool dedicated to the modeling of the thermomechanical behavior of SFR fuel subassemblies during irradiation, DOMAJEUR2. The coupling is implemented via the exchange of the diametral deformation of the claddings, calculated by DOMAJEUR2, and their associated temperature distributions, obtained with Star-CCM +. The sodium mass flow rate reduction expected in the deformed geometries is also taken into account. The developed methodology is employed to study a 7-pin SFR bundle. It is shown that, for high irradiation doses, taking the coupling into account leads to a lower end of life bundle deformation than with the traditional non-coupled simulations, consequence of the calculated temperature increase, induced by the deformation itself. The existence of operational limits on both the maximal cladding temperature and the maximal swelling in SFR highlights the relevance of these results in the context of reactor safety. Additionally, comparison of two different thermomechanical models, one based on 1D pipe finite elements and the other on volumetric finite elements, shows that they are in good agreement. Finally, the high temperature gradients on the circumference of the fuel pin claddings are shown to have a minor effect on their diametral strain.