Thermal–hydraulic numerical simulation of fuel sub-assembly using a dedicated meshing tool

As the CEA is involved in the pre-conceptual design phase of a Sodium-cooled Fast Reactor (SFR), the thermal–hydraulics modeling of sodium flow in the reactor core is a key scientific subject, not only due to the innovations proposed for the core design but also for the cost and the difficulties encountered to carry out experiments with sodium. Taking advantage of the progress made in numerical simulation and associated computational time, the sodium flow in a fuel pin sub-assembly is characterized in this work. The fuel pin sub-assembly is composed of 217 fuel pins, each wrapped by spacer wire, and surrounded by a hexagonal tube. In order to overcome the main limitation on the required number of mesh cells for modeling such a complex geometry, an original meshing tool, developed for this purpose, was necessary and is presented in this paper. This approach reveals to be of great help for modeling the sodium flow. Indeed, the pressure drop in the rod bundle is firstly evaluated. In addition, the local effects (at the scale of fuel pin) and global effects (at the scale of fuel pin bundle) for sodium velocity and temperature gradients for the alleged homogenization made by the spacer wire on the sodium flow are examined and clarified. Taking into account these results, the optimization of the fuel bundle geometry can be considered in order to homogenize the outlet temperature distribution in nominal condition. Lastly, the definition and the analysis of experiments on sub-assembly to be carried out in future experimental CEA platform, will also take advantage of this approach.