New finite element neutron kinetics coupled code system FENNECS/ATHLET for safety assessment of (very) Small and Micro Reactors

Abstract

Cores of (very) small and medium size reactor–(v)SMR – are often characterized by irregular geometries. Monte Carlo methods are not yet mature enough for transient applications. To perform coupled transient safety assessments of (v)SMRs, the Finite ElemeNt NEutroniCS (FENNECS) code is being developed at GRS. It solves the time-dependent and steady-state three-dimensional few-group diffusion equation in Galerkin finite element representation using upright triangular prisms with linear basis functions as spatial elements. FENNECS is also coupled with the GRS thermal-hydraulic system code ATHLET. For the special meshing, an external meshing tool is being developed as a Python software module. This paper considers the Heat Pipe Micro Reactor core. For the generation of cross sections to be used in FENNECS, Monte Carlo Serpent models are developed for two configurations: all absorber faces of control drums are turned out of the core (All Rods Out – ARO – configuration) and all absorber faces are turned in (All Rods In – ARI – configuration). The Serpent multiplication factors are in good agreement with the benchmark. In the FENNECS model, control drums are approximated by a 12-edges polygon or by a 24-edges polygon. The FENNECS multiplication factors agree well with Serpent reference results for both configurations. This demonstrates the applicability of FENNECS to (v)SMRs.