Benchmark and preliminary validation of the thermal-hydraulic module of the FRENETIC code against EBR-II data

This paper presents the first qualification of the thermal-hydraulic module of the recently developed FRENETIC code, in the case of a multi-assembly geometry. FRENETIC implements a coupled neutronic/thermal-hydraulic full-core model of liquid-metal-cooled fast reactors, adopting hexagonal fuel assemblies enclosed in a duct. Here we show and discuss the results of a code-to-code benchmark with the RELAP5-3D© code and of a preliminary validation by comparison against experimental data from the EBR-II reactor. The FRENETIC computational domain is restricted to a 0.61 m length of the 7 innermost rings (127 hexagonal assemblies in total, of several different types) of the EBR-II core, so that RELAP5-3D© is also used to provide suitable inlet boundary conditions to FRENETIC, as obtained during the blind phase of an international benchmark exercise. Both steady state and transient conditions are considered. The results of the different codes and of the experiment are compared with particular reference to the Na temperature and to the pin surface temperature measured at different axial and radial locations in the core. Good agreement is shown between the results of the two codes, except in a few situations where, however, the discrepancies can be explained by differences in the two models. The comparison between the FRENETIC results and the experiment shows that, within the relatively large uncertainties in input, the code is able to reasonably bracket the evolution of the coolant temperature at the outlet of the heated region.