Spatial analysis of unprotected transients in heterogeneous sodium fast reactors

The assessment of point kinetics models validity is particularly important for highly heterogeneous cores, such as the novel low void fast reactor concept ASTRID. This article presents a benchmark study between CEA and IAEC on the importance of spatial effects during unprotected transients in the ASTRID core. The analysis is performed using two different neutronics spatial models: one based on a deterministic, discrete ordinate neutronics code, while the other employs the Transient Fission Matrix approach, which allows for fast estimation of both reactivity and flux redistribution during the transient with a precision similar to that of the Monte Carlo code. Both models have been coupled to the thermal-hydraulics and applied to an ASTRID representative geometry. This application case is supposed to be sensitive to power redistributions. Even so, the analysis shows good agreement between spatial and point kinetics for unprotected loss of flow and unprotected top of power transients. Some small discrepancies between the results of both codes are found to originate from differences in the calculated spatial distribution of the neutronic reactivity feedback coefficients during stabilization, highlighting the importance of exact spatial description of neutronic feedback coefficients.