Bold application of sNDM to REA in a SSCR core with azimuthal mesh

Abstract

Previously applied to REA (Rod Ejection Accident) in several PWR-like cores, the minimalistic Nodal Drift Method (NDM) has recently been generalized to sNDM (super NDM). Both developed and validated on a heat-up transient of the KRUSTY experiment made of a few homogeneous parts, sNDM basically feeds the one-group diffusion approximation with so-called corrective Surface Factors (SF) for internodal currents from MCNP F1 tallies. In order to specify its practical usefulness for exploratory design studies, sNDM at its turn is put to the demanding test of REA in a PWR-like core. The chosen test case is a 600 MWth D2O/H2O-cooled thorium-fueled Spectral Shift Control Reactor (SSCR) core retrieved from previous studies, whose main results on conversion (by our MC-based tool SMURE) and safety (by NDM) are first summed up. Enhanced MCNP core models at HFP, CZP and HZP (respectively Hot Full, Cold Zero and Hot Zero Power) are detailed that have been specially adapted to a 2D radial-azimuthal mesh of few large nodes towards an even simpler REA calculation by sNDM. Other settings, necessary at BOT (Beginning Of Transient from HZP), include fuel and coolant thermal feedbacks as well as the global conductance of a lumped thermal model. Last but not least, the special cases of a few SF found variable between BOT and EOL (End Of Launch at t = 0.05 s) are addressed by an iterative transient calculation based on their linear interpolation. This method is proven effective at the cost of accepting an irreducible discrepancy for the radial exchange rate of the ejected node, provided that a proper so-called global way of computing all SF is used. Finally, main transient results are given until EOT equilibrium (End Of Transient at t = 300 s), with various sanity checks (including a partial safety one)