Cross-section modelling effects on pressurised water reactor main steam line break analyses

Because of the strong asymmetric overcooling effects occurring during a PWR main steam line break (MSLB) event, an accurate analysis of this transient requires the use of 3-D kinetics methods. An assessment has been made of the relative performance of the two kinetics solvers currently employed at PSI for such analyses, viz. CORETRAN and SIMULATE-3 K. For the purpose, the simulation of a hypothetical MSLB in a real operated PWR MOX cycle has been considered, employing consistent 3-D core models with specified thermal-hydraulic boundary conditions at the lower and upper plenums. Although the employed cross-section library is in both codes based on the same set of homogenised 2-group cross-sections prepared with CASMO-4, significant differences are shown to occur due to the smaller moderator reactivity coefficient calculated in CORETRAN. It is found that this stems largely from differences in the cross-section formalism, i.e. the manner in which feedback dependencies are modelled and interpolated for the cross-section sets. In particular, the CORETRAN cross-section formalism induces an inadequate treatment of coupled feedback effects, principally between boron density and moderator temperature, which renders the MSLB dynamics predictions quite sensitive to the methodology employed during the cross-section preparation. As such, transient-specific cross-section libraries need to be produced for reliable MSLB analysis in this case. The cross-section model for SIMULATE-3 K, on the other hand, is shown to be adequate for accurately capturing the coupled reactivity effects occurring during an MSLB. In this case, the sensitivity of the results to other sources of uncertainties becomes more apparent, e.g. to those related to the neutron data and/or the thermal-hydraulic boundary conditions. Considering that many other state-of-the-art advanced kinetics solvers have cross-section formalisms similar to that of CORETRAN, effects of the type currently investigated need to be taken into account while developing methodologies for assessing neutronics-related uncertainties in best-estimate transient analysis.