A comparative study on MELCOR and MAAP simulations of a severe accident scenario in a Nordic boiling water reactor

This study juxtaposes the results from two independent simulations of a station blackout scenario in a Nordic boiling water reactor (BWR) using the MAAP and MELCOR codes. The purpose is to better understand and analyse code differences in general and the existing MELCOR and MAAP models of a Nordic boiling water reactor (BWR) in particular. The figures of merit in the comparative study are chronology of events till the actuation of filtered containment venting (FCV), corium temperature and relocation, reactor pressure vessel (RPV) pressure, containment pressure and hydrogen generation. Observed from the limited scope of comparisons, the overall accident progression and the failures of fuels and RPV lower head are consistent with the findings of the MAAP-MELCOR crosswalk study previously performed by EPRI and Sandia National Laboratories ( Luxat et al., 2016 ; Andrews et al., 2017 ). The MELCOR model predicted a later vessel breach and melt ejection into the containment than the MAAP model did, while the MAAP model predicted earlier actuation of the FCV system even though the MELCOR model showed a faster containment pressure increase after RPV failure due to greater hydrogen accumulation. The two codes also differed in their predictions of post-venting containment response, with the MELCOR simulation featuring a significant containment pressure oscillation with a cyclic period of 3–4 h, caused by difference in modelling of pool hydrodynamics in the lower drywell and the wetwell. Since containment pressure evolution is important to safety, this phenomenon merits further investigation.