MAAP-MELCOR crosswalk phase 1 study

The Modular Accident Analysis Program, Version 5 (MAAP5) and Methods of Estimation of Leakages and Consequences of Releases (MELCOR) are used integral plant response analysis computer codes. Both programs have been developed over the past 30 years for the purpose of simulating a range of beyond design basis accidents. They are supported by extensive benchmarking against numerous separate effect experiments that reflect, to varying degrees, conditions expected to arise in light water reactor accidents. Such separate effect tests, however, do not completely represent the novel physics that can arise through the interaction of multiple phenomena and physical processes at a reactor scale. Furthermore, aside from the Three Mile Island, Unit 2 (TMI-2) core damage event, there is limited information available to evaluate reactor scale behavior. Both MAAP5 and MELCOR have developed models to capture reactor scale accident progression that, to a certain extent, extrapolate from separate effect experiments, with assessment against the TMI-2 event. Due to the limited information available to assess these extrapolated reactor scale models, differences in MAAP5 and MELCOR code predictions do exist, most notably in the simulation of in-vessel core melt progression. While these differences are not necessarily influential for key metrics evaluated in Probabilistic Risk Assessments (PRAs), they can have a more pronounced impact on studies assessing the efficacy of accident management measures. This paper reports the first phase of a MAAP-MELCOR crosswalk designed to identify the key core melt progression modeling differences [1]. The results of this study highlight the impact that assumptions about reactor scale, in-vessel core debris morphology have on a) the potential for high temperatures to develop above the reactor core and in the main steam lines, b) the magnitude and extent of the period for in-vessel hydrogen generation, and c) the rapidity with which a degraded core can be recovered. These examples play critical roles in the evolution of challenges to the RPV pressure boundary and containment, and are ultimately central to the evaluation of accident management effectiveness.