Significance of the OECD-MCCI Program in Relation to Severe Accident Uncertainties Evaluation

Over the past two decades, a substantial amount of research has been conducted — in the United States during the early days and later in other countries as well — to improve the understanding of severe accident phenomena for more realistic assessment of the associated risk and to address epistemic uncertainties. This research has resulted in resolution of issues from a risk perspective (e.g., containment failure attributable to an energetic fuel-coolant interaction or the so-called alpha-mode failure, direct containment heating from high-pressure melt ejection, etc.), implementation of severe accident management strategies (e.g., flooding drywell in a Mark-I containment to prevent liner failure), and improved assessment of the source term and uncertainties associated with hydrogen production (both inand ex-vessel). An unresolved issue that has received increased attention in recent years relates to melt coolability (in particular, ex-vessel coolability) and consequent containment integrity and fission product release into the environment. The Organization for Economic Cooperation and Development Melt Coolability and Concrete Interaction (OECD-MCCI) program is experimentally investigating the effectiveness of cooling ex-vessel core debris using an overlying water pool. This paper discusses the significance of the OECD-MCCI program in relation to severe accident uncertainties. Specifically, this paper discusses the important findings of the experimental program with regard to an assessment of epistemic uncertainties in coolability and core-concrete interaction (CCI) models that are used to extrapolate the results to plant conditions.