Enhanced Ex-Vessel Analysis for Fukushima Daiichi Unit 1: Melt Spreading and Core-Concrete Interaction Analyses with MELTSPREAD & CORQUENCH

Criticality Characteristics and Sensitivity Analysis of Fukushima Debris Beds Containing MCCI Products

“…During the Fukushima accident, the integrity of the RPVs in units 1, 2 and 3 could not be retained and vessel failure is believed to have occurred in all three units. Several reports state that MCCI very likely occurred at least in units 1 and 3 [14,15]. A benchmark study of the accident progression at 1FNPS (BSAF project) was performed with the participation of sixteen organizations and eight countries (France, Germany, Korea, Russia, Spain, Switzerland, the U.S., and Japan) [16].…”

Light Water Reactor Sustainability Program Reactor Safety Technologies Pathway Technical Program Plan

“…Thus, there is a need to employ an exvessel spreading model (MELTSPREAD) as well as account for melt-concrete interactions with below vessel structure and water on the drywell floor. Regarding local changes in debris surface elevation due to natural mechanisms during core concrete interaction (CCI), this topic can be informed not only by extending the CORQUENCH debris coolability model [11,12], but also by physical insights and observations gained from core-debris coolability experiments that are ongoing at ANL supported by international partners.…”

“…The answer to the second question relates not only to the extent that the core debris has spread (which determines the melt depth and available surface area for cooling), but also to the longer-term core debris coolability issue [11]; (i.e., the fraction of the overall decay heat in the core debris that is dissipated by boiling to overlying coolant versus ablation of underlying concrete). If the core debris is quenched and rendered permanently coolable, then the minimum water injection rate would be determined by the functional need to remove all the decay heat in the core debris that has relocated.…”

“…With this background, the principle objective of this work is to make modest upgrades to existing analytical tools (i.e., MELTSPREAD and CORQUENCH -which have been used in the Fukushima accident analyses [11]) in order to provide a technical basis for supporting development of water throttling strategies for BWRs that are aimed at keeping ex-vessel core debris covered with water while preserving the wetwell vent path. Specifically, there is currently a gap in analysis capability for evaluating core melt relocation and cooling behavior that accounts for several important factor that include: (1) the influence of below vessel structure and pre-existing water on the containment floor on melt stream breakup and subsequent spreading behvior, and (2) the effect of water throttling on spreading and long term debris coolability.…”

“…Upgrade and document the core debris coolability model: Incorporate appropriate modeling upgrades into an existing core debris coolability code [12] to evaluate: (1) the effect of water throttling rate (including water-starved situations) on local debris cooling behavior, but more importantly, (2) spatially dependent cavity erosion behavior across the extent of the containment by implementing a multi-nodal scheme that can account for non-uniform cavity geometries such as the Mark I. This latter proposed upgrade, which is considered to be state-of-the art, was carried manually as part of an earlier study [11]; this proposal is to implement software to automate this process for more general applications. As part of this effort, appropriate documentation would be developed for these modeling upgrades in the form of a users manual so that the upgraded code can be released to interested users to support industry R&D activities.…”