Insights into corium coolability phenomena-top flooding vs. bottom flooding

During a postulated severe accident in a nuclear reactor, the core can melt and form a melt pool in the vessel. In the absence of adequate cooling strategies, the melt can fail the vessel and the molten corium can be relocated in the containment cavity forming a melt pool. In order to arrest further progression of severe accident, complete quenching of the molten corium pool is necessary. Most common way to deal with ex-vessel scenario is to flood the melt pool with large quantity of water. The mechanism of coolability is much more complex involving multi-component, multiphase heat, mass and momentum transfer associated with wide variation of thermal and mechanical properties of materials with temperature. In addition to this, the melt will start attacking the concrete basemat which brings about the decomposition of the concrete, generating lots of gases like water vapor, CO₂ etc. These gases react with the molten metals in the pool which results in chemical heat addition and generation of hydrogen which poses a potential danger of containment pressurization and detonation. Simultaneously, the concrete may start ablating and will get mixed with the corium changing its physical properties like density, solidus and liquidus temperature etc. In order to ensure complete coolability, much more knowledge about the phenomena occurring is needed. In this paper, a mechanistic model has been developed for the corium coolability with top flooding conditions and a model has been presented on bottom flooding condition. The quenching rate and water ingression in top flooding condition has been predicted. Discussion on coolability with top flooding vis-à-vis bottom flooding is also presented.