A model for recovery of a badly degraded core

The hydrogen source term that can be generated in-vessel during a severe accident has a strong implication on the size of the containment of a nuclear reactor. This source term can be generated during the main phases of a postulated accident. On the other hand, recovery of an overheated core can produce a large hydrogen source term since large amounts of steam can be generated by the quenching process, enabling Zircaloy oxidation in regions that were previously steam starved. An accurate model would provide guidelines on the system response to operator actions to inject water into the core at different stages of the accident. A model that attempts to simulate the recovery of a badly degraded core has to consider the cooling effect of the water together with the potential to overheat the debris due to oxidation of metallic Zircaloy by the steam. The basic assumption of this model is that the heat transfer rate from a degraded core region to the coolant can be related directly to the porosity. The model was applied to the B-loop transient in the Three Mile Island Unit 2 (TMI-2) accident using the core conditions just prior to the transient (174 min) as calculated by MAAP. It is shown that, with this model, injection of water into a melting core could actually save the reactor vessel from being breached. Additionally, the hydrogen generation would not be larger than that without recovery action