Heatup of the TMI-2 (Three Mile Island Unit 2) lower head during core relocation

According to current perceptions of the Three Mile Island Unit 2 (TMI-2) accident, corium largely relocated into the reactor vessel lower head at {approximately}224 min into the accident. Defueling examinations have revealed that the corium relocated from the molten core region to the lower head predominantly by way of drainage through the core former region (CFR) located between the vertical baffle plates immediately surrounding the fuel assemblies and the core barrel. An analysis has been carried out to assess the heatup of the reactor vessel lower head during the core relocation event, particularly the potential for a melting attack on the lower head wall and the in-core instrument nozzle penetration weldments. The analysis employed the THIRMAL computer code developed at Argonne National Laboratory (ANL) to predict the breakup and quenching or corium jets under film boiling conditions as well as the size distributions and quenching of the resultant molten droplets. The transient heatup and ablation of the vessel wall and penetration weldments due to impinging corium jets was calculated using the MISTI computer code.