Thermal attack of a PWR lower vessel head during a core meltdown accident

In the event of a core meltdown accident in a light water reactor, the core debris may relocate into the bottom of the reactor vessel where it could accumulate and begin to attack the lower head wall. The failure mode and timing of the reactor vessel could have an important impact on containment performance. Several types of failure modes have been suggested, ranging from the local failure of instrument guide tube penetrations to the gross creep rupture of the entire lower head. The mode of bottom head failure depends on the nature of the thermal and pressure loading that the lower head experiences, which in turn is dependent on the accident sequence and the characteristics (quantity, composition, temperature, and timing) of the core materials (corium) that arrive on the bottom head. The purpose of the current study is to provide a framework for evaluation of the thermal attack of the bottom head wall (in the absence of local penetrations) by a corium pool accumulated in the lower plenum. A transient model for the molten pool behavior and thermal attack of the lower head has been developed. The model has been applied to the analysis of a lower head failure comparisonmore » exercise (pressurized water reactor (PWR) case 2), which was specified by a Committee on the Safety of Nuclear Installations (CSNI) Task Group on October 23, 1990.« less