MAAP 5.04 ex-vessel CRD tube heat transfer and melting model

Enhancements are made in the Boiling Water Reactor (BWR) version of MAAP 5.04 to consider ex-vessel Control Rod Drive (CRD) housing heat transfer and melting. The in-vessel CRD tube model is extended below the reactor vessel and conduction and radiative heat transfer between the lower head, CRD tubes, pedestal gas and pedestal wall are considered. The possibility of corium melt flowing into collapsed CRD tubes and causing creep rupture of CRD tubes outside the reactor vessel, leading to vessel failure, is considered. Lastly, ex-vessel CRD tube melting due to the corium jet exiting the breached reactor vessel after vessel failure is considered. The BWR Station Blackout (SBO) scenario was simulated using the new code. The results show that explicitly modeling the ex-vessel CRD tube heat transfer did not affect the time of vessel failure. Also, vessel failure due to corium ingress into ex-vessel CRD tubes was not predicted. A small, but nontrivial mass of steel from ablated ex-vessel CRD tubes was added to the debris bed on the pedestal floor after vessel failure.