FINITE-ELEMENT ANALYSES OF A BWR VESSEL AND PENETRATION UNDER SEVERE ACCIDENT CONDITIONS

This study assesses the two-dimensional thermal response of a BWR vess el and drain line penetration to three types of debris bed: primarily metallic, primarily ceramic and metallic and ceramic layered, with sen sitivity studies for the most severe case. Structural finite element a nalysis evaluates vessel elastic, plastic and creep response for two c ases which bound the thermal challenge to the vessel. Thermal analysis results indicate that drain line failure does not occur for the case when metallic debris relocates to the lower head; structural analysis predicts that the vessel also remains intact for this case. In cases w here ceramic debris relocates to the lower head, drain line temperatur es peak near values where failure may occur within several minutes; wh ereas vessel failure is not predicted for 3.5 to 4.0 hours. Sensitivit y studies indicate that large porosity debris or high heat removal rat es from the vessel and drain line outer surfaces can preclude failure temperatures from occurring.