During severe nuclear accidents, penetrations at the reactor pressure vessel (RPV) lower head may fail under combined thermal loads and internal pressure. While existing severe accident codes primarily consider melting and ejection of the penetration, results from the OECD Lower Head Failure (OLHF) experiments indicate that weld cracking should also be accounted for. This study investigates the penetration weld failure mechanism under severe accident conditions and proposes a model for failure assessment applicable to accident codes. Finite element analysis is conducted to examine the thermo-mechanical behavior and identify the governing weld failure mechanism. Based on these results, the penetration is simplified into a two-dimensional plane-strain edge-crack representation, and a crack model is applied to evaluate stresses and strains. Comparative analysis between the finite element and crack models is performed to assess the applicability of the simplified approach. Although discrepancies are observed, the crack model can still be used for weld failure assessment when suitable weighting factors are introduced.
Juraeva et al. (Fri,) studied this question.