In severe accident conditions, the molten core material forms an internally heated debris bed and eventually becomes a molten pool of corium, which will cause or induce thermal and mechanical loads to the reactor vessel lower head (RVLH) resulting in penetrations leading to failure. A good understanding of the mechanical behavior of the RVLH is essential for estimating structural integrity and improving accident mitigation strategies.

Coupled thermomechanical analysis using ANSYS, a general-purpose finite element method analysis code, was used to evaluate the possibility and timescale of failure. A two-dimensional axisymmetric finite element model was adopted based on APR1400 design data with relevant material properties including creep data.

From the study, it was found that the possibility of plastic and creep failure of the RVLH for the APR1400 was considerably low for a full-core meltdown of the reactor core under ex-vessel cooling conditions with an ambient temperature of 130°C and constant decay heat from the corium, but the lower head may fail unless the increased internal pressure can be reduced on time. Plastic failure can be a major cause of lower head failure of a reactor vessel in high internal pressure conditions and creep failure is not negligible, since failure mechanisms under long-lasting periods are considered. This study found that the APR1400 RVLH failure time is around 220 h using 15% creep strain failure criteria from the postulated accident condition.