A simple model evaluation of contamination integrity against ex-level steam explosion on advanced PWR

The guideline for consideration of severe accidents on containment design for the next-generation LWR was published in 1999 in Japan. The guideline specifies the probabilistic performance targets for the frequency of events in which the capability of containment to retain fission products cannot satisfy the requirements. Also the guideline specifies that assessment should be made from the deterministic point of view in order to ensure that the containment has a proper safety margin against possible loads that may be produced during a severe accident. Pressurized water reactors (PWRs) in operation in Japan adopt the wet cavity formula as a severe accident management, which focuses on the control of molten core concrete interaction (MCCI) during a severe accident. Therefore, there may be a possibility of an ex-vessel steam explosion in case of the reactor vessel failure. Seeing that the wet cavity formula will pe adopted in the next-generation of PWRs, we have decided to develop a method of assessing the containment integrity against ex-vessel steam explosion in order to check the validity of future containment designs for the guideline. First, we conducted a simple evaluation on an APWR, the latest designed PWR in Japan, though it is not categorized as a next-generation LWR. The explosion energy was assumed to be a function of the mass of the dropping melted core and the conversion ratio. The strength of the reactor cavity wall was assumed to be equivalent to the total strain energy which would accumulate by the time one reinforcing bar element would first reach the failure strain in FEM analyses. As a result of this study, the explosion energy would be less than the wall strength; thus, the containment integrity would be maintained even if an ex-vessel steam explosion were to occur. (authors)