Thermal attack of core debris on a PWR reactor vessel: Chapter 15

In order to provide realistic estimates of the small risk posed by a nuclear power plant it is necessary to be able to model beyond design basis accidents in some detail. This paper is concerned with one small part of the accident sequence - the thermal attack of hot core debris on the reactor vessel. According to early risk assessments this phase of the accident has a limited influence on the rest of the progression; the principal effect was considered to be the delay in contact of debris with the containment floor. It was concluded in WASH-1400 that uncertainties in the length of this delay were not particularly significant. The recognition of a dispersive mode of debris relocation (produced by failure of small penetrations through the vessel prior to gross failure) for small break and transient sequences has now modified this view, but as yet little emphasis has been placed on debris conditions during the attack