DEBRIS INTERACTIONS IN-REACTOR VESSEL LOWER PLENA DURING A SEVERE ACCIDENT .2. INTEGRAL ANALYSIS

The integral physico-numerical model for the reactor vessel lower head response has been exercised for the TMI-2 accident and possible sever e accident scenarios in PWR and BWR designs. The proposed inherent coo ling mechanism of the reactor material creep and subsequent water ingr ession implemented in this predictive model provides a consistent repr esentation of how the debris was finally cooled in tile TMI-2 accident and how the reactor lower head integrity was maintained during the co urse of the incident. It should be recalled that in order for this str ain to occur, the vessel lower head had to achieve temperatures in exc ess of 1000 degrees C. This is certainly in agreement with the tempera tures determined by metallographic examinations during the TMI-2 Vesse l Inspection Program. The integral model was also applied to typical P WR and BWR lower plena with and without structures under pressurized c onditions spanning the first relocation of core material to the reacto r vessel failure due to creep without recovery actions. The design app lication results are presented with particular attention being focused on water ingression into the debris bed through the gap formed betwee n the debris and the vessel wall. As an illustration of the accident m anagement application, the lower plenum with structures was recovered after an extensive amount of creep had damaged the vessel wall. The co mputed lower head temperatures were found to be significantly lower (b y more than 300 K in this particular example) with recovery relative t o the case without recovery. This clearly demonstrates the potential f or in-vessel cooling of the reactor vessel without a need to externall y submerge the lower head should such a severe accident occur as core melting and relocation.