PREDICTION OF THERMOPLASTIC FAILURE OF A REACTOR PRESSURE-VESSEL UNDER A POSTULATED CORE MELT ACCIDENT

This paper presents the lower-head failure calculations performed for a postulated accident scenario in a commercial nuclear power plant. A postulated 1 inch break in the primary coolant circuit leads to dry-ou t and subsequent meltdown of the core. The reference plant is a pressu rized-water reactor without penetrations in the reactor vessel lower h ead. The molten core material accumulates in the lower head, eventuall y causing failure of the vessel. The analysis investigates flow condit ions in the melt pool, temperature evolution in the reactor vessel wal l and structure mechanical evaluation of the vessel under strong therm al loads and a range of internal pressures. The calculations were perf ormed using the ADINA finite-element codes. The analysis focusses on t he failure processes, and time and mode of failure. The most likely mo de of failure at low pressure is global rupture due to gradual accumul ation of creep strain over a large part of the heated area. In contras t, thermoplasticity becomes important at high pressure or following a pressure spike and can lead to earlier local failure. In situations in which part of the heat load is concentrated over a small area, result ing in a hot spot, local failure occurs, but not until the temperature s are close to the melting point. At low pressure in particular, the h ot spot area remains intact until the structure is molten across more than half of the thickness. (C) 1997 Elsevier Science Ltd.