Analysis of likelihood of lower head failure and ex-vessel fuel coolant interaction energetics for AP1000

A one-dimensional model is formulated to assess the thermal response of the Westinghouse Advanced Plant (AP1000) lower head based on two bounding melt configurations. Melt Configuration I involves a stratified light metallic layer on top of a molten ceramic pool, and melt Configuration II represents the conditions that an additional heavy metal layer forms below the ceramic pool. The approach consists of the specification of initial conditions; determination of the mode, the size and the location of lower head failure based on heat transfer analyses; computer simulation of the fuel coolant interaction processes; and finally, an examination of the impact of the uncertainties in the initial conditions and the model parameters on the fuel coolant interaction energetics through a series of sensitivity calculations. The results of the calculations for melt Configuration I show that the heat flux remains below critical heat flux (CHF) in the molten oxide pool, but the heat flux in the light metal layer could exceed CHF because of the focusing effect associated with presence of the thin metallic layers. The thin metallic layers are associated with smaller quantities of the molten oxide in the lower plenum following the initial relocation into the lower head. The calculations show that the lower head failure probability due to the focusing effect of the stratified metal layer ranges from ∼0.04 to ∼0.30. On the other hand, the thermal failure of the lower head at the bottom location for melt Configuration II is assessed to be highly unlikely. Based on the in-vessel retention analysis, the base case for the ex-vessel fuel coolant interaction (FCI) is assumed to involve a side failure of the vessel involving a metallic pour into the cavity water. The FCI sensitivity calculations intended to assess the implications of the uncertainties in initial conditions and the FCI modeling parameters show that the FCI loads range from a few MPa to upward of 1000 MPa (maximum pool pressure) with corresponding impulse loads ranging from a few kPa s to a few hundred kPa s.