Analysis of In-Vessel Retention and Ex-Vessel Fuel Coolant Interaction for AP1000 (NUREG/CR-6849)
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Manuscript Completed: July 2004
Date Published: August 2004
H. Esmaili, M. Khatib-Rahbar
Energy Research, Inc.
P.O. Box 2034
Rockville, MD 20847-2034
S. Basu, NRC Project Manager
NRC Job Code Y6696
Division of Sytems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
The present study assesses the likelihood and potential location of the reactor pressure vessel lower head failure following low-pressure severe accident scenarios in AP1000, and subsequently, the energetics of ex-vessel fuel coolant interaction. 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 thermal analysis of lower head shows that the lower head failure probability at the side of the vessel due to the focusing effect of the thin stratified metal layer overlying the ceramic pool ranges from ~0.04 to ~0.30, depending on the uncertainties in late phase melt progression and the modeling parameters. On the other hand, the failure of the lower head due to thermal attack at the bottom location is assessed to be highly unlikely. Furthermore, two-dimensional ex-vessel fuel coolant interaction analyses show that the calculated impulse loads (and maximum pressures) for AP1000 remain below those that were estimated for AP600 using similar methods.