AN ANALYSIS OF MOLTEN-CORIUM-INDUCED FAILURE OF DRAIN PIPES IN BWR MARK II CONTAINMENTS

Abstract

This study has focussed on mechanistic simulation and analysis of potential failure modes for inpedestal drywell drain pipes in the Limerick1 and WNP-2₂ boiling water nuclear reactor (BWR) MARK 11 containments. Physical phenomena related to surface tension breakdown, heatup, melting, ablation, crust formation and failure, core material relocation into drain pipes with simultaneous melting of pipe walls were modeled, and analyzed. The results of analysis have been used to assess the possibility of drain pipe failure and the resultant loss of pressure-suppression capability. Estimates have been made for the timing and amount of molten corium released to the wetwell. The study has revealed that significantly different melt progression sequences can result depending upon the failure characteristics of the frozen metallic crust which forms over the drain cover during the initial stages of debris pour. Another important result is that it can take several days for the molten fuel to ablate the frozen metallic debris layer— if the frozen layer has cooled below 1100 K before fuel attack.

KEYWORDS:

• Boiling water reaction Mark II
• Corium
• Drain pipes.