The Potential for AP1000 Reactor Vessel Failure Induced by a Stratified Debris Bed with a Bottom Metal Layer during IVR

摘要

In-vessel retention of molten core debris (IVR) is an important severe accident management strategy employed in the AP1000 Passive Light Water Reactor. The reactor vessel is externally cooled with water, preventing vessel failure and retaining molten debris in the lower plenum. Analysis of the heat loads from a metal-over-oxide lower plenum debris bed configuration demonstrates significant margin-to-failure. However, an alternate debris bed configuration with a dense bottom metal layer below the oxide layer can be postulated to form due to debris material interactions in the lower plenum. While the mechanisms that allowed such interactions to occur are not predicted to occur during the in-vessel debris relocation predicted for the AP1000, the alternate debris bed configuration is analyzed as a low probability uncertainty in the AP1000. Two potential vessel failure modes are addressed in the analysis: failure due to high heat loads from the bottom metal layer and failure due to high heat loads caused by thinning of the top metal layer. For bounding analyses of both of these potential failure modes, the AP1000 vessel is predicted to remain intact.