ICONE11-36542 LATE-PHASE MELT CONDITIONS AFFECTING THE POTENTIAL FOR IN-VESSEL RETENTION IN HIGH POWER REACTORS

Abstract

If cooling is inadequate during a reactor accident, a significant amount of core material could become molten and relocate to the lower head of the reactor vessel, as happened in the Three Mile Island Unit 2 accident. In such a case, concerns about containment failure and associated risks can be eliminated if it is possible to ensure that the lower head remains intact so that relocated core materials are retained within the vessel. Accordingly, in-vessel retention (IVR) of core melt as a key severe accident management strategy has been adopted by some operating nuclear power plants and planned for some advanced light water reactors. However, it is not clear that currently proposed external reactor vessel colling (ERVC) without additional enhancements can provide sufficient heat removal for high power reactors (i.e., reactors with power levels above 1000 MWe). Consequently, a joint United States/Korean International Nuclear Energy Research Initiative (I-NERI) has been launched to develop recommendations to improve the margin for IVR in high power reactors. This program is initially focussed on the Korean Advanced Power Reactor-1400 MWe (APR1400) design. However, recommendations will be developed that can be applied to a wide range of existing and advanced reactor designs. The recommendations will focus on modifications to enhance ERVC (improved data, vessel coatings to enhance heat removal, and an enhanced vessel/insulation configuration to facilitate water ingress and steam venting) and modifications to enhance in-vessel debris coolability (enhanced in-vessel core catcher configuration, thickness, and material). In this paper, late-phase melt conditions affecting the potential for IVR of core melt in APR1400 were established as a basis for developing the I-NERI recommendations. The selection of 'bounding' reactor accidents, simulation of those accidents using the SCDAP/RELAP5-3D^[○!C]code, and resulting latephase melt conditions are presented.