Paris, 18 October 2002
The OECD Nuclear Energy Agency (NEA) has just received information on the results of two successful tests carried out at the Argonne National Laboratory in the United States as part of an international project to study the emergency cooling of the debris of a molten nuclear reactor core following a severe accident. The OECD Project on Melt Coolability and Concrete Interaction (MCCI), managed by the US Nuclear Regulatory Commission (USNRC), involves participation from 13 OECD countries in a US$4.8 million programme being carried out over 2002-2005.
These tests are intended to assess the extent to which water ingress through cracks and pores in the crust of a molten debris will enhance the cooling of the melt, i.e. to provide data on long-term core cooling of the molten mass by water ingress mechanism. The two tests were performed with two different melt compositions, but under otherwise identical test conditions. Preliminary data from the experiments indicate that enhanced cooling was achieved in both tests, thus confirming water ingress can be one of the viable cooling mechanisms. The results will be used to develop cooling models for incorporation into analytical codes, which will be used in the safety analysis of both operating and future reactors. Ultimately, the outcome will provide the basis to determine the efficacy of severe accident management strategies of flooding the reactor cavity in the event of an ex-vessel core melt release.
In a core melt accident, if the molten core is not retained inside the vessel despite severe accident mitigation actions, the core debris will relocate to the reactor cavity region and interact with the structural concrete - potentially resulting in failure of structures through erosion or over-pressurisation. This could result in the release of fission products into the environment. Although this is a highly unlikely event, the radiological consequences could be substantial enough to warrant an effective mitigation strategy for preventing such a release. The severe accident management guidance for operating light water reactor plants includes, as one of several strategies, flooding the reactor cavity in the event of a core melt release outside the steel vessel. The MCCI Project aims to provide experimental data on these severe accident phenomena.