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In a core melt accident, if the molten core is not retained in-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 basemat failure through erosion or overpressurisation. This would result in the release of fission products into the environment. Although this is a late release event, the radiological consequences could be substantial enough to warrant an effective mitigation strategy for preventing such a release. The severe accident management guidance (SAMG) for operating light water reactor plants includes, as one of several strategies, flooding the reactor cavity in the event of an ex-vessel core melt release.
The Melt Coolability and Concrete Interaction (MCCI) Project was dedicated to provide experimental data on this severe accident phenomena and to resolve two important accident management issues:
verify that molten debris that has spread on the base of the containment can be stabilised and cooled by water flooding from the top;
assess the two-dimensional, long-term interaction of the molten mass with the concrete structure of the containment, as the kinetics of such interaction is essential for assessing the consequences of a severe accident.
To achieve these basic objectives, supporting experiments and analyses were performed at Argonne National Laboratory (ANL), with a view to providing an understanding the phenomena, and to produce a consistent interpretation of the results relevant to accident management.
Previously, an internationally-sponsored programme has already been carried out at ANL to address the corium coolability issue. The MCCI project aimed to complete this earlier research and achieve the following technical objectives:
resolve the ex-vessel debris coolability issue through a redirected programme which focuses on providing both confirmatory evidence and test data for the coolability mechanisms identified in previous ANL integral effect tests;
address remaining uncertainties related to the long-term, two-dimensional, melt-concrete interaction under dry cavity conditions.
The first MCCI experiments focused on water ingress mechanisms, as these are thought to be the most effective ones for cooling the melt. These experiments have demonstrated how cooling of the melt by water is affected by the concrete-melt composition and that cooling of the melt by water is reduced at increasing concrete content, i.e. cooling by water flooding is more effective in the early phase of the melt-concrete interaction. The effect of concrete type, such as siliceous and limestone types (used respectively in Europe and the United States), has also been addressed. Material properties such as porosity and permeability have been derived from these tests.
A first melt-concrete interaction test with siliceous concrete in 2003 produced unexpected results (a strong asymmetry in concrete ablation), although the associated analytical exercise proved very valuable in helping to understand code capabilities and shortcomings. A second test was carried out in 2004 at 30% lower power than the first on limestone concrete (instead of the siliceous concrete used in the first test). The strength of the solid upper crust, a parameter that is of great interest for modelling and understanding MCCI at plant scale, was also determined during these experiments. A third test with siliceous concrete was successfully carried out in 2005, yielding excellent data on axial and radial concrete ablation.
The first phase of the programme (MCCI-1) was completed in 2005. The experiments on water ingress mechanisms showed that cooling of the melt by water is reduced at increasing concrete content, implying that water flooding is more effective in the early phase of the melt-concrete interaction. The effect of concrete type, i.e. siliceous and limestone types (used respectively in Europe and the United States), was also addressed in the first phase of the programme. Material properties such as porosity and permeability were derived. Tests also showed appreciable differences in ablation rate for siliceous and limestone concrete, which is a relevant finding that requires confirmation. A workshop on the results of MCCI-1 took place on 10-11 October 2007 in Cadarache. This project was followed by the MCCI-2 project.
Project participants: Belgium, Czech Republic, Finland, France, Germany, Hungary, Japan, Norway, Republic of Korea, Spain, Sweden, Switzerland and the United States.
Project period: January 2002-December 2005
Project management: US Nuclear Regulatory Commision (USNRC)
For more detailed information visit http://www.oecd-nea.org/jointproj/mcci.html
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M. T. Farmer, S. Lomperski, D. Kilsdonk, R. W. Aeschlimann, and S. Basu:
A Summary of Findings from the Melt Coolability and Concrete Interaction (MCCI) Program, Paper 7544, 2007 International Congress on Advances in Nuclear Power Plants (ICAPP'07), Nice, France, 13-18 May 2007.
U.S. Nuclear Regulatory Commission
11545 Rockville Pike, Rockville, MD 20852 U.S.A.
Mitchell T. Farmer
Manager, Engineering and Development Laboratory
Argonne National Laboratory
Argonne, IL 60439 USA
Stephen W. LOMPERSKI
Reactor Engineering Division
Argonne National Laboratory
Argonne, IL 60439 USA
Keywords: core concrete interactions, experiment, nuclear reactor safety, reactor materials, severe accidents.