Melt Coolability and Concrete Interaction Project Overview

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.

Achieving these two programme objectives will lead to improved severe accident management guidelines for existing plants as well as better containment designs for future plants.

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.

A new three-year programme (MCCI-2) has now been adopted. Emphasis will be placed on two dimensional core-concrete interaction experiments, as they provide the integrated effect of many processes. The MCCI-2 project involves organisations from 12 member countries. A meeting of the project steering bodies was held in Paris in April 2006. On this occasion, the test conditions for the three-year programme were discussed. A second meeting was held in March 2007 to review the results of the first project tests and to further specify the test matrix.

This first test of the quenching series current programme, conducted in January 2007, was a small-scale water ingression test with the same small-scale water ingression and crust strength (SSWICS) apparatus that was used for the MCCI-1 Project, but adapted to provide gas injection during quench to simulate the flow of concrete decomposition gases. The corium composition and operating conditions duplicate that of SSWICS-6, a fully oxidised PWR corium melt containing 15 wt% siliceous concrete quenched at a system pressure of 1 bar. The resulting data provides an opportunity to compare the cooling rate and crust morphology of corium quenched with and without sparging gases. The first test of the core-concrete interaction series, CCI-4, conducted in May 2007, utilised a similar test apparatus and experiment boundary conditions, including a limestone/common sand concrete test section. However, instead of a fully oxidised PWR melt, a partially-oxidised BWR core melt containing ~8 wt% structural steel constituents was used for the CCI-4 test. This test thus evaluated the effect of elevated melt metal content on two-dimensional cavity erosion and debris cooling behaviour.

Project data

Complete project data is available to NEA member countries and the Russian Federation, data abstracts are public.

Project participants

Belgium, Czech Republic, Finland, France, Germany, Hungary, Japan, Norway, Republic of Korea, Spain, Sweden, Switzerland and the United States.

Project period

April 2006 to December 2009

Budget

US$1.2 million/year.

Project Management

US Nuclear Regulatory Commission (USNRC)

Publications

M. Farmer, S. W. Lomperski, S. Basu, The results of the CCI-2 reactor material experiment investigating 2-D core-concrete interaction and debris coolability, NURETH-11, Avignon, France, October 2005.

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.

M. T. Farmer and S. Lomperski, Status and Future Direction of the MCCI Program, Second European Review Meeting on Severe Accident Research (ERMSAR-2007), Forschungszentrum Karlsruhe (FZK), Germany, 12-14 June 2007.

OECD/NEA Melt Coolability and Concrete Interaction (MCCI) Project