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 type of 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 provide an understanding of the phenomena, and to produce a consistent interpretation of the results relevant to accident management.

Previously, an internationally-sponsored programme had 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.

MCCI phases

First phase (2002-2005)

The first MCCI experiments focused on water ingress mechanisms, as these are thought to be the most effective ones for cooling the melt. The experiments demonstrated how cooling of the melt with water was affected by the concrete-melt composition and that cooling the melt with water was reduced by 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), had also been addressed. Material properties such as porosity and permeability had been derived from these tests as well.

In 2003, a first melt-concrete interaction test with siliceous concrete 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, France. 

Phase 1 final report is available at OECD MCCI project final report.

Second phase (2006-2009)

After successful completion of the first phase at Argonne National Laboratory (ANL), a second phase (MCCI-2) using the same ANL facilities was set up. The MCCI‐2 Project was carried out from 2006 to early 2010 to help bridge data gaps not fully covered during MCCI‐1. Testing fell into four categories:

• combined effect tests to investigate the interplay of different cooling mechanisms and to provide data for model development and code assessment
• tests to investigate the effectiveness of new design features that enhance debris coolability
• tests to generate additional 2‐D core‐concrete interaction data for model development and code validation
• an integral test at larger scale to confirm synergistic effect of different cooling mechanisms and to provide data for validation of severe accident codes.

Aside from these tests, a supporting task analysis was carried out to further develop and validate debris coolability models that formed the basis for extrapolating the experiment findings to plant conditions. In total, ten tests were conducted in this program and they were all successful.

Four category one tests were performed using the Small Scale Water Ingression and Crust Strength (SSWICS) apparatus. Tests were conducted to provide additional crust strength data to confirm the concept of a floating crust boundary condition at plant scale and to investigate the effect of gas sparging on water ingression cooling of corium. Crust strength tests (2) showed that the strength of un‐sectioned crust samples was consistent with that of the sectioned specimens tested in MCCI‐1. Gas sparging tests (2) showed that the presence of sparging significantly increases the cooling rate of a solidifying corium pool over those observed when sparging is absent.

Category two tests to examine the effectiveness of design features for augmenting coolability, i.e. melt stabilisation concepts, were of two cooling types:

• water‐cooled basemat test (WCB‐1) addressed cooling with external water‐cooled surfaces
• SSWICS‐12 and SSWICS‐13 tests addressed water ingress into the melt volume by fragmentation.

Category three tests provided additional 2‐D core-concrete interaction data. CCI tests in both the MCCI and French VULCANO facilities had shown a marked dependence of cavity erosion behaviour on the concrete type. Tests with limestone/common sand (LCS) concrete generally exhibited a radial/axial power split of ~1; conversely, siliceous tests exhibited splits that were significantly greater than one. The CCI‐4 test was conducted with LCS concrete, but with increased metal content (structural and cladding) to evaluate effect on cavity erosion behaviour. The CCI‐5 test was conduced with siliceous concrete, but the apparatus was modified to increase lateral scale to diminish the wall effects to the greatest extent possible. Test aspect ratio (cavity width/melt depth) increased from 1 to 3.7.

Category four was an integral test to validate severe accident codes. The large-scale CCI‐6 test was conducted with early flooding to focus on debris coolability. Key features were:

• 70 cm x 70 cm cross section, 28 cm melt depth: 900 kg (63/25/6/6 wt % UO2/ZrO2/Cr/concrete)
• siliceous concrete
• cavity flooding ~ 1 minute after melt contact with basemat.

The design incorporated an embedded array of water injection nozzles at a depth of 27.5 cm into the concrete. If debris did not quench, then a second test phase would have been initiated to provide additional category two data on bottom water injection cooling. Results demonstrated that: 1) early cavity flooding significantly enhances debris coolability, even for siliceous concrete; and 2) melt eruptions are a viable cooling mechanism for siliceous concrete. The test was terminated on the basis of debris quench well before water injection of the nozzles was reached.

Phase 2 final report is available at OECD MCCI-2 Project. Final Report

A concluding seminar of the MCCI-2 Project was held in Cadarache, France from 15 to 17 November 2010.

Project data

The data packages can be requested at MCCI and MCCI-2.