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:
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:
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.
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:
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:
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:
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.
A concluding seminar of the MCCI-2 Project was held in Cadarache, France from 15 to 17 November 2010.
The data abstract is public.
Belgium, Czech Republic, Finland, France, Germany, Hungary, Japan, Norway, Korea, Spain, Sweden, Switzerland and United States
MCCI: Jan 2002 to Dec 2005
MCCI-2: April 2006 to Dec 2009
MCCI: USD 1.2 million/year
MCCI-2: USD 1.1 million/year