Left: Test facility for qualification of melt spreading tests. Photo: Argonne National Laboratory (ANL).
Right: Example of corium melt pouring for a melt spreading preparatory test. Photo: Argonne National Laboratory (ANL).

The NEA Reduction of Severe Accident Uncertainties (ROSAU) project aims to reduce knowledge gaps and uncertainties associated with severe accident progression and mitigation. Following the Fukushima Daiichi accident, systematic re-evaluations of assumed accident initiators were carried out worldwide. These re-evaluations examined plant responses to severe accident conditions and operator actions for coping with and terminating severe accident progression. Based on these analyses, gaps were identified in two areas of severe accident progression where additional knowledge was needed and uncertainties could be reduced. These areas are: 1) the spreading of core melt in the containment cavity after the reactor pressure vessel rupture and the effect of metal content in the melt on molten core-concrete interaction; and 2) in-vessel and ex-vessel core melt and debris coolability. The status of technical knowledge in these two areas, focusing on ex-vessel configurations, had been thoroughly reviewed in State-of-the-Art Report on Molten Corium Concrete Interaction and Ex-Vessel Molten Core Coolability and in Status Report on Ex-Vessel Steam Explosion.

The ROSAU Project, supported by 15 partners in 8 countries, comprises 10 tests in the above two areas. Experiments are conducted at the Argonne National Laboratory (ANL) in the United States, under the co-ordination of the US Nuclear Regulatory Commission, and include five large underwater melt spreading tests (MST) with up to 300 kg of molten prototypic material in a newly designed facility and five smaller melt-core-concrete interaction (MCCI) and coolability tests (DCAM) with up to 80 kg of molten prototypic material. Tests are conducted at temperatures up to 2 500°C, with different metal and concrete contents in the melts, with different cooling water flow rates and sub-cooling. In addition to these experiments and with the support of project partners, the ANL will carry out an analytical activity to refine and validate models and codes for each test category, so as to form the technical basis needed to extrapolate experiment findings to plant conditions.

The project started in June 2019 and is planned to end by mid-2024. By the end of 2022, 3 DCAM tests had been performed and provided new results of interest to enhance modeling of core melt cooling by water. The first MST test was performed in early 2023 and in the meantime preparatory drop tests with 50 kg of corium melt where conducted to qualify the test facility which already brought results of interest for code validation. The next DCAM and MST tests are planned to be performed by June 2023.

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