The core loss during a severe accident (COLOSS) project is a 3-year shared-cost action which started in February 2000. The project is concerned with the consequences that core degradation, occurring under severe accident conditions, may have on H2 production, melt generation and the source term. Unresolved in-vessel risk-relevant issues are studied, through a large number of experiments such as (a) UO2 and MOX dissolution by molten zircaloy and burn-up effects, (b) simultaneous dissolution of UO2 and ZrO2 in rod geometry, (c) oxidation of U-O-Zr mixtures, (d) oxidation of pure B4C material and (e) degradation and oxidation of B4C control rods. A parallel effort is devoted to model developments for severe accident (SA) computer codes. These codes are finally used for plant calculations to assess SA code capabilities and to apply results produced in this project to evaluate their consequences on key SA sequences occurring in different plants such as PWR-1300, BWR, VVER-1000, EPR and in the TMI-2 accident. Following significant results have been produced at the mid-term of the project: Several B4C oxidation experiments have improved the understanding of oxidation mechanisms. Preliminary models have been developed and implemented in SA codes. Separate-effects tests (SET) on simultaneous UO2 and ZrO2 dissolution and on U-O-Zr oxidation by steam enabled progress to be made on the understanding and modelling of these interactions. There is experimental evidence that the oxidation of mixtures can contribute significantly to the large H2 production occurring during the reflood of a reactor core under severe accident conditions. Two large-scale tests CODEX-B4C and QUENCH-07 have been carried out with a central B4C control rod. The B4C effects on VVER and PWR core degradation and on the related gas production have been evaluated. Twelve plant calculations of key SA sequences illustrate the current status of SA codes to predict core degradation, in particular B4C effects.
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