The COLOSS project was a 3-year shared-cost action, which started in February 2000. The work-programme performed by 19 partners was shaped around complementary activities aimed at improving severe accident codes. Unresolved risk-relevant issues regarding H2 production, melt generation and the source term were studied through a large number of experiments such as (a) dissolution of fresh and high burn-up UO2 and MOX by molten Zircaloy, (b) simultaneous dissolution of UO2 and ZrO2, (c) oxidation of U-O-Zr mixtures, (d) degradation-oxidation of B4C control rods. Corresponding models were developed and implemented in severe accident computer codes. Upgraded codes were then used to apply results in plant calculations and evaluate their consequences on key severe accident sequences in different plants involving B4C control rods and in the TMI-2 accident. Significant results have been produced from separate-effects, semi-global and large-scale tests on COLOSS topics enabling the development and validation of models and the improvement of some severe accident codes. Break-throughs were achieved on some issues for which more data are needed for consolidation of the modelling in particular on burn-up effects on UO 2 and MOX dissolution and oxidation of U-O-Zr and B 4C-metal mixtures. There was experimental evidence that the oxidation of these mixtures can contribute significantly to the large H2 production observed during the reflooding of degraded cores under severe accident conditions. The plant calculation activity enabled (a) the assessment of codes to calculate core degradation with the identification of main uncertainties and needs for short-term developments and (b) the identification of safety implications of new results. Main results and recommendations for future R&D activities are summarized in this paper.
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