The COLOSS project is a 3-year shared-cost action witch started in February 2000. The objective is concerned with the consequences that core degradation, occurring under Severe Accident (SA) 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 at different scales, such as a) burn-up effects on UO2 and MOX dissolution, 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-oxidation of prototypic B4C control rods of different designs. A parallel effort is devoted to model developments for SA codes. These codes are finally used for plant calculations to assess SA code capabilities and to apply results and models produced by the project to evaluate the consequences of key SA sequences occurring in different plant such as PWR-1300, BWR, VVER-1000, EPR and in the TMI-2 accident. Significant experimental and analytical results have already been produced: - Many experiments were carried out on the B4C oxidation and the related gas production. - Preliminary results on U-O-Zr oxidation by steam indicate that the oxidation of mixtures can contribute significantly to the large H2 peak production during large oxidation phases. - Two large-scale tests CODEX-B4C and QUENCH-07 with respectively VVER and PWR bundles, each characterised by a central B4C control rod, have been carried out with success. - A preliminary B4C model has been developed and implemented in several SA codes. A 1st set of 12 plant calculations of key SA sequences has been carried out and analysed.
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