Core Loss During a Severe Accident (COLOSS Project)

The COLOSS project was a shared-cost action, co-ordinated by IRSN within the Euratom Research Framework Programme 1998-2002. Started in February 2000, the project lasted three years. 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 by molten Zircaloy,
c) oxidation of U-O-Zr mixtures by steam,
d) degradation-oxidation of B₄C control rods.

Significant results have been produced from separate-effects, semi-global and large-scale tests on COLOSS topics. Break-throughs were achieved on some issues. Nevertheless, more data are needed for consolidation of the modelling on burn-up effects on UO₂ and MOX dissolution and on oxidation of U-O-Zr and B₄C-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.

Based on the experimental results obtained on the COLOSS topics, corresponding models were developed and were successfully implemented in several severe accident codes. Upgraded codes were then used for plant calculations to evaluate the consequences of new models on key severe accident sequences occuring in different plants designs involving B₄C control rods (EPR, BWR, VVER-1000) as well as in the TMI-2 accident.

The large series of plant calculations involved sensitivity studies and code benchmarks. Main severe accident codes in use in the EU for safety studies were used such as ICARE/CATHARE, SCDAP/RELAP5, ASTEC, MELCOR and MAAP4. This activity enabled
a) the assessment of codes to calculate core degradation
b) the identification of main uncertainties and needs for short-term developments and
c) the identification of safety implications of new results.