Behaviour of nuclear reactor pit concretes under severe accident conditions

In the hypothetical case of a nuclear reactor severe accident, a mixture of high temperature molten materials (UO2, ZrO2,..) coming from the core, named corium, could flow out of the reactor vessel, and interact at 2000-3000 deg. C with the reactor pit concrete. A series of experiments have been conducted in which 20 to 40 kg of corium are molten and poured inside a concrete crucible. Concrete ablation is studied by monitoring more than 100 thermocouples imbedded in the concrete and by post-test analyses. The major result of these tests is a different behavior depending on the nature of the concrete aggregates: for concretes made mainly from siliceous aggregates, the ablation rate is much larger on the horizontal direction than downwards while it is isotropic with larger limestone content. (authors)