Main achievements of the European R/D on the issue of air ingression during a LWR severe accident

During a severe accident, simulations have showed that air can penetrate the reactor vessel and can have significant implication for the source term and specifically towards ruthenium fission product, which is highly radio-toxic. Under very oxidising conditions, like those induced by air, ruthenium can form volatile oxide species from metallic ruthenium still present in the fuel. These ruthenium oxides can be transported through the reactor coolant system and are able to reach the containment building. Due to various radio-chemical reactions in the containment, the species RuO4 can persist in the gaseous phase during several days. Potential release of RuO4 (g) in the environment may occur due to the French ultimate procedure of containment venting, leading thus to a probable increase of the total efficient dose in the environment. Under air atmosphere, high ruthenium oxides releases have been experimentally observed and now are well estimated by calculations