Study of fission gas behaviour in L.M.F.B.R. fuel pins during fast transients

Among the different gas or vapour sources present inside a nuclear fuel heated at high temperature, the fission gas appears to be the most efficient driving force for fuel disruption and motion phenomena at moderate fuel enthalpies before or during fuel melting. The preirradiation conditions of the fuel and the duration of the heating ramp are major parameters which might influence effects of fission gas in transient conditions. The small scale in-pile and out-of-pile experiments SILENE and ECLAT bring quantitative information on kinetics of fission gas release out of solid or melting fuel and allow to adjust the parameters of a model of fission gas behaviour describing fission gas migration from the grains of the fuel matrix to the plenum. Global validation of the model is achieved on the basis of experimental results deduced from close to prototypic in pile experiments in CABRI with preirradiated fuel pins