Models of fission gas behaviour in fast reactor fuels under steady state and transient conditions

Abstract

Two different physical mechanisms have been used in the past as the basis of models describing fission gas release and swelling in an operating fast reactor fuel. This has led to confusion in the interpretation of experimental data and to a proliferation of apparently conflicting models. This work aims at resolving some of these difficulties and shows clearly that the real situation can only be described completely by a model which combines the two basic mechanisms, each dominating in a different temperature regime. The rate theory is used to describe the nucleation and evolution of the fission-gas bubble population. At lower temperatures the model is based upon the notion of the random motion of single gas atoms and gas bubbles and includes the effects of re-solution and coalescence within the bubble population whilst at higher temperatures the directed motion of bubbles in a temperature gradient, again including re-solution and coalescence, is shown to be the most important mechanism. There are further difficulties within each of these overall schemes and we highlight in particular the sensitivity of the results of the random motion model to the re-solution rate and bubble mobility.