Fission Gas Behaviour during Power Transients in High Burn-Up LWR Nuclear Fuels Studied by Electron Microscopy

Abstract

A Transmission Electron Microscope study has been made of UO₂ nuclear fuel samples which have been subjected to short term in-reactor power transients, involving an increase of the fuel temperature of about 300°C. Under steady state operating conditions most of the fission gas (Xe, Kr) is retained in solution in the oxide fuel matrix, or is precipitated into a population of very small fission gas bubbles (<3nm diameter). This bubble population is continuously subjected to re-solution and re-nucleation, and large fission gas bubbles are not able to grow. In contrast, in fuels with about 4.5% FIMA burn up, containing about 0.5 at% Xe and 0.06 at% Kr as fission gases, subjected to an increase in temperature through a power transient, a population of large fission gas bubbles grows, which is almost invariably associated with the dislocation networks. The implications of the growth of this fission gas bubble population is discussed.