Fission product release from dynamically heated mixed oxide reactor fuels

Knowledge of the fraction and rates of fission product release from molten oxide fuel that has been heated on a millisecond time scale are needed for thermal transient calculations in fast reactor safety analysis. Release of fission products in the vapor phase can produce pressurization leading to fuel motion in such safety studies. In the present work, experiments have been carried out to investigate such gas release behavior for thermal transients in the millisecond range. A series of experiments is described in which fuel samples are heated to temperatures in the 50000K range in less than one millisecond by fission heating in the Sandia Pulsed Reactor. The oxide sample is contained by a pressure vessel inside of which a transducer monitors the pressure of vapor phase material over the fuel as a function of time. Oxide fuel in the form (U075, Pu025)O2 previously irradiated to a burnup of 5.3% and retaining approximately 30% of the fission gas was selected for the sample material. Analysis based upon an equilibrium chemical model of the oxide is used to determine upper bounds on the pressure generated in the vapor phase for comparison with experimental measurements. The pressures observed in these experiments for times in the millisecond range are lower than the equilibrium calculations. This owes at least partially to heat transfer and mass diffusion effects. 8 figures