Structure and composition changes in nuclear fuel in simulated loss-of-flow transients

Abstract Nuclear fuel which had been preirradiated in the steady state in the EBR II to 5% burn-up was reirradiated under transient power conditions at Sandia National Laboratories to investigate mode and timescale of fuel response under simulated loss-of-flow conditions. The structure of the fuel and concentration of some important fission products were examined at Kernforschungszentrum Karlsruhe both before and after exposure to the transient power. The large increase of the fuel volume during the transient heating is caused mainly by grain boundary separation in the peripheral zone and by swelling of bubbles and pores in the internal fuel zone. These processes release about 50% of the xenon and about 15% of the cesium retained in the fuel after the steady-state irradiation. Thus, the gases and volatile fission products retained after the transient represent a potential for foaming of molten fuel.