Release of short-lived fission products from UO2 fuel: Effects of operating conditions

To support accident “source-term” studies, we have carried out a series of in-reactor tests to determine the behaviour of short-lived fission products (Xe, Kr, I) in operating UO₂ fuel under a variety of conditions. The linear power range examined was 1762 kW/m to a maximum burnup of about 220 MW. h/kg U.Under normal operating conditions, release of the short-lived xenons and kryptons was described by a λ(decay constant)^?0.5 relationship, characteristic of diffusion; there was a strong temperature-dependence of release in the thermally-activated diffusion range. Release during shutdown and startup transients was dependent on prior operating temperature, while under oxidizing conditions enhancement of release up to a factor of 4.5 was observed. Under loss-of-coolant conditions, release was described by a λ^?1 relationship, characteristic of movement of stored inventory. Most release accompanied the re-wet portion of the transient, consistent with enhancement due to fuel cracking. No iodines exited from the fuel elements under normal, oxidizing or loss-of-coolant conditions.