Cs and I release source terms from irradiated LWR fuel

Abstract Knudsen cell experiments have been performed to investigate the status of fission cesium and iodine in irradiated UO 2 . For those experiments UO 2 samples with a burnup = 33 GWd/t(U) are taken from PWR fuel rods unloaded from a commercial power reactor. The temperatures of a noticeable release decreases from 1800°C at very low burnup to 1400°C at 33 GWd/t(U). The time pattern of Cs and I exhalation is composed of a transient release depending on the accessible surface and temperature followed by a release which mainly depends on the temperature and the square root of time. The temperature dependence of the second release can be described by an Arrhenius function using an activation energy quite similar to that known for the fission gas release. The characteristic release over time pattern is nearly identical for Cs and I exhalation. Also the ratio of release over inventory is nearly the same for both fission products.