Burnup and fission gas release rate determinations on irradiated UO₂ fuel

Chemical examinations were being made on the natural uranium UO₂ pellets which were irradiated in GETR for one and two months, respectively. The pellets were contained in a stainless steel inner capsule and an aluminum outer capsule, the activities when they arrived at us were 40 and 120 c, respectively. The determination of the release rate of gaseous fission products (⁸⁵Kr) and burnup calculation were performed at these samples. The release rate of ⁸⁵Kr were determined by comparing the activities of fission gases which were trapped when the capsules were drilled in a vacuum container with those which were trapped by melting a piece of the pellets. The rate were found to be 0.1∼0.13% which means that gaseous fission products were very well retained in these UO₂ pellets. The burnup values were determined by using ¹³⁷Cs as an indicator and were 0.06 and 0.13%, respectively. These data agreed very well with calculated values from the activities of Al-Co monitoring wires which were irradiated attached to the capsules. The chemical analysis of various parts of pellets were made and no relocation of ¹³⁷Cs was noticeable. (author)