Mechanisms of in-pile fission gas release from UO$sub 2$

In-pile release mechanisms of fission gas from UO/sub 2/ at low temperatures were studied. The release of /sup 133/Xe, /sup 135/Xe, /sup 138/Se, 2/ pellet was measured at temperatures ranging from 250 to 930 deg C using a graphite specimen holder. The release from the holder, in which a fraction of fission gas recoil-implanted, was subtracted to obtain the net release from the UO/sub 2/ pellet. Knock-out release from the UO/sub 2/ was measured directly, and it was found that it was not the main release mechanism, at least not for short-lived nuclides. A pseudo-recoil'' release model is proposed to explain the low temperature release under irradiation. In the model, some of the defects produced by fission fragments act as shortlived carriers for fission gas. (6 figures, 23 references) (auth)