AbstractIrradiated UO2graphite fuel samples in which most of the fission products had recoiled into the graphite matrix were heated after irradiation, and then leached with nitric acid. The leach-ability of non-gaseous fission products was influenced by fission product concentration, by irradiation temperature, and largely by temperature and period of the heating. A possible rate-controlling mechanism for the fission product loss during the heating is discussed, and the results obtained are compared with some of the earlier work. The data may be interpreted as indicating that the fission products migrate through the graphite crystal to its surface according to a fast and a slow migration step. Escape from the graphite matrix by volatilization is apparently less rapid than the fast migration step; volatilization may be the rate-limiting mechanism in the loss process.