Post-irradiation examinations of several ceramic nuclear fuels are summarized, emphasizing the identification and characterization of fission induced defects. The lattice parameter and electrical resistivity were significantly altered with fission damage in all fuels. A remarkable reduction, however, was found in the irradiation induced changes of both physical properties in uranium dioxide (UO2), uranium nitrite (UN), and uranium sesquicarbide (U2C3) at high fission doses, probably because of a preferential annihilation of extended defects in the damage process. An effect of the extended defects in irradiated single crystalline UC was a greater increased resistivity at low temperature, indicating a contribution of interstitial-type defects. The irradiation induced physical properties, on the other hand, were annealed out, within a few recovery steps in all the fuels, exhibiting some dependence on the extent of the fission damage. It was of great interest that a drastic change was observed in the magnetic property changes in some magnetic substances, such as uranium monosulfide (US) and UN. All the magnetic parameters changed with fission damage and recovered to the original values in successive annealing processes.

Fission fragment-induced hardening of uranium mononitride (UN) was examined by the Knoop indentation technique. Measurements were done at a variety of indentation loads (25 to 500 g) and for specimens with various fission doses (2.7 x 1021 to 5.3 to 1024 f/m3). The hardness showed a gradual increase with fission dose until 1023 f/m3. After that, however, for irradiations in JRRs (a lower thermal neutron flux and low irradiation temperature), the hardness attained a maximum at around 5 x 1023 f/m3 followed by a successive decrease. By contrast, in the Japan Material Test Reactor (JMTR), in which the thermal neutron flux and irradiation temperature were 7 x 1017 n/m2s and 400°C, respectively, the hardness increased monotonically up to 5.3 x 1024 f/m3. In the thermal annealing process, on the other hand, the hardness exhibited two recovery steps at 650 and 950°C, to which two types of extended defects caused by fission fragment damage could be attributed. It is concluded here that dislocation loops and vacancy clusters contributed to the fission fragment-induced hardening of UN.