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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.