Radiation effects of (Th,U)O2 solid solution

Sintered pellets of (Th,U)O2 were irradiated by JMTR to study the fission damage effects from measurements of microhardness and lattice parameter. Specimens were the solid solution of (Th0.6 U0.4)O2 and (Th0.8 U0.2)O2 with density of 94∼97%TD. As a mechanical test, knoop hardness was measured. From the load dependence an intermediate plateau region was found where hardness values were almost independent of the load. Specimens were irradiated with thermal neutron flux of 1.0 x 1014 n/cm2.s and fission dose ranged from 1.3 x 1016 to 2.8 x 1018 fiss./cm3. As a result of the post-irradiation test, three stages of the dose dependence were found. Microhardness and lattice parameter gradually increased up to 1 x 1017 fiss./cm3 and took a maximum value at 8 x 1017 fiss./cm3, then decreased at a higher dose. Comparison with the radiation behavior of UO2 suggests that interstitial atoms and clusters formed by the fission damage cause the increase of microhardness and lattice parameter. At higher doses, however, interstitial atoms are presumed to recombine with excess vacancies, hence the third stages of a recovery occurs. (author)