High temperature fast-neutron irradiation of pyrolytic-carbon-coated ThO 2 microspheres

Abstract

We determined the effects of fast-neutron damage alone on four types of pyrolytic-carbon coatings with spherical geometry. The coatings, which ranged in density from 1.75 to 2.7 g/cm 3 and had Bacon anisotropy factors from 1.0 to 1.4, were irradiated in the Dounreay Fast Reactor to 4.5 × 10 21 neutrons/cm 2 (> 0.18 MeV) at about 1200°C. A porous buffer layer prevented interaction between the coating and the sol-gel ThO 2 kernels. Essentially no fissioning occurred in the ThO 2 in the fast-neutron spectrum. We found that only the coatings with the highest anisotropy factor failed. Coatings with an initial density of 1.8 g/cm 3 or less densified while coatings with initial density of 1.9 g/cm 3 or greater swelled. Metallographie examination revealed that the preferred orientation increased in the coatings that densified. These changes must be considered in mathematical models describing coated-particle behavior.