Densification of uranium dioxide at low burn-up

Uranium dioxide has been irradiated in the Harwell Materials Test Reactors at temperatures of between 350 and 15500C and ratings between 6 and 47 W/g(U). Burn-ups ranged from 2.5 to 2600 MWd/t(U). Pore size distributions were measured before and after irradiation using quantitative optical and scanning electron microscopy. It was shown that between 1000 and 15500C the sub-micron porosity was almost completely removed after only a few hours irradiation. It was not possible to determine whether or not the sintering rate was temperature-sensitive above 10000C due to the high sintering rate at these temperatures. Below 10000C pore sintering was much slower and showed a slight temperature dependence. (orig.)