Effect of Green Density on the Initial‐Stage Sintering Kinetics of UO 2s

The kinetics of the initial‐stage sintering of a UO 2 powder in H, were investigated over the range 600° to 1200°C by dilatometry. Decreasing green density below #40% of theoretical leads to progressively reduced rates of sintering. Initiation of the effect is related to a critical value of the ratio of pore size to particle size at which pores become stabilized and inhibit local shrinkage. Green density did not affect the sintering law or the apparent activation energy for sintering. A constant apparent activation energy for sintering of 86±8 kcal/mol was found, indicating that diffusion of the same species is rate‐controlling over this temperature range. Above 900°C, the rate‐controlling mechanism is consistent with grain‐boundary diffusion of the uranium ions.