AbstractWide variations in sintering behaviour have been observed for uranium dioxide powder compacts prepared from ammonium diuranate (ADU) powder precursor and calcined at various temperatures. In general, it is observed using X-ray diffraction that powders calcined below 800°C are non-crystalline and they are also highly sinterable (active), whereas powders calcined above 800°C have poor sinterability and are designated inactive. A model has been proposed for measurement of the activation energy values for sintering of compacts of urania powder based upon a modification derived from Frenkel's sintering model as applicable to non-crystalline powders, from which the shrinkage rate at zero time (t→0) can be obtained. Moreover, the associated effect on sintering due to the non-crystalline to crystalline transition with progressive thermal treatment is eliminated using this model. The activation energy values for sintering of the non-crystalline powders at various calcination temperatures are in good agreement and, below 800°C, values of ~167·5 kJ mol−1are obtained. For powders calcined above 800#x00B0;C, values of ~314·1 kJ mol−1are obtained.MST/812