Phasengleichgewichte in den systemen UO 2UO 3NdO 1,5 und NpO 2+xNdO 1,5

Abstract

In the UO 2UO 3NdO 1.5 part of the UNdO ternary system, the following four single-phase regions are observed at 1250°C: • β-U 3O 8 which does not show any solubility for A-NdO 1·5; • a fluorite phase (U, Nd)O 2±x which extends over the largest part of the system. In this phase the valency of uranium varies from +4 to +6 with the limiting compositions MeO 1·6 and MeO 2·25; • a rhombohedral phase wtth the limiting compositions UO 3 · 6NdO 1·5 and UO 2 · 6NdO 1·5, respectively. This phase shows no variation in the U : Nd-ratio; • A-NdO 1·5 which does not take up uranium oxides into solid solution. The single phase regions are separated by the corresponding two-phase-regions. The phase width of the fluorite phase increases with increasing temperature. At 1550°C and p(O 2) = 1 atm it reaches from 13 to 81 mole-% NdO 1·5 as many be seen in the phase diagram of the pseudobinary system UO 2+x-NdO 1·5. In the pseudobinary system NPO 2+x-NdO 1·5 (p(O 2) = 1 atm) four single-phase regions are observed between 1100°C and 1550°C: two fluorite phases, a rhombohedral phase having NpO 3 · 6NdO 1·5 as the oxygen-rich composition, and A-NdO 1·5. The thermal stability of the stoichiometric ternary oxides (Np 0·5, Nd 0·5)O 2·00 with pentavalent Np and of NpO 3 · 6NdO 1·5 with hexavalent Np is less than that of the isostructural uranium compounds.