Chemical state, phases and vapor pressures of fission-produced noble metals in oxide fuel

Abstract

The phase diagram of the ternary Mo-Ru-Pd system at 1723 K in vacuum (10 −4 to 10 −5 Pa) was investigated by means of optical microscopy, X-ray diffractometry and electron probe microanalysis. It was found that the ϵ-phase region decreases with decreasing ruthenium content and finally coexists with α and β phases and that the σ-phase region is very small. The phase diagram calculated from the regular solution model proposed by Kaufman and Bernstein was compared with the phase diagram obtained experimentally. The vapor pressure over pure palladium metal and that over the ϵ-phase of Mo-Ru-Pd alloys were measured by mass spectrometry in the temperature range of 1567–1775 K. The chemical activity of palladium in Mo 0.450Ru 0.450Pd 0.100 and Mo 0.225Ru 0.675Pd 0.100 determined from the vapor pressure of Pd(g) was compared with that calculated on the basis of the regular solution model. The oxygen partial pressure for the oxidation of molybdenum, ruthenium and palladium in Mo-Ru-Pd alloy was calculated. The oxygen partial pressure thus calculated for Mo-Ru-Pd alloys was compared with that for the oxidation of UM 3 alloys (M = Ru, Rh, Pd). It was found that UPd 3 and URh 3 are not oxidized but URu 3 is oxidized during the irradiation of UO 2 fuel. The oxidation experiment on noble metal alloys carried out under controlled oxygen potential showed that the molybdenum in the alloy is oxidized selectively at high oxygen potential. The prior ϵ-phase changes into two-phase mixtures of α- and ϵ-phases after oxidation. The oxygen potential necessary to oxidize molybdenum in the alloys was estimated from the molybdenum activity calculated by the regular solution model.