Synthesis and characterization of uranium-bearing britholites

Unlike thorium which incorporation is easy and quantitative in the britholite structure, that of tetravalent uranium appears rather difficult even after heating at high temperature. Indeed, only about 3.5 wt% of uranium (instead of 10.3 wt% expected) are incorporated in the britholite structure when using a manual grinding of the initial mixture while it is increased to 5.9 wt% when performing mechanical grinding/heating cycles. The optimized grinding and heating conditions can be fixed to 15 min at 30 Hz and to 1400 oC for 6 h, respectively. All the samples prepared at 1400 oC are found to be composed by (Nd, U)-britholite and calcium uranate CaU2O5+y which formation results from that of CaUO4 above 800 oC consequently to the direct reaction between UO2 and CaO. The incorporation of tetravalent uranium in (Nd, U)-britholites begins simultaneously to the transformation of CaUO4 into CaU2O5+y above 1100 oC. Due to these redox reactions, the incorporation of uranium remains partial even though it is significantly increased at 1400 oC and mainly proceeds through diffusion phenomena. Two main methods can be used to improve significantly the incorporation of uranium in (Nd, U)-britholites. The first one deals with the compaction of the powdered initial mixture prior to perform the heating treatment at 1400 oC. The second one is based on the simultaneous incorporation of tetravalent thorium and uranium, leading to the formation of (Nd, Th, U)-britholites. Like for uranium, such methods could be of significant interest in the field of the incorporation of other tetravalent actinides, e.g. neptunium or plutonium, which could be also stabilized in several oxidation states