Synthesis and characterization of thorium-bearing britholites

In the field of the immobilization of tri- and tetravalent minor actinides, apatites and especially britholites were already proposed as good candidates. In order to simulate tetravalent minor actinides, the incorporation of thorium, through dry chemical routes, was studied in britholite samples of general formula Ca9Nd1−xThx(PO4)5−x(SiO4)1+xF2. The study showed that the incorporation of thorium was effective whatever the thorium reagent used or the grinding conditions considered. Nevertheless, it appeared necessary to use mechanical grinding (30 Hz, 15 min) before heating treatment (T = 1400 °C, 6 h) to improve the reactivity of powders and the sample homogeneity. In these conditions, the incorporation of thorium in the britholite structure occurred above 1100 °C. The heating treatment at 1400 °C led to single phase and homogeneous compounds. This work also underlined the necessity to prefer the coupled substitution (Nd3+,PO43-)⇔(Th4+,SiO44-) instead of (Nd3+, F−) ⇔ (Th4+, O2−) in order to prepare pure and single phase samples in all the range of composition examined.