Elaboration and characterization of britholites loaded with tetravalent actinides

摘要

Due to a natural occurrence, a good resistance to radiation damage and a low solubility in neutral and basic conditions, Nd-britholite of formula Ca_9Nd(PO_4)_5(SiO_4)F_2 was already considered as a potential host matrix for the specific immobilization of trivalent actinides. The incorporation of tetravalent actinides like Th, U or Ce in the structure was examined through the elaboration of Ca_9Nd_(1-x)An~(Ⅳ) _x(PO_4)_(5-x)(SiO_4)_(1+x)F_2 samples. This study was the early beginning of the incorporation of ~(239)Pu and/or ~(238)Pu in order to evaluate the effects of a-decay on the britholite structure. The preparation of the samples was realized through dry chemistry methods using mechanical grinding in order to optimize the conditions of synthesis. The results showed that the incorporation of thorium in britholite structure was successful up to 20 wt.% leading to the formation of solid solutions. For U(Ⅳ), the incorporation was incomplete: only 5-8 wt.% of uranium was incorporated instead of 10 wt.% expected. The secondary phase, which was uranium enriched, was identified as the calcium uranate CaU_2O_(5+y). The incorporation of Pu(Ⅳ) was studied using Ce(Ⅳ) as a surrogate. The results were quite good: major part of cerium was introduced in the structure but was partly reduced in Ce(Ⅲ) during the heating treatment.