Precipitation of lanthanides and actinides oxalates in presence of additives orientation of the microstructure.
Résumé
Spent nuclear fuel consists in valuable elements such as uranium and plutonium accompanied with fission products and minor actinides. An interesting option is the comanagement of actinides in a closed fuel cycle in order to fabricate fuel pellets or transmutation targets. Such a process implies the separation of actinides and their conversion into oxides. Oxalic precipitation is a way to transfer actinides from a solution to a solid phase which can be calcinated into oxides. As fuel pellets shaping is strongly influenced by the oxide morphology which, in turn, depends on the oxalate precursor morphology, this work aims to control the structure and microstructure of actinide oxalates. The influence of additives on the precipitation of Nd(III) oxalates, surrogates of An(III) oxalates, was investigated. The rod like morphology usually obtained for Nd(III) oxalates could be modified into cushions with addition of glycerol. The use of glycerol was extended to the study of the hydrazinium/Nd(III)/Th(IV)/oxalate system. Without Th spherulites and agglomerated sheets are observed. An increase of the Th content leads to additional hexagonal-based rods which proportion increases so as to remain the only morphology beyond 50 % of Th. Those morphologies are typical of three crystal phases: a hydrated Nd(III) oxalate, a mixed Th/Nd oxalate from the hexagonal series and a mixed Th/Nd oxalate from the triclinic series which was herein obtained for the first time in precipitation conditions