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Jianwei Wang1

1, Louisiana State University, Baton Rouge, Louisiana, United States

Apatite-structured materials have been considered for immobilization of a number of fission products from reprocessing nuclear fuel because of their chemical durability as well as compositional and structural flexibility. It is hypothesized that the effect of beta decay on the stability can be mitigated by introducing appropriate electron acceptor at the neighboring sites in the structure. Decay series 137Cs 137Ba and 90Sr → 90Y → 90Zr were investigated using a spin-polarized DFT approach to test the hypothesis. Apatites with compositions of Ca10(PO4)6F2 and Ca4Y6(SiO4)6F2 were selected as model systems for radionuclides Cs and Sr incorporation respectively. Ferric iron was introduced in the structure as an electron acceptor. Calculated electron density of states suggests that the extra electron is localized at the ferric iron, which changes its oxidation state and becomes ferrous iron. The calcualtions show that there are minor changes in the crystal and defect structure of CsFeCa8(PO4)6F2 with Cs+ and Fe3+ substitutions undergoing Cs → Ba transmutation, and of Ca3SrY4Fe2(SiO4)6F2 with Sr2+ and Fe3+ substitutions undergoing Sr → Y → Zr transmutations. The results on calculated cohesive energy suggest that transmutations of Cs+ → Ba2+ and Sr2+ → Y3+ → Zr4+ in both apatite compositions are energetically favorable, which are consistent with the minor structure distortions. Stability improvement by incorporating ferric iron is significant with respect without variable valence ions. The results confirm the structural and compositional adaptability of apatites upon beta transmutations. The study suggests that apatite-structured materials could be promising nuclear waste forms to mitigate the beta decay induced instability, by incorporating variable valence cations such as ferric iron in the structure. The study demonstrates a methodology which evaluates the structural stability of waste forms incorporating fission products undergoing beta decay.

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