Materials in Nuclear Energy Systems (MiNES) 2021: Nuclear Fuel Cycles- Session III
Program Organizers: Todd Allen, University of Michigan; Clarissa Yablinsky, Los Alamos National Laboratory; Anne Campbell, Oak Ridge National Laboratory

Tuesday 1:30 PM
November 9, 2021
Room: Conference Center A
Location: Omni William Penn Hotel

Session Chair: Jenifer Shafer, Colorado School of Mines

1:30 PM  Invited
Beta Transmutations in Apatite with Ferric Iron as an Electron Acceptor – Implication for Nuclear Waste form Development: Jianwei Wang¹; Jie Lian²; ¹Louisiana State University; ²Rensselaer Polytechnic Institute
    Apatite-structured materials are considered for immobilization of fission products from reprocessing nuclear fuel because of their chemical durability and compositional and structural flexibility. It was hypothesized that the effect of beta decay on stability can be mitigated by introducing an 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 for radionuclides Cs and Sr incorporation respectively. Ferric iron was introduced in the structure as an electron acceptor. Electron density of states, crystal and defect structure, and energies 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.

2:10 PM
The Effect of Phase Structure on the Aqueous Corrosion of Yttrium Disilicate: Keith Bryce¹; Kun Yang¹; Jie Lian¹; ¹Rensselaer Polytechnic Institute
    In deep geological repositories, rare earth elements may react with silicates found in the surrounding engineered barrier systems to form rare earth disilicates (RE₂Si₂O₇). This study investigates the chemical durability of Y₂Si₂O₇ polymorphs with different phases (amorphous, alpha, beta, and gamma) in dense sintered pellets by semi-dynamic leaching tests. The gamma phase showed the lowest leaching rates for both Y and Si, followed by beta, alpha and the amorphous pellet, suggesting a strong impact of phases on chemical durability of disilicates. Incongruent leaching was observed for all phases with a preferential release of Y from the matrix. Microstructural analysis of the post leaching sample revealed variations in the level of corrosion of the surface grains for the beta and gamma pellets.

2:30 PM
Predicting Phase Stability of Potential Actinide-bearing Hollandite Waste Forms Using First Principles Calculations: Amir Mofrad¹; Matthew Christian¹; Juliano Shcorne-Pinto¹; Theodore Besmann¹; ¹University of South Carolina
    Fission product, cesium-137 with a half-life of approximately 30 years, has been well known as problematic for waste forms. Titanate-based hollandites have long been considered effective phases for sequestering cesium, having the formula A_1.33(Ti⁴⁺,B)₈O₁₆, where A is an alkali/alkaline earth element and B is a metal. Due to the beta decay of ¹³⁷Cs, hollandites must be energetically stable not only to immobilize cesium, but also to remain stable when it is replaced by its decay product, barium. In this study, we have investigated the structural stability of actinide (U, Np, and Pu)-bearing hollandites, that is, when actinides co-exist at B-sites with titanium, to allow their consideration as both a cesium/barium and actinide waste form. The predicted formation enthalpies from density functional theory have provided insight into the stability of these hollandites as a function of composition which are being used to target synthesis efforts.

2:50 PM Break