Structural, thermodynamic, electronic and elastic properties of Th1-xUxO2 and Th1-xPuxO2 mixed oxides

Incorporation of Th4+ and Sr2+ into Rhabdophane/Monazite by Wet Chemistry: Structure and Phase Stability

Rhabdophane is an important permeable reactive barrier to enrich radionuclides from groundwater and has been envisaged to host radionuclides in the backend of the nuclear fuel cycle. However, understanding of how An4+ and Sr2+ precipitate into rhabdophane by wet chemistry has not been resolved. In this work, Th4+ and Sr2+ incorporation in the rhabdophane/monazite structure as La1-2xSrxThxPO4·nH2O solid solutions is successfully achieved in the acid solution at 90 °C. Some specific issues such as lattice occupation of Th4+ and Sr2+, precipitation reaction kinetics, and crystal growth affected by starting stoichiometry are discussed in detail, along with investigating the chemical stability of La1-2xSrxThxPO4·nH2O precipitations and associated La1-2xSrxThxPO4 monazite. The results reveal that the excess of Sr2+ appears to be a prevailing factor with a suggested initial Sr: Th ≥ 2 to obtain the stability domain of La1-2xSrxThxPO4·nH2O (x = 0∼ 0.1). A rapid ion removal associated with a nucleation process has been observed within 8 h, and Th4+ can be removed more than 98% after 24 h in 0.01 mol/L solutions. From structural energetics based on density functional theory, the lattice occupation of Th4+ and Sr2+ is energetically favorable in nonhydrated lattice sites of [LaO8], although two-thirds of lattice sites are associated with [LaO8·H2O] hydrated sites. Intriguingly, the crystal transformation from rhabdophane to monazite associated with the transformation from [SrO8] to [SrO9] polyhedra can greatly improve the leaching stability of Sr2+.

Evidence of vacancy ordered structures in PuO2-x and AmO2-x from first-principles calculations

A combination of first-principles calculations and cluster expansion method is used to study ordering of oxygen vacancies in PuO_2-x and AmO_2-x. Vacancy ordered stable/metastable structures of composition Pu₈O₁₅ (PuO_1.875), Pu₆O₁₁ (PuO_1.833), Pu₈O₁₄ (PuO_1.75) and Am₁₀O₁₉ (AmO_1.90), Am₈O₁₅ (AmO_1.875), Am₁₀O₁₈ (AmO_1.80), Am₈O₁₃ (AmO_1.625) are identified in PuO_2-x and AmO_2-x, respectively, from cluster expansion calculations. A comparison of formation enthalpies of vacancy ordered and vacancy disordered structures shows that Am₈O₁₅ (AmO_1.875) and Am₈O₁₃ (AmO_1.625) are more stable by 52 and 55 meV per atom, respectively, compared to their disordered counterparts. Similarly, vacancy ordered Pu₈O₁₅ (PuO_1.875) and Pu₈O₁₄ (PuO_1.75) structures are more stable compared to the disordered structures by 10 and 8 meV per atom, respectively. In contrast, the disordered PuO_1.625 structure is more stable compared to the cluster expansion generated structures. The vacancy ordered structures are mechanically stable and their bulk modulus, Young's modulus, shear modulus and Poisson's ratio are reported.