Atomistic Modeling of the Solid-State Chemistry of Actinide Materials.

Abstract

Materials that incorporate actinides are critical to the nuclear fuel cycle, either as nuclear fuels or nuclear waste forms. In this thesis, I examine four materials: i) ThO2-UO2 solid solutions, ii) binary ThO2-CeO2-ZrO2 solid solutions, iii) Np-doped studtite, iv) Np-doped boltwoodite. Computational methods, particularly density functional theory (DFT) calculations and Monte-Carlo (MC) simulations, are used to determine the energetics and structures of these actinide-bearing materials. The solid-solution behavior of nuclear fuels and nuclear waste forms indicate the thermodynamic stability of the material, which is important for understanding the in-reactor fuel properties and long-term stability of used fuel.

The ThxU1-xO2 and ThxCe1-xO2 binaries are almost completely miscible; however, ΔGmix reveals a small tendency for the systems to exsolve (e.g., ΔEexsoln(ThxU1-xO2) = 0.13 kJ/(mol cations) at 750 K). Kinetic hindrances (e.g., interfacial energy) may inhibit exsolution, especially at the low temperatures necessary to stabilize the nanoscale exsolution lamellae observed in the ThxU1-xO2 and CexZr1-xO2 binaries. Miscibility in the Zr-bearing binaries is limited. At 1400 ˚C, only 3.6 and 0.09 mol% ZrO2 is miscible in CeO2 and ThO2, respectively.

The incorporation of minor amounts of Np5+,6+ into uranium alteration phases, e.g., studtite [UO2O2(H2O)4] or boltwoodite [K(UO2)(SiO3OH)(H2O)1.5] , may limit the mobility of aqueous neptunyl complexes released from oxidized nuclear fuels. Np6+-incorporation into studtite requires less energy than Np5+-incorporation (e.g., with source/sink = Np2O5/UO3 ΔEincorp(Np6+) = 0.42 eV and ΔEincorp(Np5+) = 1.12 eV). In addition, Np6+ is completely miscible in studtite at room temperature with respect to a hypothetical Np6+-studtite. Electronic structure calculations provide insight into Np-bonding in studtite. The Np 5f orbitals are within the band gap of studtite, resulting in the narrowing of the band gap from 2.29 eV for studtite to 1.09 eV for Np-incorporated studtite.

Three charge-balancing mechanisms for the substitution of Np5+ for U6+ were compared: i) addition of H+ [ΔEincorp(bolt) = 0.79 eV; ΔEincorp(stud) = 1.12 eV], ii) interlayer coupled substitution [ΔEincorp(bolt) = 1.40 eV], iii) intra-layer coupled-substitution [ΔEincorp(bolt) = 0.86 eV]. Solid-solution calculations of the intra-layer coupled-substitution mechanism, where Np5+ and P5+ substitute for U6+ and Si4+, predict an incorporation limit of 585 ppm at 300 °C.