Theodore Besmann

It has been demonstrated that monazite-type materials are excellent candidates for nuclear waste forms, and hence, their facile synthesis is of great importance for the needed sequestration of existing nuclear waste. The synthesis of monazite, LaPO ₄ , requires inconveniently high temperatures near 1000°C and generally involves the conversion of the presynthesized rhabdophane, LaPO ₄ •nH ₂ O, to the LaPO ₄ monazite phase. During this structure transformation, the rhabdophane converts irreversibly to the thermodynamically stable monoclinic monazite structure. A low-temperature (185° to 260°C) mild hydrothermal acid-promoted synthesis of monazite is described that can both transform presynthesized rhabdophane or assemble reagents to the monoclinic monazite structure. The pH dependence of this reaction is detailed, and its applicability to the Ln PO ₄ ( Ln  = La, Ce, Pr, Nd, Sm-Gd), Ca _0.5 Th _0.5 PO ₄ , and Sr _0.5 Th _0.5 PO ₄ systems is discussed. The crystal growth of Ca _0.5 Th _0.5 PO ₄ and Sr _0.5 Th _0.5 PO ₄ is described, and their crystal structures were reported. In situ x-ray diffraction studies, performed as a function of temperature, provide insight into the structure transformation process.

It has been demonstrated that monazite-type materials are excellent candidates for nuclear waste forms, and hence, their facile synthesis is of great importance for the needed sequestration of existing nuclear waste. The synthesis of monazite, LaPO ₄ , requires inconveniently high temperatures near 1000°C and generally involves the conversion of the presynthesized rhabdophane, LaPO ₄ •nH ₂ O, to the LaPO ₄ monazite phase. During this structure transformation, the rhabdophane converts irreversibly to the thermodynamically stable monoclinic monazite structure. A low-temperature (185° to 260°C) mild hydrothermal acid-promoted synthesis of monazite is described that can both transform presynthesized rhabdophane or assemble reagents to the monoclinic monazite structure. The pH dependence of this reaction is detailed, and its applicability to the Ln PO ₄ ( Ln  = La, Ce, Pr, Nd, Sm-Gd), Ca _0.5 Th _0.5 PO ₄ , and Sr _0.5 Th _0.5 PO ₄ systems is discussed. The crystal growth of Ca _0.5 Th _0.5 PO ₄ and Sr _0.5 Th _0.5 PO ₄ is described, and their crystal structures were reported. In situ x-ray diffraction studies, performed as a function of temperature, provide insight into the structure transformation process.

The extensively evaluated and consistent thermodynamic database, the Molten Salt Thermal Properties Database—Thermochemical (MSTDB-TC), was used along with additional thermodynamic values from other sources as examples of ways to examine molten salt reactor (MSR) fuel behavior. Relative stability with respect to halide potential and temperature for likely fuel and fission product components were mapped in Ellingham diagrams for the chloride and fluoride systems. The Ellingham diagrams provide a rich, visual means for identifying halide-forming components in proposed fuel/solvent salt systems. Thermochemical models and values from MSTDB-TC and ancillary sources were used in global equilibrium calculations to provide compositions for a close analysis of the behavior of a possible Molten Chloride Salt Fast Reactor and a Molten Salt Reactor Experiment-type system at high burnup (100 GWd/t). The results illustrated the oxidative nature of burnup in MSRs and provided information about redox behavior and possible control.

A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation.

A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation.

Molten salt reactors (MSRs) utilize salts as coolant or as the fuel and coolant together with fissile isotopes dissolved in the salt. It is necessary to therefore understand the behavior of the salts to effectively design, operate, and regulate such reactors, and thus there is a need for thermodynamic models for the salt systems. Molten salts, however, are difficult to represent as they exhibit short-range order that is dependent on both composition and temperature. A widely useful approach is the modified quasichemical model in the quadruplet approximation that provides for consideration of first- and second-nearest-neighbor coordination and interactions. Its use in the CALPHAD approach to system modeling requires fitting parameters using standard thermodynamic data such as phase equilibria, heat capacity, and others. A shortcoming of the model is its inability to directly vary coordination numbers with composition or temperature. Another issue is the difficulty in fitting model parameters using regression methods without already having very good initial values. The proposed paper will discuss these issues and note some practical methods for the effective generation of useful models.

Single crystals of CsGa₇O₁₁, RbGa₇O₁₁, and RbGa₄In₅O₁₄ were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa₇O₁₁ and RbGa₇O₁₁ adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO₆ octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa₄In₅O₁₄ is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa₇O₁₁ and RbGa₇O₁₁. CsGa₇O₁₁ and RbGa₄In₅O₁₄ were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa₇O₁₁ decomposes to GaO(OH) xH₂O when heated in warm aqueous solutions. CsGa₇O₁₁ undergoes ion exchange in both an aqueous solution of RbCl and a RbNO₃ melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO₃ melt.

The Linde Type A (LTA) zeolite capability to exchange alkaline earth ions is analyzed using DFT calculations, considering the systems to be in water and vacuum, investigating the LTA's potential as a material for radioactive decontamination processes.

Crystals of three new uranium(v) containing oxyfluorides were grown out of an alkali fluoride flux and adopt a perovskite-type structure and are examined by SXRD, PXRD, XANES, XPS, EDS, magnetic susceptibility measurements, DFT calculations, and UV-vis spectroscopy.