Spectroscopic evaluation of UVI-cement mineral interactions: ettringite and hydrotalcite.

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U LIII-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U LIII-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.

Objective colour analysis from digital images as a nuclear forensic tool.

A digital colour image may be composed of hundreds of thousands of pixels, every pixel exhibiting a single colour. Each colour can be described as a combination of red, green and blue (RGB) components, of discrete values between 0-255. The RGB data contained within the pixels of an image could, therefore, be used to quantitatively establish the colour of nuclear material powders from digital images, particularly for use in nuclear forensics applications, where there is a need for consistent, objective analysis. This paper sets out a standard method for the photography and analysis of digital images of uranium oxide powder, for the objective quantification of colour by mean RGB values. Eight heat treated (up to 550°C) powder samples of studtite ([(UO2)(O2)(H2O)2]·2H2O) were photographed at room temperature and analysed by the RGB method. Hue, saturation and value of the coloured samples were obtained alongside mean RGB values, both of which were used to successfully determine the heating temperatures of unknown specimens of studtite.

Low-Temperature Nitridation of Fe3O4 by Reaction with NaNH2.

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH3 gas. One such method is the reaction between metal oxides and NaNH2 at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe3O4 by reaction with NaNH2 at 170-190 °C, via a pseudomorphic reaction. The more reduced Fe3O4 precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe2O3. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and 57Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-Fe2+xN, accompanied by 29-39 mol % FeO1-xNx and 8-14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH2, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO1-xNx oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH2, with either careful control of H2O concentration in the system or postsynthetic hydrolysis and crystallization.

Crystal and Electronic Structures of A2NaIO6 Periodate Double Perovskites (A = Sr, Ca, Ba): Candidate Wasteforms for I-129 Immobilization.

The synthesis, structure, and thermal stability of the periodate double perovskites A2NaIO6 (A= Ba, Sr, Ca) were investigated in the context of potential application for the immobilization of radioiodine. A combination of X-ray diffraction and neutron diffraction, Raman spectroscopy, and DFT simulations were applied to determine accurate crystal structures of these compounds and understand their relative stability. The compounds were found to exhibit rock-salt ordering of Na and I on the perovskite B-site; Ba2NaIO6 was found to adopt the Fm-3m aristotype structure, whereas Sr2NaIO6 and Ca2NaIO6 adopt the P21/n hettotype structure, characterized by cooperative octahedral tilting. DFT simulations determined the Fm-3m and P21/n structures of Ba2NaIO6 to be energetically degenerate at room temperature, whereas diffraction and spectroscopy data evidence only the presence of the Fm-3m phase at room temperature, which may imply an incipient phase transition for this compound. The periodate double perovskites were found to exhibit remarkable thermal stability, with Ba2NaIO6 only decomposing above 1050 °C in air, which is apparently the highest recorded decomposition temperature so far recorded for any iodine bearing compound. As such, these compounds offer some potential for application in the immobilization of iodine-129, from nuclear fuel reprocessing, with an iodine incorporation rate of 25-40 wt%. The synthesis of these compounds, elaborated here, is also compatible with both current conventional and future advanced processes for iodine recovery from the dissolver off-gas.