Jamie L. Weaver

Chemical Trends in Solid Alkali Pertechnetates

Insight into the solid-state chemistry of pure technetium-99 (99Tc) oxides is required in the development of a robust immobilization and disposal system for nuclear waste stemming from the radiopharmaceutical industry, from the production of nuclear weapons, and from spent nuclear fuel. However, because of its radiotoxicity and the subsequent requirement of special facilities and handling procedures for research, only a few studies have been completed, many of which are over 20 years old. In this study, we report the synthesis of pure alkali pertechnetates (sodium, potassium, rubidium, and cesium) and analysis of these compounds by Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), solid-state nuclear magnetic resonance (static and magic angle spinning), and neutron diffraction. The structures and spectral signatures of these compounds will aid in refining the understanding of 99Tc incorporation into and release from nuclear waste glasses. NaTcO4 shows aspects of the relatively higher electronegativity of the Na atom, resulting in large distortions of the pertechnetate tetrahedron and deshielding of the 99Tc nucleus relative to the aqueous TcO4-. At the other extreme, the large Cs and Rb atoms interact only weakly with the pertechnetate, have closer to perfect tetrahedral symmetry at the Tc atom, and have very similar vibrational spectra, even though the crystal structure of CsTcO4 is orthorhombic while that of RbTcO4 is tetragonal. Further trends are observed in the cell volume and quadrupolar coupling constant.

Vitrified hillforts as anthropogenic analogues for nuclear waste glasses : project planning and initiation

Nuclear waste must be deposited in such a manner that it does not cause significant impact on theenvironment or human health. In some cases, the integrity of the repositories will need to sustain fortens to hundreds of thousands of years. In order to ensure such containment, nuclear waste is frequentlyconverted into a very durable glass. It is fundamentally difficult, however, to assure the validity ofsuch containment based on short-term tests alone. To date, some anthropogenic and natural volcanicglasses have been investigated for this purpose. However, glasses produced by ancient cultures for thepurpose of joining rocks in stonewalls have not yet been utilised in spite of the fact that they might offersignificant insight into the long-term durability of glasses in natural environments. Therefore, a projectis being initiated with the scope of obtaining samples and characterising their environment, as well asto investigate them using a suite of advanced materials characterisation techniques. It will be analysedhow the hillfort glasses may have been prepared, and to what extent they have altered under in-situconditions. The ultimate goals are to obtain a better understanding of the alteration behaviour of nuclearwaste glasses and its compositional dependence, and thus to improve and validate models for nuclearwaste glass corrosion. The paper deals with project planning and initiation, and also presents some earlyfindings on fusion of amphibolite and on the process for joining the granite stones in the hillfort walls.Keywords: ageing, amphibolite, analogue, anthropogenic, Broborg, glass, hillfort, hill-fort, leaching,long-lived, nuclear, rampart, waste.

Chemometric analyses of XANES data collected on 99 Tc bearing silicate glasses

A challenge to the incorporation of 99Tc into a glass matrix is that 99Tc is volatile at vitrification temperatures. Understanding how this volatilization occurs requires knowledge of the multiple chemistries which Tc may take during vitrification. This paper presents an overview of how the localized chemistry of 99Tc has been determined in a series of 99Tc-bearing glasses by chemometric analyses of X-ray absorption near edge spectra (XANES). Linear combination fitting and principal component analysis of the glasses’ XANES spectra suggested that the glasses contained 3–4 chemically distinct Tc environments. The identity of the detected chemistries were pertechnetates, (Tc(VII) as isolated oxyanions, or locally coordinated by Na or K), or isolated Tc(IV) species. The linear combination fitting distribution of local Tc sites agrees with reanalyzed Raman spectra, suggesting that as targeted KTcO4 incorporation increases, a significant ion exchange takes place, and speciation in the glass changes to favor Tc(VII) formation, specifically NaTcO4. Based on the statistical suggestion that not all Tc environments are accounted by the available standards, a new mechanism for the behavior of Tc during vitrification is proposed.