Abstract
Development of a sustainable fuel cycle, which must include closing the back-end by reprocessing and/or disposing of used nuclear fuel, is a key component of the nuclear energy renaissance. Technetium-99 (β = 293.7 keV, t1/2 = 2.1 × 105 years), a byproduct of 235U and 239Pu fission, comprises a significant component of radioactive waste (~5% fission yield), and its worldwide inventory has increased steadily. Treatment and immobilization of 99Tc contained in reprocessed nuclear waste represents a major challenge. One potential approach to managing this highly mobile and long-lived radionuclide is immobilization into micro- and mesoporous crystalline solids, specifically sodalite. We synthesized and characterized the structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and the structure of a guest-guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x. Perrhenate was used as a chemical analogue for pertechnetate. Bulk analyses of each solid confirm a cubic sodalite-type structure (𝑃43𝑛, No. 218 space group) with rhenium and technetium in the 7+ oxidation state. High-resolution nanometer scale characterization measurements provide first-of-a-kind evidence that the ReO4- anions are distributed in a periodic array in the sample, nanoscale clustering is not observed, and the ReO4- anion occupies the center of the sodalite β-cage in Na8[AlSiO4]6(ReO4)2. We also demonstrate, for the first time, that the TcO4- anion can be incorporated into the sodalite structure. Lastly, thermochemistry measurements for the perrhenate sodalite were used to estimate the thermochemistry of pertechnetate sodalite based on a relationship between ionic potential and the enthalpy and Gibbs free energy of formation for previously measured oxyanion-bearing feldspathoid phases. These results not only provide the data required to evaluate the long-term chemical stability of pertechnetate sodalities, but also an approach to estimate thermodynamic constants for a range of micro- and mesoporous crystalline solids.
