2, University of South Carolina, Columbia, South Carolina, United States
Hierarchical waste form materials are a novel approach to nuclear waste sequestration. Their inherent ability to contain various structural motifs within a larger framework or structure make them an interesting candidate to hold various transuranic or fission product elements all within a single entity. Salt-inclusion materials, SIMs, are a class of hierarchical material that consist of a covalent oxide framework containing voids filled by ionic salts potentially of radionuclides of important fission products. The framework allows for structural variability forming uranyl based silicate, germanate, vanadate or borate networks, as well as europium and gadolinium silicates. To widen the class of materials, ion exchange of existing SIM’s can be performed to include targeted isotopic compositions important in nuclear waste. It is therefore of interest to understand the role of the pore sizes created by the salt inclusions and their involvement in ion exchange mechanisms. Moreover, the preparation of these framework materials can take atomic size or charge into consideration during synthesis, though little is known about their thermodynamic stability, including formation enthalpies or Gibbs energies. To date there is no published literature on the thermodynamic properties of SIMS. This work investigates the thermodynamic stability using estimation/correlation techniques such as volume based thermodynamics (VBT) to determine values for the SIM’s, including their separate framework and salt constituents. We use structural information from crystallographic data and build a thermodynamic cycle to calculate entropies, enthalpies and Gibbs energies of formation. Similarly, ion exchange energetics can be predicted by applying hydration enthalpies to VBT. These calculations allow for the determination of relative material stability and the tendency for ion exchange. The results can guide experimental efforts and can be coupled with calorimetric data when available. We aim to provide a library of Gibbs energy values for a set of systems that encompass a multitude of different frameworks and potential salt inclusions to effectively inform the sequestration of radionuclides for waste management. We also investigate the dependence of pore sizes and channel dimensionality within the SIM’s and relate these properties to their framework configuration and propensity for ion exchange.
This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.