Performance assessment of nuclear waste disposal options requires implementation of effective buffer materials. Buffer materials must meet longevity requirements and scavenge challenging radioisotopes, e.g., iodine-129 (I-129) and technetium-99 (Tc-99), both long-lived, highly mobile, anionic species in oxic environments. In many proposed nuclear waste repositories, heat generating radioactive waste will be surrounded by clay buffer material, which swells to fill the gap between the waste package and the host geology, restricting radionuclide transport to diffusive processes for several hundred years. The clay can be functionalized to further the limit release of Tc-99, as pertechnetate (TcO 4 − ), and I-129, as iodate (IO 3 − ) and iodide (I − ). Here, we review clay buffer functionalization and the potential to enhance sequestration of anionic radionuclides that have an increased risk of leaching out of a disposal facility. Functionalization with quaternary amines, nanosilver, bismuth-based materials, silane, and aluminum pillars have been demonstrated to improve the removal of targeted contaminants from solution. This review defines key research questions that remain: (i) can functionalized clay immobilize TcO 4 − , IO 3 − , and I − to the same extent as the reactive components themselves? (ii) Does functionalization affect the ability of the clay barrier to perform its safety functions of protecting the waste canister and limiting radionuclide transport? In addition to radionuclide retention, a multimodal analytical approach is required to assess functionalized clay performance, and recommended techniques are reviewed here. The key parameters that can influence radionuclide sequestration by functionalized clay are also evaluated. Theoretical approaches, including both density functional theory calculations and molecular dynamics simulations, can inform the energetics and kinetics of radionuclide interactions with functionalized clays. This combined experimental and theoretical approach can link atomistic and microscopic processes to predict the macroscopic physical properties of the functionalized clays.

Multicomponent doping of ceria with four cations is used as a preliminary investigation into the ionic conductivity of high-entropy-doped ceria systems. Different compositions of Ce1-x(Ndx/4Prx/4Smx/4Gdx/4)O2-δ (x = 0.05, 0.10, 0.15, and 0.20) are synthesized using the oxalate co-precipitation method yielding single-phase oxalate precursors. X-ray diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy are used to characterize the precipitated oxalates. Simultaneous thermal gravimetric analysis and differential scanning calorimetry reveal a two-step decomposition of the oxalates into the doped oxide. The ionic conductivity of the samples is measured from 250 °C to 600 °C using electrochemical impedance spectroscopy. All samples exhibit similar grain conductivity values at 600 °C, comparable to singly doped samples. However, an increase in total conductivity is observed with an increase in doping concentration up to 15% followed by a decrease beyond this concentration. These findings suggest that multicomponent doping may not significantly enhance the grain conductivity of doped ceria beyond conventional single and co-doped compositions but can modulate the grain boundary conductivity and thus the total conductivity of ceria ceramics.

Synthesis of advanced ceramics requires a high degree of control over the particle size and stoichiometry of the material. When choosing a synthesis method for complex oxides it is important to begin with the correct precursors and solvents to achieve high purity nanoparticles. Here, we detail the selection process for precursors and solvents for liquid-phase precipitation synthesis. Data for metal nitrate, chloride, acetate, and oxalate precursors has been compiled to assist future synthesis. The role of hydration within the precursors is discussed as it affects the final stoichiometry of the material. Melting temperatures are also compiled for these compounds to assist in material selection. The solubility of the precursors in different solvents is examined to determine the correct solvent during synthesis. As an example, using the methodology presented here, two different materials are synthesized based on commonly available precursors. A catalyst based on a quaternary perovskite and an advanced ionic conductor based on a high entropy fluorite oxide are synthesized using precipitation methods and their characterization is detailed.

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