Residual impurities affect the stability of hydrogen atoms in irradiated gibbsite: Study

During Cold War-era plutonium production at what is now the Department of Energy’s Hanford Site in Washington State, aluminum was used extensively as fuel cladding material. The waste products generated by fuel processing are currently stored in underground holding tanks, where aluminum is present as boehmite and gibbsite.

These irradiated fuel wastes create a complex, extreme environment, and a recent study examined how trace species, in this case residuals from synthetic precursors, affect the presence of radicals in gibbsite. Researchers also investigated irradiated boehmite, which is known for its ability to trap and stabilize hydrogen atoms. The paper is published in the journal Physical Chemistry Chemical Physics.

Studying how aluminum (oxy)hydroxides could function as radiolytic substrates in hydrogen production is a key research interest for energy sector security and decarbonization. It is also vital for effective management and storage of legacy radioactive wastes.

This study demonstrates that hydrogen atoms generated in gibbsite nanoplates by gamma radiation are affected by residual impurities. The concentration of these hydrogen atoms, trapped in the interstitial layers of gibbsite, is lower and decays faster than in boehmite. Providing experimental evidence for the behavioral differences of hydrogen atoms in gibbsite versus boehmite can help clarify the highly complex nature of radioactive waste.

Generating and stabilizing gamma-radiation-induced hydrogen atoms in gibbsite nanoplates is directly affected by residual ions left over from different precursors used during synthesis, including aluminum nitrate (Al(NO₃)₃) and aluminum chloride (AlCl₃). Both gibbsite and boehmite are present in legacy waste mixtures at the Hanford Site, so this study also investigated the stability of hydrogen in boehmite, which can both trap and stabilize hydrogen atoms within its structure.

Diffuse reflectance infrared Fourier transform spectroscopy was used to analyze hydroxyl groups in gibbsite synthesized from nitrate and chloride precursors, as well as nitrate in gibbsite from Al(NO₃)₃. X-ray photoelectron spectroscopy demonstrated the presence of chloride residuals in AlCl₃-based gibbsite. While the surface concentrations of these residual ions (~0.4 atom percent of nitrate and ~0.6 atom percent of chloride) were too low to affect the bulk properties of gibbsite, they were significant enough to affect how the material responds to radiolysis.

Using a variety of radiation doses, the study investigated the presence of hydrogen atoms in gibbsite through electron paramagnetic resonance spectroscopy. All doses produced hydrogen atoms in the Cl-gibbsite, while the NO₃-gibbsite did not contain any detectable hydrogen atoms. This indicates that although hydrogen atoms are likely formed in both cases, they are only trapped when nitrate is not present.

Additionally, boehmite irradiated by the same doses contained far greater concentrations of hydrogen atoms with slower limiting decay rates (331 s^-1 for gibbsite compared to 0.11 s^-1 in boehmite). These results indicate boehmite’s superior ability to trap and stabilize hydrogen atoms within its crystalline structure. Overall, the findings illustrated here help provide experimental evidence that increases our understanding of the extreme and complicated environments encountered in irradiated fuel wastes.