Comparative structural investigations of nuclear waste glass alteration layers and sol-gel synthesized aerogels

Joelle T Reiser,Daniel K Schreiber,Joseph V Ryan,Matthew J Olszta,Marc H Weber,Jan Ilavsky,Nathalie A Wall

doi:10.1038/s41529-020-0109-y

Joelle T Reiser, Daniel K Schreiber + Show 5 more

Open Access

https://doi.org/10.1038/s41529-020-0109-y

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Abstract

While various glass alteration layer formation mechanisms have been debated in recent years, the glass alteration community generally agrees that more information on physical properties of the alteration layers is needed to further the understanding of their impacts on overall glass alteration. In this work, pore volumes and solid structures of glass (International Simple Glass, ISG) alteration layers formed in solutions of various pH conditions in initially dilute conditions at 90 °C are evaluated with positron annihilation spectroscopy, small-angle X-ray scattering, and scanning transmission electron microscopy. Pore volumes of alteration layers formed at pH 9 were found to be at their lowest near the surfaces of the alteration layers. Solid structures of alteration layers are compared with those of synthetic aerogels of comparable compositions produced under various pH conditions. Alteration layers formed at pH 11 on ISG were shown to contain large structures (>10 nm) similar to synthetic aerogels created under neutral and basic conditions whereas alteration layers formed at pH 9 did not. Available dissolved silica species defined by silica solubility were proposed to have the greatest impact on alteration layer structure.

Highlights

While commercial nuclear reactors provide reliable energy, resulting wastes require safe disposal to ensure the safety of humans and the environment
Since the alteration layer is generated from completely free silica species in solution, the proposed model is similar to what is observed in other silica–water systems.[11,18,19]
The alteration experiment samples are identified by a three character code (XXX), where the first character represents the target pH of the reactant solution shows schematic descriptions of sample cross sections for 7I7, 9I7, and 11I7 scaled as a function of implantation energies calculated using thicknesses measured on scanning electron microscopy (SEM)

Summary

INTRODUCTION

While commercial nuclear reactors provide reliable energy, resulting wastes require safe disposal to ensure the safety of humans and the environment. In silica–water systems, several variables impact the polymerization behavior, primarily pH and the presence of background salts, which influence pore and particle sizes of the final gel.[19,20,23,24] Multiple polymerization processes, Ostwald ripening and aggregation, contribute to the formation of large silica networks where the predominance of various processes varies with pH. The hydrolysis and condensation reactions are similar to from ICP-OES analysis and measured thicknesses from SEM those proposed for alteration layer formation for both inter-diffusion and dissolution/precipitation models, the starting silica species differ, whereas the dissolution/precipitation model presents initial micrographs of the altered portions of 3I7, 5I7, 7I7, 9I7, and 11I7.39 (See Supplementary Table 3 for EThB and measured thickness values.) The trend of alteration layer thickness as a silica species similar to those formed during sol-gel synthesis. A parameter related to the energy distribution known as the R parameter is used to qualitatively determine open volume (without differentiating between pore size and porosity) where a larger R value

Methods and Supplementary

DISCUSSION

METHODS