Glass - Back to the Future!

Presenting Author:
Claire Corkshill
<[email protected]>

article posted 18 March 2016

Claire Corkhill is a Vice Chancellor’s Research Fellow at the University of Sheffield, with a research background in mineralogy and geochemistry. Her research focuses on understanding the mechanisms and kinetics of nuclear waste dissolution under conditions relevant to geological disposal of these materials, which includes an understanding of coupled geochemical processes within the engineered barrier. One of her key research topics involves the state-of-the-art determination of nuclear waste glass durability, using a range of surface sensitive analytical techniques, such as vertical scanning interferometry and µ-XAS.

An evaluation of the mechanisms and kinetics of UK simulant nuclear waste glass dissolution under high pH conditions

Claire L. Corkhill1*, Daniel J. Backhouse1, Nathan Cassingham1, Colleen Mann1, Luke Boast1, Jack Clarke1, Clare L. Thorpe1, Russell J. Hand1 & Neil C. Hyatt1#
1Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK

Under the generic scenario envisaged for the geological disposal of vitrified UK high and intermediate level waste glass (HLW and ILW, respectively), high pH environments, formed through corrosion of the engineered barrier system, are expected to dominate the ground water chemistry thousands of years into the future. For example, under slow flow conditions, bentonite clay from high level waste backfill will buffer the groundwater to a pH between 7.0 and 10.5. There is also potential for leachate from concrete vault structures (~ pH 13) to interact with HLW. Furthermore the likely co-location of vitrified ILW with cementitious UK ILW may lead to the interaction of vitrified products with a high pH (ca. pH 10 – 12) groundwater derived from interaction with cementitious waste and backfill in the ILW repository in the long-term. It is therefore necessary to build a complete kinetic and mechanistic understanding of UK HLW and ILW glass dissolution in high pH, high-Ca solutions and cement leachates. We highlight the main findings of a number of recent and ongoing studies that aim to evaluate and understand how the engineered barrier system of a geological disposal facility may influence the dissolution rate and the release of radionuclides from vitrified wasteforms. Briefly, we discuss results from batch experiments detailing the mechanisms of alteration layer formation in Ca-rich systems (Fig. 1), including SEM / TEM / SAED and synchrotron µ-XRD analysis of the phases controlling gel layer formation (Fig. 2), supported by thermodynamic modelling using the geochemical code PHREEQC; we describe the influence of cement leachate solutions on the dissolution of glass compositions relevant to the vitrification of UK ILW (e.g. arising from plutonium contaminated waste); and we review the application of single pass flow through methods to derive the fundamental parameters necessary to model the dissolution kinetics of UK HLW glass, for example, activation energy (Ea), pH power law coefficient (?) and the intrinsic rate constant (k0). Finally, the preliminary results of a 10 year natural analogue experiment are detailed, where samples of simulant HLW and ILW sourced from several countries are undergoing dissolution in situ, within a high pH, Ca-rich cave, containing 200-year old lime slurry (Fig. 3).