The Solubility Limited Source Term for Cement - Conditioned Wastes: A Status Report
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ABSTRACT An important function of the cement backfill in a nuclear waste repository is to react with aqueous waste species and reduce their solubility. However, to quantify backfill performance it is first necessary to prove the existence and establish the nature of the chemical solubility controls. This can be done by characterising the solubility-limiting phases, determining their solubility and thermodynamic functions, and assessing their stability and persistence and solubility during backfill degradation. Much of the necessary data must be acquired experimentally. The title paper describes briefly the progresss of experimental work on selected species including nickel, chromium(III,VI), tin(IV) molybdenum(VI), uranium(VI), Ce(III,IV), thorium, actinide simulants (III, IV) and chloride. Data needs are assessed and although much experimental work remains to be done, methodologies have been developed which will expedite progress. The expectation of a more quantitative performance assessment of cement barriers is, therefore, attainable. INTRODUCTION Cements may be used as barriers to retard or prevent the migration of potentially-soluble radioactive waste species. The barrier function may be physical, chemical, or both. Numerous physical barrier systems compete with cements: for example, clay, steel, copper, glass, etc. either singly or in combination. However, contenders for chemical barrier systems are fewer. Most of the candidate materials perform by conditioning permeating water to a high pH; the high pH precipitates a range of polyvalent cationic species as hydroxides and
hydrous oxides. However, difficulties have arisen in including solubility data in performance assessments: clearly solubility limits exist, but are they reproducible and, if so, what is the defining solid phase? How stable will the defining solid phases be under repository conditions, both now and in the future, as the barrier degrades? These questions and other related issues need to be quantified to validate the concept of a solubility-limited source term and implement its acceptance into performance assessments. Limited progress has been made in this direction. Natural analogue studies, especially of uranium, provide some insight into the mineralogy of phases but the data are mainly on occurrence, not solubility, and are likely to be relevant to near-neutral pH. Actinides are virtually absent in nature and laboratory experiments with actinides are difficult and expensive, with the result that much work has been done with actinide simulants. These simulants give valuable experience and highlight key features and processes for subsequent studies using the actinides themselves. Experimental work aimed at characterising the solubility - limiting phase has drawbacks: it is difficult to characterise dilute, solubility-limiting phases in cement matrices which are themselves fine-grained and have a high content of amorphous phases. Consequently, much work has also been done interfacing simulants for cements with actinides. 1225 Mat. Res. Soc. Sy
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