Assessment of the Performance of Cement-Based Composite Material for Radioactive Waste Immobilization
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ASSESSMENT OF THE PERFORMANCE OF CEMENT-BASED COMPOSITE MATERIAL FOR RADIOACTIVE WASTE IMMOBILIZATION
M. ATKINS, J. COWIE, F.P. GLASSER, T. JAPPY, A. KINDNESS AND C. POINTER, Department of Chemistry. University of Aberdeen, Meston Building, Old Aberdeen, AB9 2UE, Scotland.
ABSTRACT The problem of predicting the future performance of cement-based systems is complicated by a poor understanding of the behaviour of cement systems at long ages, as well as of the complex interactions which can occur between cement and waste components - active as well as inactive - and with cement blending agents including fly ash, glassy slags and natural pozzolans. The progress achieved in developing a predictive capability is reviewed. Considerable success has been achieved in modelling the chemically-related features of cement based systems in terms of pH, Eh, and of element partition between solid and aqueous phases. The behaviour of model radwaste elements iodine and uranium - has been studied in depth and indicates that both sorption and precipitation occur. U(VI), in particular, forms solubilitylimiting compounds, e.g. uranophane. But in general, presently-available data are inadequate to predict many cement-waste stream interactions; future progress in modelling is likely to rely heavily on additions to the data base. The repository environment will also condition chemical exchanges in cement-based systems. Progress is being made in predicting the impact of C02, a common ground water component, on the performance of cement systems. INTRODUCTION Cement, based materials have many conceptual advantages for the immobilization of low-to-intermediate level radioactive wastes: these are summarised in Table I. The ability of cement systems to tolerate wet wastes, often with little or no pretreatment, combined with low cost and an established track record in civil engineering, make cements an attractive choice of matrix. Indeed, the past two decades have seen several cementTable I
CONCEPTUAL ADVANTAGES OF CEMENT BASED I MMOBILIZATION SYSTEMS -
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Engineering properties and materials handling, including automated methods, are proven technology. Some guide to specification available from civil engineering codes, e.g. ASTM. Tolerant of water and, within limits, other soluble species, thus compatible with many waste streams. Perform well in fire and accident scenarios. Resistant to low levels of radiation. Have chemical as well as physical potential for immobilization. Cheap, widely available and relatively durable; some historical documentation available: regarded by engineers and informed public as an "acceptable" matrix.
Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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based systems tailored to specific immobilization requirements and repository conditions. These formuations are frequently applications-led: examples include free-flowing grouts, "saltcretes" for service in salt-rich geologic strata, blends with blast furnace slag having low heat evolution and low permeability, etc. Much is known about the p
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