Long-Term Predictions of the Concentration of a-isosaccharinic Acid in Cement Pore Water
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Long-Term Predictions of the Concentration of a-isosaccharinic Acid in Cement Pore Water Martin A. Glaus1, Luc R. Van Loon1, Bernhard Schwyn2, Sarah Vines3, Steve J. Williams3, Peter Larsson4, Ignasi Puigdomenech4 1 Laboratory for Waste Management, Paul Scherrer Institut (PSI), CH-5232 Villigen, Switzerland 2 National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettingen, Switzerland 3 Nuclear Decommissioning Authority (NDA), Radioactive Waste Management Directorate, Curie Avenue, Harwell Didcot, Oxon, OX11 0RH, UK 4 Swedish Nuclear Fuel and Waste Management Co (SKB), Box 5864, SE-102 40 Stockholm, Sweden ABSTRACT The long-term prediction of the equilibrium concentration of α-isosaccharinic acid (αISA) in cement pore water is a crucial step in the assessment of the role of cellulose in the safety of a cementitious repository. The aim of the present contribution is to summarise recent efforts in identifying the most important processes leading to the formation or degradation of α-ISA and in predicting its most likely concentrations in cement pore water. The issues considered are the kinetics involved in the formation of α-ISA, reactions of α-ISA with dissolved or solid compounds that may lead to limitations of its pore water concentrations and the chemical stability of α-ISA in a heterogeneous alkaline environment. Some new results are presented showing that α-ISA is degraded to low-molecular weight organic compounds in the presence of oxygen, whereas such processes occur only to a minor extent under anaerobic conditions. It is concluded that the processes involved in the degradation of cellulose under alkaline conditions are not sufficiently understood to explain fully the observed concentrations of α-ISA in longterm experiments. INTRODUCTION Cellulose is a main constituent of the organic matter in low- and intermediate level radioactive wastes. The degradation of cellulosic material in the cementitious alkaline environment of the repositories for these wastes results in the formation of water-soluble species. Among the various reaction products of the alkaline degradation of cellulose, α-isosaccharinic acid (α-ISA) is one of the most important complexants formed [1-3]. Cellulose degradation products may thus have a detrimental effect on the barrier function of cement because they enhance the solubility and decrease the sorption of many cations of the transition metal, lanthanide and actinide series. The long-term estimation of the equilibrium concentration of αISA in cement pore water is consequently an important step in the assessment of the role of cellulose in the safety of such a repository. Van Loon and Glaus [3] proposed a reaction scheme which was used to predict quantitatively the equilibrium concentration of α-ISA in cement pore water. The scheme comprises the degradation reactions of cellulose that lead to the formation of α-ISA and reactions that lead to the disappearance of α-ISA from the solution phase by precipitation or sorption reactions [4,5]. For a typical reference case of cellulos
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