The Geochemical Behaviour of Nickel in the Repository Environment
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THE GEOCHEMICAL BEHAVIOUR OF NICKEL IN THE REPOSITORY ENVIRONMENT
DAVID SAVAGE
Intera Information Technologies Ltd., 47 Burton Street, Melton Mowbray, Leicestershire LE13 1AF, United Kingdom
ABSTRACT
59Ni may make significant contributions to calculated doses in performance assessment (PA) studies of geological repositories for radioactive waste, due principally to the adoption of relatively high near-field solubilities. Ni concentration values used in PA are typically 1-6 orders of magnitude greater than those measured in natural groundwaters. This difference is a result of the utilisation in PA of solubility data for pure nickeliferous solids, and in some instances, the selection of solids chemically incompatible with the disposal environment. The geochemical behaviour of Ni is reviewed. It is considered highly unlikely that Ni will form discrete pure solids in the repository near-field, so that partition as a trace element into solid solutions should be considered. Henry's law behaviour of trace elements is discussed and geochemically-realistic Ni solubility values for HLW and L/ILW disposal scenarios calculated. It is concluded that Ni will behave according to the chemical conditions of the disposal environment, which should be reflected in the choice of solubility data for PA. The adoption of solid solution modelling for other elements of interest to PA should be considered. INTRODUCTION
Radionuclides of nickel (59Ni and 63Ni) are present in both high-level and intermediate-level radioactive waste streams. The half-life of 63Ni is relatively short (96 years), but 59Ni has a 75,000 year half-life and may make significant contributions to calculated doses in PA studies of geological repositories for radioactive waste on a timescale of 105-106 years. Retardation behaviour is relatively good (KD'S of 0.1-1 m 3 kg-i on geological materials are typical), so that calculated doses are primarily a result of the adoption of relatively high near-field solubility values. Although it is acknowledged that over-estimation of radionuclide solubilities in this manner errs on the side of conservatism, it will be argued here, using Ni as an example, that solubility data adopted for PA should also be geochemically realistic, and hence, defendable in the widest possible context. NICKEL IN PERFORMANCE ASSESSMENT STUDIES
Recent separate PA calculations carried out for the disposal of HLW in crystalline rock in Finland, Japan, Sweden and Switzerland all assumed Ni concentrations in repository pore fluids in excess of 10-4 moles kg-1 H 20 (> 6 mg kg-I H 20)i11l In all of the PA studies except that of Switzerland (Ni assumed to be unlimited by solubility), Ni concentrations were taken to be limited by the solubility of a pure Ni solid. Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
1160
Table I: Studies of Ni concentrations in groundwater. Rock Type
Groundwater Characteristics
Concentration Range (moles kg-1 H20)
Data Source
various aquifers
various
2-300 x 10-8
[3]
granite
neutral, oxic,
10-10
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