Mineralogical Characterisation of Sediments at the Drigg Low Level Radioactive Waste Disposal Site and the Influence on
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0LQHUDORJLFDO&KDUDFWHULVDWLRQRI6HGLPHQWVDWWKH'ULJJ/RZ/HYHO5DGLRDFWLYH:DVWH 'LVSRVDO6LWHDQGWKH,QIOXHQFHRQ*URXQGZDWHU&KHPLVWU\ M.G. Randall1, D.P. Trivedi1, J. Graham1, J.S. Small1 and C. Hughes2 1 BNFL, Research and Technology, Risley, Warrington WA3 6AS UK 2 Department of Earth Sciences, University of Manchester, Manchester, M13 9PL UK. $%675$&7 Petrographic microscopy, scanning and transmission electron microscopy, X-ray diffractometry, surface area and cation exchange analyses have been undertaken on a suite of ‘bulk’ and size separated sediment fractions from the Drigg site. The data indicate a complex phyllosilicate assemblage consisting of illitic and vermiculite material derived from the complex weathering history of the siliceous, micaceous, feldspathic and chloritic parent clasts. Coarse grains range from clean fresh looking ‘polished’ clasts to heavily coated with iron oxides and oxyhydroxides. The surfaces of most igneous clasts (granites and volcanics) are usually highly altered to micaceous (‘illitic’), chloritic and vermiculitic clays. Carbonates, such as calcite, dolomite and siderite, are present, particularly in the more shallow formations. The properties of the sediments have a clear impact on the evolution of groundwater chemistry. Redox conditions are consistent with equilibrium between iron oxyhdroxides and siderite, both present in Drigg sediments. Drigg groundwater samples are close to equilibrium with calcite, which not only controls the concentration of dissolved calcium and bicarbonate ions, but also the groundwater pH. Concentrations of potassium and magnesium are controlled by ion exchange reactions, while both mineralogical and groundwater evidence show that some weathering of aluminosilicate minerals is occurring, although equilibrium is not reached. ,1752'8&7,21$1'%$&.*5281' The Drigg site is an operational facility for the near-surface disposal of LLW. The site is located in north-west England and is owned and operated by British Nuclear Fuels plc (BNFL). Disposals are carried out under the terms of an authorisation granted by the UK Environment Agency. The authorisation is periodically subject to formal regulatory review. In September 2002 BNFL produced an updated post-closure safety case (PCSC) [1,2] supported by a postclosure radiological safety assessment (PCRSA) for the Drigg site. In support of the post-closure safety case, BNFL has undertaken a programme of research into the characterisation of environmental processes that impact on radionuclide mobility. Of these processes, radionuclide behaviour in the geological environment plays an important role in the assessment of the release of radioactivity from a disposal site to the biosphere. The physical and chemical characteristics of the sediments encountered at Drigg play a large part in determining the mobility of radionuclides, through contributing to the evolution of groundwater chemistry and by providing sorption substrates. Therefore it is important that the mineralogy of the Drigg sediments is well un
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