Near-field/Far-field Interface of a Near-Surface Low-Level Radioactive Waste Site
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I.R. Beadle1, J. Graham1, S. Boult2, V.L. Hand2, P. Warwick3, M.G. Randall1, D.P. Trivedi1, and P.N. Humphreys1. 1. British Nuclear Fuels plc, Risley, Warrington, Cheshire, WA3 6AS, UK. 2. University of Manchester, Department of Earth Sciences, Manchester, UK. 3. University of Loughborough, Department of Chemistry, Loughborough, LE11 3TU, UK. $%675$&7 Experimental and Modelling studies have been used to investigate the biogeochemical processes occurring at the interface zone between the near-field and far-field of the Drigg LowLevel radioactive Waste (LLW) trenches. These have led to a conceptual model of interface biogeochemistry, which has subsequently been modelled by the BNFL code known as the Generalised Repository Model (GRM). GRM simulations suggest that as organic rich leachate migrates into the far-field, iron III minerals such as iron hydroxide will dissolve, and reduced iron minerals such as siderite will precipitate due to microbial processes. Batch and column experimental studies have verified this conceptual understanding. Some unexpected processes, such as the presence of nitrate enhancing sulphate reduction, were also observed ,1752'8&7,21 The Drigg site is an operational facility for the near-surface disposal of Low-Level radioactive Waste (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 post-closure radiological safety assessment (PCRSA) for the Drigg site. The Drigg site includes two disposal systems: 1) An original system operated from 1959 to 1995 comprising a series of trenches excavated into glacial clays, back filled with LLW and covered with an interim water resistant cap. 2) Current disposal of compacted waste placed in steel ISO-freight containers, with void space filled with highly fluid cement based grout, and disposed of in a series of concrete vaults. The objective of this paper is to outline the approach used in the PCRSA to assess the extent and influence of a near-field plume on the Drigg far-field. The paper will primarily consider the interface between the Drigg disposal trenches and the surrounding geosphere, however the PCRSA also considers the trench/vault and vault/geosphere interface. Experimental studies have been used to support conceptual model development, and key results are presented. The modelling results presented are focussed on establishing the extent of the interface region and the biogeochemical evolution over extended time periods that are not possible in experimental systems. '5,**1($5),(/')$5),(/'&21&(378$/02'(/ In order to demonstrate that all of the factors that could possibly affect the site have been evaluated, the PCRSA considers the
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