A Comparison of Site Characterisation Data and Modelling Results from a Radiological Assessment of the Drigg Low Level R
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$&RPSDULVRQRI6LWH&KDUDFWHULVDWLRQ'DWDDQG0RGHOOLQJ5HVXOWVIURPD5DGLRORJLFDO $VVHVVPHQWRIWKH'ULJJ/RZ/HYHO5DGLRDFWLYH:DVWH'LVSRVDO6LWH J. S. Small, P.K. Abraitis, I.R. Beadle, T.L Johnstone, P. Kelly, C.L. Pettit and G.A.Stevens, BNFL, Research and Technology, Risley, Warrington WA3 6AS UK $%675$&7 The biogeochemical processes controlling release of radionuclides from solid low level radioactive waste (LLW) disposed at the Drigg site have been represented in a computational model in order to determine the effects of varying major element chemical parameters and solubility limits of radionuclides. The model is used to provide a source term for radiological safety assessment calculations for the groundwater pathway. Site data has been compared to the results for the initial stages of the model in order to build confidence in the conceptual and mathematical model used to derive the source term for 234U, the key dose contributor, and other radionuclides which are influenced by the biogeochemical processes. Measured aqueous concentrations of radionuclides whose release from LLW is sorption controlled are compared to the model results as a basis to discuss the uncertainty in sorption input parameters used. ,1752'8&7,21 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. 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 a highly fluid cement based grout, disposed of in a series of concrete vaults. The PCRSA includes a post-closure engineering design [1], which includes a cap covering the whole of the trench and vault disposal areas and drainage features to control groundwater flow in the near-surface glacial sediments on which the site is placed. The PCRSA considers the radiological consequences through the groundwater and gaseous pathways, and the possible effects of human intrusion and disruptive events. The objective of this paper is to outline the conceptual and mathematical models used in the PCRSA to calculate the source term flux of radionuclides in groundwater. Modelling results presented are focussed on processes in the trench environment, which for the short-term (100 years post-closure), are compared to analyses of major chemical element species and radionuclides present in samples of leachate collected from the Drigg trenches. '5,**1($5),(/'&21&(3
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