Gas Generation and Migration from Radioactive Waste Repositories
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GAS GENERATION AND MIGRATION FROM RADIOACTIVE WASTE REPOSITORIES P.J. AGG, P.J. NASH, W.R. RODWELL AND T.R. LINEHAM AEA Technology, 424.4 Harwell, Didcot, Oxfordshire, OX11 ORA, UK ABSTRACT During the post-closure phase of a repository for low- and intermediate-level radioactive wastes significant quantities of gas will be generated, primarily by the anaerobic corrosion of metals and the degradation of organic wastes. A description is given of recent work carried out within the Nirex Safety Assessment Research Programme to address the generation of gas within a repository and the migration of gas from the repository to the biosphere. Theoretical modelling capabilities have been developed to address both of these issues. In order to gain confidence in such theoretical models, it is important to validate model predictions against observed experimental data. Preliminary experiments carried out to help validate the modelling approaches are described. INTRODUCTION It is anticipated that the UK repository for low- and intermediate-level radioactive waste (LLW and ILW respectively) will be sited at a depth of several hundred metres in watersaturated low-permeability fractured rock formations. In assessing the safety of the repository, two important issues to be considered are the generation of gas within the repository and the migration of gas from the repository to the biosphere. In this paper a description is given of recent work carried out within the Nirex Safety Assessment Research Programme (NSARP) to address these issues. During the post-closure phase of the repository significant quantities of gas will be generated, principally by the coupled processes of metal corrosion and microbial degradation of organic, notably cellulosic wastes. By comparison, the amount of radiolytic gas generated within a repository containing LLW and ILW is expected to be negligible [1]. The principal gases formed by these mechanisms are hydrogen, carbon dioxide, methane and hydrogen sulphide. In addition, it is likely that some of the generated gas will be radioactive, principally as a result of the incorporation of the isotopes 3H and 14 C that will be present within the waste [2]. Although the volume of radioactive gases will be negligible, the potential radiological impact of these gases in the biosphere must be considered. Gases may also be generated by these mechanisms during repository operations, and these must be accommodated by the repository ventilation system. If gas were to be contained within the repository, a build-up of pressure would occur. This could represent a threat to the integrity of the engineered structure and host rock, and lead to a disturbance of the pressure-head gradients and groundwater flows in the repository environment. On the other hand, if gas were to escape from the repository into the geosphere, various possible consequences should be considered. For example, there are potential radiological and flammability hazards associated with the release of radioactive and flammable gases to the biosphere. Also,
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