Key Issues Influencing the Performance of Near-Field Barriers
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KEY ISSUES INFLUENCING THE PERFORMANCE OF NEAR-FIELD BARRIERS
P.W. TASKER, S.J. WISBEY and C.B. BOYLE AEA Decommissioning & Radwaste, Harwell Laboratory, Didcot, Oxon., OXI1 ORA, UK. ABSTRACT In developing a national strategy for the disposal of radioactive wastes, each country will consider isolation options that are appropriate to the types of waste that are produced. The options are developed in response to specific national regulatory requirements, and thus will focus on different aspects of performance. However, there are a number of technical concerns that are common to all programmes. The major issues concerning the behaviour of the chemical and physical barriers in the near field of a radioactive waste repository are discussed in this paper. The description of key issues has been divided into the following categories: barrier design, barrier evolution, scientific understanding through modelling, and validation of performance. The near-field barriers are selected and designed to provide appropriate radionuclide containment and control. Factors affecting the evolution of these barriers, such as container degradation and gas generation, determine the subsequent release of radionuclides to the human environment. Modelling repository evolution is therefore an integral feature of performance assessments, and issues such as the treatment of inhomogeneities and non-equilibrium chemistry may need to be addressed. However, the use of mathematical and computer models implies a requirement for validation. The use of demonstration experiments and natural analogues builds confidence in the predictions of repository performance models, and provides a degree of validation for otherwise inaccessible timescales. INTRODUCTION The post-closure performance assessment of a waste disposal facility is concerned with the radiological impact of the disposed radionuclides after it has been closed and sealed. Most facilities will be subject to a period of institutional control, during which remedial measures may be taken if necessary, but those facilities that are designed to accept long-lived radionuclides will need to take account of the need for continuing reliable performance in the period following institutional control. The design and siting of the repository must be taken into account, together with the evolution of the facility and its environment over extended, even geological, timescales. In this paper, some key issues associated with the design and evolution of the facility as they affect the near-field barriers are identified. These issues relate to pathways by which radionuclides may eventually return to the surface environment. Aspects of the evolution of these pathways and barriers over the protracted timescales of interest are discussed with reference to the performance assessment work in the UK and its supporting research. These activities are funded by UK Nirex Ltd. (Nirex), who are developing a deep geological repository for the disposal of low- and intermediate-level wastes containing long-lived radionuclides. The
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