Technical feasibility of a concept radioactive waste disposal facility in Boom clay in the Netherlands

Document available in extended abstract form only. The current management strategy in the Netherlands for radioactive waste is interim storage for approximately 100 years, followed by final deep geological disposal. At present, both Boom Clay and Salt formations are being considered and investigated via the OPERA (Onderzoeks Programma Eindberging Radioactief Afval) and CORA (Commissie Opberging Radioactief Afval) research programmes respectively, instigated by COVRA (Centrale Organisatie Voor Radioactief Afval). This paper outlines the on-going investigation into the initial technical feasibility of a high-level radioactive waste disposal facility, located within a stratum of Boom Clay, as part of the OPERA research programme. The feasibility study is based on the current Belgian Super-container concept, incorporating specific features relevant to the Netherlands, including the waste inventory and possible future glaciation. The repository is designed to be situated at approximately 500 m depth in a Boom Clay stratum of approximately 100 m thickness, and will co-host vitrified High Level Waste (HLW), spent fuel from research reactors, non-heat generating HLW, Low and Intermediate Level Waste (LILW) and depleted uranium. The total footprint is designed to be 3050 m by 1300 m, and will be segregated by waste type. The waste will be stored in drifts drilled perpendicular to the main galleries and will vary in length and diameter depending upon waste type. The repository life-cycle can be considered in three phases: (i) the pre-operation phase, including the conceptual development, site investigation and selection, design and construction; (ii) the operational phase, including waste emplacement and any period of time prior to closure; and (iii) the post-operational phase. The research on the technical feasibility of the repository will investigate whether the repository can be constructed and whether it is able to perform the appropriate safety functions and meet safety requirements. The research will be based on 2D and 3D numerical studies of the reference concept, as well as possible variations on that design. It will involve analyses of the staged excavation and construction of the repository, as well as pre- and post-closure assessments of repository performance; for example, in terms of (long term) stability, deformations, stress changes, structural integrity and water/gas pathways. A significant feature of the research is that it will take account of the considerable uncertainties in the clay properties and behaviour through a reliability-based design approach. In particular the project aims to deliver: - An assessment of whether, and under which conditions, the current OPERA disposal concept is feasible; - An assessment of whether any modification to the design can enhance the safety performance against the occurrence of various failure mechanisms, such as excavation heading instability, unacceptable lining forces, excessive deformations and preferential water/gas pathways, and, if such modifications are deemed appropriate, what form these modifications should take; - An assessment of the relative importance of parameters (e.g. material properties, material behaviour, design parameters) for the safety case, accounting for parameter uncertainty using a reliability-based design approach; - The development and validation of a strategy to analyse repository performance with respect to safety, i.e. based on linking 2D and 3D numerical modelling with probability theory, which may be used for conducting more detailed feasibility studies in the future. Probabilistic techniques will be used to quantify the influence of the considerable uncertainties arising from the gaps in current knowledge, especially with regard to clay properties and behaviour. However, as knowledge is gained through other projects, including those within OPERA, the analyses may be refined as uncertainties reduce. Note that, while the probabilistic approach is an appropriate tool for quantifying the effects of uncertainties and for determining critical factors, its effectiveness is greatly enhanced by narrower (less uncertain) parameter distributions arising from increased knowledge. Hence, this approach is not a substitute for lack of knowledge; rather, it is a means of quantifying lack of knowledge and its influence on predicted repository performance. The need for quality data for Boom Clay at depths exceeding 500 m is clear and remains an urgent priority