Waste Package Scenario Modelling
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Waste Package Scenario Modelling Sarah P Watson1, David P Hodgkinson1, Peter C Robinson1, Lucy EF Bailey2 and Mike J Poole2 1 Quintessa Limited, Dalton House, Newtown Road, Henley-on-Thames, Oxfordshire, RG9 1HG, UK. 2 UK Nirex Limited, Curie Avenue, Harwell, Oxfordshire, OX11 0RH, UK ABSTRACT UK Nirex has supported a programme of work to develop models describing the postclosure evolution of intermediate-level waste packages with the objectives of: • providing support and justification for the parameters and representations used in performance assessment models; • informing future model development and packaging advice. Scenarios for the potential evolution of a waste package were developed and modelled taking explicit account of waste package heterogeneity and the time-dependence of the physical and chemical characteristics of the system. The modelling work highlighted the treatment of organic complexants and the representation of physical containment as two areas in which the impacts of time dependence and package scale heterogeneity might be particularly significant. A subsequent study of the impact of organic complexants emphasised the importance of heterogeneity in package inventory in determining the radionuclide release from the near field. The degree of containment afforded by the wasteform and the waste container has been investigated as part of a study to develop a preliminary understanding of the mixing scales within the repository. The study suggests that the most important control on the release of radionuclides from the waste packages is the integrity of the waste encapsulation grout. Interactions between neighbouring packages are to be expected, but the degree to which homogeneous (well mixed) conditions develop may be limited in both time and space. INTRODUCTION UK Nirex has developed a phased geological repository concept for the long-term management of intermediate-level radioactive wastes. The long-term safety of this concept is described in Nirex’s Generic Post-closure Performance Assessment (GPA) [1]. One of the current modelling assumptions in the GPA is that no long-term credit is taken for the performance of the waste packages themselves, instead the repository near field is modelled by assuming that the radionuclides in the wastes are dissolved in the repository pore-water, up to their solubility limits, giving rise to homogeneous near-field conditions. Nirex believes this assumption is conservative and is investigating ways of improving the modelling of the near field to allow for a more realistic representation of the evolution of the waste packages. This is important as the GPA is used as a benchmark against which waste packaging proposals are assessed as part of Nirex’s Letter of Compliance process.
This paper provides a top level summary of work reported elsewhere [2, 3, 4, 5] that contributes to the proposed development of Nirex’s post-closure assessment methodology. The overall objectives of the work were: • to provide support and justification for the parameters and representation
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