Epistemic and Aleatory Uncertainty in Recommended, Generic Rock K d Values used in Performance Assessment Studies
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Epistemic and Aleatory Uncertainty in Recommended, Generic Rock Kd Values used in Performance Assessment Studies
James Crawford, Ivars Neretnieks, and Luis Moreno Department of Chemical Engineering and Technology Royal Institute of Technology, S-10044 Stockholm, Sweden ABSTRACT Over the past decade or so there has been an explosion in the number of sorption modelling approaches and applications of sorption modelling for understanding and predicting solute transport in natural systems. The most widely used and simplest of all models, however, is that employing a constant distribution coefficient (Kd) relating the sorbed concentration of a solute on a mineral surface and its aqueous concentration. There are a number of reasons why a constant partitioning coefficient is attractive to environmental modellers for predicting radionuclide retardation, and in spite of all the shortcomings and pitfalls associated with such an approach, it remains the leitmotif of most performance assessment transport modelling. This paper examines the scientific basis underpinning the Kd-approach and its broad defensibility in a performance assessment framework. It also examines sources of epistemic and aleatory uncertainty that undermine confidence in Kd-values reported in the open literature. The paper focuses particularly upon the use of so-called “generic” data for generalised rock types that may not necessarily capture the full material property characteristics of site-specific materials. From the examination of recent literature data, it appears that there are still a number of outstanding issues concerning interpretation of experimental laboratory data that need to be considered in greater detail before concluding that the recommended values used in performance assessments are indeed conservative. INTRODUCTION A number of countries are currently planning deep geological repositories for high-level nuclear waste. In Sweden, the storage concept (the KBS-3 system) involves encapsulation of spent fuel rods in corrosion-resistant iron-copper canisters surrounded by a bentonite clay buffer at a depth of approximately 500 metres in crystalline bedrock. The canisters are to be located in deposition holes that will be bored in the floors of a system of tunnels comprising the repository. Performance assessment (PA) calculations are frequently based upon the assumption that one or more engineered containment structures are initially defective or become defective after some time. Radionuclides may then escape the repository and be transported to the biosphere by water flowing in the fractured rock surrounding the repository. When this occurs, the properties of the surrounding rock volume that comprises the natural barrier for radionuclide migration is of overwhelming importance for the safety of the repository. There is abundant evidence in the open literature that the transport of radionuclides is retarded by matrix diffusion and sorption on mineral surfaces within rocks both in laboratory [1-
5] and field studies [6] as well as in natural an
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