Abstract.Planning for the disposal of spent nuclear fuel is at an advanced stage in several nations around the world. Licensing of the disposal facility requires correspondingly detailed assessment of the future performance of the facility. With increased site-specific detail available to the assessment, local characteristics play an increasingly important role in determining the potential radiological risk posed by releases to the biosphere. In this paper we go beyond existing reference biosphere models and investigate the potential for specific accumulation mechanisms. The implications for the modelling carried out in long timescale performance assessment are discussed. BACKGROUNDPost-closure safety assessments for nuclear waste repositories (Performance Assessments, PAs) involve radioecological analysis of the release to the surface environment from an underground source(s). Models for PA applications have been under development for over thirty years. At early stages of a waste disposal programme generic modelling is appropriate; the international Reference Biospheres Methodology has been developed to construct stylised biospheres based on present-day biosphere analogues [1]. Traditionally, long-term safety assessments treat the geosphere and biosphere as distinct model domains, coupled only by the nuclide-specific biosphere dose conversion factor.In more advanced disposal programmes, for example in Sweden, an integrated modelling approach has recently been developed [2] in which "landscape models", based on detailed site-specific data, are used to evaluate radionuclide accumulations in the biosphere, together with associated doses. In this approach land uplift induced by climate evolution (glaciation cycles) and a number of connected biosphere objects are considered. However, the coupling of geosphere and biosphere models remains a weak aspect and structures within ecosystem models are greatly simplified [3].A transparent and credible dose assessment must demonstrate that all relevant features, processes and events have been handled appropriately in the assessment model. The focus on "landscape modelling" in contemporary assessments has overshadowed the need for suitably representative models of radionuclide transport in both the traditional biosphere objects and in deeper components of the geosphere-biosphere interface. Independent PA modelling carried out by SSM emphasises the internal dynamics of landscape objects. Once activity has reached the landscape drainage system, contaminants are lost fairly rapidly from the area around the release point. What biosphere accumulation does occur, takes place predominantly in the locality of the release [4] and the degree of accumulation depends on the nature of the ecosystem into which the release occurs [5]. Biosphere processes are inherently dispersive but some promote the concentration of contaminants in relatively small volumes.

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The International Atomic Energy Agency has coordinated an international project addressing climate change and landscape development in post-closure safety assessments of solid radioactive waste disposal. The work has been supported by results of parallel on-going research that has been published in a variety of reports and peer reviewed journal articles. The project is due to be described in detail in a forthcoming IAEA report. Noting the multi-disciplinary nature of post-closure safety assessments, here, an overview of the work is given to provide researchers in the broader fields of radioecology and radiological safety assessment with a review of the work that has been undertaken. It is hoped that such dissemination will support and promote integrated understanding and coherent treatment of climate change and landscape development within an overall assessment process. The key activities undertaken in the project were: identification of the key processes that drive environmental change (mainly those associated with climate and climate change), and description of how a relevant future may develop on a global scale; development of a methodology for characterising environmental change that is valid on a global scale, showing how modelled global changes in climate can be downscaled to provide information that may be needed for characterising environmental change in site-specific assessments, and illustrating different aspects of the methodology in a number of case studies that show the evolution of site characteristics and the implications for the dose assessment models. Overall, the study has shown that quantitative climate and landscape modelling has now developed to the stage that it can be used to define an envelope of climate and landscape change scenarios at specific sites and under specific greenhouse-gas emissions assumptions that is suitable for use in quantitative post-closure performance assessments. These scenarios are not predictions of the future, but are projections based on a well-established understanding of the important processes involved and their impacts on different types of landscape. Such projections support the understanding of, and selection of, plausible ranges of scenarios for use in post-closure safety assessments.

In developing models of the biosphere for use in assessing the impacts on human health and the environment of releases of contaminants from disposal facilities for solid radioactive wastes or from contaminated legacy sites, there is a need to demonstrate that the models adopted are both comprehensive and appropriate to the assessment context. To achieve this end, it is useful to develop a structured approach to conceptual model development and it is here proposed that interaction matrices (IMs) provide a suitable framework. This process can provide a conceptual model expressed in terms of either a single IM or a nested set of IMs. The focus of the work described herein is the development of a transparent approach to translating such a set of IMs into a mathematical model, which is typically expressed as a set of ordinary differential equations complemented by algebraic expressions. Some remarks are also made on appropriate approaches to obtaining numerical solutions of these equations in circumstances where simplifications of the general equations can be justified. Overall, the intent is to provide background and guidance by providing a formal basis for the process in generalised terms.

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