Development of a model for radionuclide transport in streams for biosphere assessment purpose.

As a part of the overall safety assessment for a geological disposal of radioactive waste, models for different ecosystems are used to evaluate doses to humans and biota from possible radionuclide discharges to the biosphere. In previous safety assessments, transport modelling of radionuclides in running waters such as streams has been much simplified to the extent that only dilution of the inflow of radionuclides has been considered with no regard of any other interactions. Hyporheic exchange flow (HEF) is the flow of surface water in streams that enters the subsurface zone and, after some time, returns to the surface. HEF has been studied for decades. Hyporheic exchange and the residence time in the hyporheic zone are key parameters controlling the transport of radionuclides in a stream. Furthermore, recent studies have shown that HEF can reduce the groundwater upwelling area and increase the upwelling velocity in areas closest to the streambed water interface. In this paper, the development of an assessment model describing radionuclide transport with consideration of HEF and deep groundwater upwelling along streams is presented. An approach to parameterising the hyporheic exchange processes into an assessment model is based on a comprehensive study that has been performed in five different Swedish catchments. Sensitivity analyses are performed to explore the effect with consideration of the inflow of radionuclides with regard to HEF and deep groundwater upwelling in a safety assessment perspective. Finally, we include some suggestions for the application of the assessment model to long-term radiological safety assessments.

Identification of areas of interest for radiological impact assessments in an evolving landscape context.

Regulations concerning potential future health impacts of the final disposal of radioactive wastes in geological disposal facilities are written in terms of annual dose to individuals who are representative of small groups living in the landscape in the vicinity of the repository site. As disposal programmes in Sweden and Finland have progressed towards licensing and construction, so too has detail describing the state and evolution of surface biosphere and the landscape around the proposed disposal sites increased. Simple and generic biosphere dose assessment models in early iterations have grown in complexity with increasing site-specific detail that aims to capture the radiologically significant features of the landscape into which future releases of radionuclides might credibly occur. Current dose assessment models used in support of license applications for disposal programmes in Sweden and Finland are highly complex and their application consequently lacks transparency. An alternative simpler approach to characterising landscape objects for dose assessment models would be beneficial in that it would offer an additional line of reasoning and would add clarity, thereby supporting the decision-making process of the regulatory authorities. In the context of coastal Fennoscandia, landscape change is relatively rapid and dramatic with post-glacial landrise transforming areas of the coastal seabed into terrestrial ecosystems over a period of a few thousand years, global sea level rise notwithstanding. The locations of the geosphere-biosphere interfaces for deep geologic disposal can be estimated with some precision but the nature of the receiving ecosystems at the time of the release is less certain. The approach described here provides a statistical quantification of key morphological characteristics of areas in the landscape where doses could arise, so as to better express uncertainties in dose modelling. The proposed method assumes that the variation in the morphology of potential release locations can be described by the variation in landscape objects seen in the landscape on a wider scale, providing a statistical description of the possible landscape objects, so allowing a more comprehensive range of potential future evolutions to be addressed. Our understanding of the evolution of the landscape, based on the kinds of terrain and ecosystem development models used by POSIVA in Finland and SKB in Sweden, suggests that objects identified in present-day maps can be used as analogues for a statistical characterisation of objects in the future landscape; objects identified in the observed topography and bathymetry can therefore serve as the basis for the statistical description of landscape dose objects over the period during which doses are likely to arise. Using digital elevation models around a disposal site in Finland, we show that the statistical descriptions of landscape dose objects at three times over a period of 10 kyear of the evolved landscape are sufficiently similar to establish the suitability of the approach. The aim of this statistical analysis is to supplement current methods for defining radiological assessment models so as to provide additional numerical support to both the simpler and more complex methods employed by implementors and regulators. The method has been developed in the context of the Swedish and Finnish regulatory review process and is referred to in the IAEA's revised BIOMASS methodology. We briefly address how the method might be applied in other landscape contexts.

The significance of agricultural vs. natural ecosystem pathways in temperate climates in assessments of long-term radiological impact.

Recent developments in performance assessment biosphere models have begun to emphasise the importance of natural accumulation pathways. In contrast to the agricultural pathways, the database for natural ecosystem pathways is less well developed, leading to a mismatch in quality of representations of the two types of system. At issue is the lack of reliable soil-plant and animal ingestion transfer factors for key radionuclides in natural ecosystems. The relative importance of the agricultural vs. natural ecosystem pathways is investigated here, in the context of a temperate site in present day, Eastern France. The BIOMASS Candidate Critical Group (CCG) methodology has been applied to map a set of eight candidate critical groups derived from the present-day societal context onto physical locations within a simple model of a river catchment system. The overall assessment model has been implemented using the Aquabios code. Annual individual dose to each of the CCGs has been calculated for each of the key radionuclides (79Se, 94Nb, 99Tc, 129I, 135Cs and 237Np) released to the valley aquifer and river. In addition to the traditional agricultural pathways, lifestyle groups exploiting natural habitats are explicitly addressed. Results show the susceptibility of different candidate critical groups to different radionuclides. A reference database typical of those employed in long-term performance assessment models is employed. Doses from external exposure (94Nb) and dust inhalation (237Np) are shown to dominate agricultural food consumption by factors of more than six, but, with the reference data set, foodstuffs obtained from natural ecosystems do not contribute significantly to critical group dose and, at most, show similar exposures to the agricultural pathways. This may lead to the conclusion that natural food can be ruled out of consideration in performance assessment models. However, systematic parametric sensitivity studies carried out on soil-plant and animal ingestion transfer factors restrict the validity of this observation and demonstrate the limitations of existing databases. Remaining uncertainties can be reduced by improving structural models for performance assessment and by better characterisation of sources of locally obtained foods. Improved characterisation of radionuclide accumulation in natural ecosystems in temperate as well as alternative future climate states should complement the modelling approach.