Application of a generic biosphere model for dose assessments to five European sites.

The BIOMOSA (BIOsphere MOdels for Safety Assessment of radioactive waste disposal) project was part of the EC fifth framework research programme. The main goal of this project was to improve the scientific basis for the application of biosphere models in the framework of long-term safety studies of radioactive waste disposal facilities and to enhance the confidence in using biosphere models for performance assessments. The study focused on the development and application of a generic biosphere tool BIOGEM (BIOsphere GEneric Model) using the IAEA BIOMASS reference biosphere methodology, and the comparison between BIOGEM and five site-specific biosphere models. The site-specific models and the generic model were applied to five typical locations in Europe, resulting in estimates of the annual effective individual doses to the critical groups and the ranking of the importance of the exposure pathways for each of the sites. Uncertainty in the results was also estimated by means of stochastic calculations based on variation of the site-specific parameter values. This paper describes the generic model and the deterministic and stochastic results obtained when it was applied to the five sites. Details of the site-specific models and the corresponding results are described in two companion papers. This paper also presents a comparison of the results between the generic model and site-specific models. In general, there was an acceptable agreement of the BIOGEM for both the deterministic and stochastic results with the results from the site-specific models.

A comparative radiological assessment of five European biosphere systems in the context of potential contamination of well water from the hypothetical disposal of radioactive waste.

In the framework of the BioMoSA project for the development of biosphere assessment models for radioactive waste disposal the Reference Biosphere Methodology developed in the IAEA programme BIOMASS was applied to five locations, situated in different European countries. Specific biosphere models were applied to assess the hypothetical contamination of a range of agricultural and environmental pathways and the dose to individuals, following contamination of well water. The results of these site-specific models developed by the different BioMoSA partners, and the individual normalised dose to the exposure groups were compared against each other. Ingestion of drinking water, fruit and vegetables were found to be among the most important pathways for almost all radionuclides. Stochastic calculations revealed that consumption habits, transfer factors, irrigation rates and distribution coefficients (Kd(s)) were the most important parameters that influence the end results. Variations in the confidence intervals were found to be higher for sorbing elements (e.g. (36)Cl, (237)Np, (99)Tc, (238)U, (129)I) than for mobile elements (e.g. (226)Ra, (79)Se, (135)Cs, (231)Pa, (239)Pu). The influence of daughter products, for which the distribution into the biosphere was calculated individually, was also shown to be important. This paper gives a brief overview of the deterministic and stochastic modelling results and the parameter sensitivity. A screening methodology was introduced to identify the most important pathways, simplify a generic biosphere tool and refine the existing models.

Development and comparison of five site-specific biosphere models for safety assessment of radioactive waste disposal.

This paper describes the development and application of site-specific biosphere models that might be used for assessment of potential exposures in the framework of performance assessment studies of nuclear waste disposals. Model development follows the Reference Biosphere Methodology that has been set up in the framework of the BIOMASS study. In this paper, the application is to real sites at five European locations for which environmental and agricultural conditions have been described and characterised. For each of the sites a biosphere model has been developed specifically assuming a release of radionuclides to waters that are used by humans, for example as drinking water for humans and cattle and as irrigation water. Among the ingestion pathways, the intakes of drinking water, cereals, leafy vegetables, potatoes, milk, beef and freshwater fish are included in all models. Annual individual doses were calculated, and uncertainties in the results were estimated by means of stochastic calculations. To enable a comparison, all results were normalised to an activity concentration in groundwater of 1 Bq m(-3) for each of the radionuclides considered ((36)Cl, (79)Se, (99)Tc, (129)I, (135)Cs, (226)Ra, (231)Pa, (230)Th, (237)Np, (239)Pu, and (238)U), i.e. those that are usually most relevant in performance assessment studies of nuclear waste disposals. Although the results do not give answers in absolute terms on potential future exposures, they indicate the spectrum of exposures that might occur in different environments and specify the interaction of environmental conditions, human habits and potential exposure.

Evaluation and ranking of restoration strategies for radioactively contaminated sites.

An international project, whose aim was the development of a transparent and robust method for evaluating and ranking restoration strategies for radioactively contaminated sites (RESTRAT), was carried out under the Fourth Framework of the Nuclear Fission Safety Programme of the EU. The evaluation and ranking procedure used was based on the principles of justification and optimisation for radiation protection. A multi-attribute utility analysis was applied to allow for the inclusion of radiological health effects, economic costs and social factors. Values of these attributes were converted into utility values by applying linear utility functions and weighting factors, derived from scaling constants and expert judgement. The uncertainties and variabilities associated with these utility functions and weighting factors were dealt with by a probabilistic approach which utilised a Latin Hypercube Sampling technique. Potentially relevant restoration techniques were identified and their characteristics determined through a literature review. The methodology developed by this project has been illustrated by application to representative examples of different categories of contaminated sites; a waste disposal site, a uranium tailing site and a contaminated freshwater river.

A sensitivity study of the SCK.CEN BIOSPHERE model for performance assessment of near-surface repositories.

At SCK.CEN a model has been developed on behalf of NIRAS/ONDRAF for the performance assessment of near-surface repositories, consisting of several submodels. This article deals with the submodels BIOSPHERE, describing the transfer and accumulation of the radionuclides in the biosphere and DOSE, calculating effective individual doses to the critical group. An extensive literature review was performed in order to determine best-estimate values and uncertainty ranges (probability density functions) of biosphere parameter values, specific to conditions that may prevail at potential disposal sites in Belgium. In this paper the BIOSPHERE and DOSE models are described and default and site-specific values (probability density functions where appropriate) of the parameters involved are indicated for the radionuclides 129I, 239Pu, and 94Nb. A combined uncertainty/sensitivity analysis based on the pdf of the site-specific parameter values has been carried out. Median values and 95% confidence intervals of the site-specific doses are indicated and most influential parameters to the uncertainty identified. Site-specific median dose values are also compared with generic doses.

Assessment of dose to man from releases of 99Tc in fresh water systems.

The objective of this paper is to evaluate the dose to man from releases of 99Tc in a fresh water system and to identify the biospheric transfer parameters to which the total dose is the most sensitive. Only internal exposure is taken into account, as the external irradiation leads to a negligible dose contribution. Two release modes were considered: continuous (routine) releases and accidental releases. The concentrations in the biospheric compartments subsequent to routine releases were calculated according to International Atomic Energy Agency procedures. For the accidental releases, a more dynamic approach was adopted, especially for the milk and meat compartments. A routine-release scenario typical for the Mol site has been applied, and the biospheric compartment leading to the highest dose contribution was shown to be the irrigated grain. The biospheric transfer parameters to which the first-year doses were the most sensitive consisted mainly of the mass interception factor for grain and the milk transfer factor. The doses in following years were very dependent on the value of the root zone removal rate. The accidental-release scenario resulted in committed dose equivalent that are strongly influenced by the time of year at which the release occurs.