Selenium-79 in the Biosphere
Selenium-79 (Se-79) is an important radionuclide in some types of radioactive waste. It is mobile and long-lived and potentially could migrate into the biosphere following release from radioactive waste disposal facilities. Special attributes are its complex chemistry, notably the chemical form is dependent upon the redox state with some compounds being volatile. Selenium is an essential element, but can also be toxic with only a small concentration range between deficiency and toxicity.
The data necessary for radiological assessment of Se-79 are scarce and can be unreliable, particularly in relation to its behaviour in the terrestrial soil-plant system.
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International Forum on Selenium-79 in the Biosphere
A workshop was held to discuss those attributes of Se-79 of particular relevance to radiological assessments for geological waste repositories. The objective was to provide an open forum for presentation and discussion of the behaviour of selenium in the biosphere, with particular focus on the environmental processes involved in selenium migration and accumulation in the biosphere and how radioecological assessments of Se-79 could be improved.
The workshop was held on 5-6 May 2008, hosted by Nagra, Wettingen, Switzerland, and involved both presentation and discussion of data requirements for dose assessment and knowledge and on-going research in the field of Se-79 behaviour in the biosphere.
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Se-79 in soils and plants
A work programme was established, which aimed to develop a better understanding of the processes affecting Se-79 accumulation in soils and uptake into plants. The work programme - intended to be used as the basis for the development of improved assessment models and to provide a better basis for selection of parameters for existing models - included:
- A review of the current knowledge (and new research) on Se-79 behaviour in the soil-plant system; Consideration of the Features, Events and Processes (FEPs) of potential relevance to the assessment of doses to hypothetical critical groups arising in the long term due to releases to the terrestrial biosphere from a repository for radioactive waste disposal;
- Development of an interaction matrix which shows how the FEPs can be combined to create a conceptual assessment model. The intention is to provide a transparent and traceable record of how information has been included, or indeed, excluded, with recorded justification, from the assessment model; and,
- Identification of data which need to be determined site specifically, as opposed to reliance upon site generic sources.