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The effect of I-129 speciation on deposition and transfer to food crops - RP0153

The atmospheric emissions of many radionuclides from Sellafield have declined over a number of years. By contrast emissions of 129I have remained relatively constant since the early 1980's at around 10 to 40 GBq per year. The resultant does (possible 10's of *sV pa, primarily to milk and/or green vegetable consumers in 1993) is now a relatively substantial contributor to overall terrestrial food chain doses to consumers local to Sellafield: yet the deposition and uptake factors for 129I are rather less certain than those for nuclides emitted as small particles. As the same time, the start-up of THORP operations is likely to increase substantially total 129I emissions, possibly with a differing chemical form to current emissions. Determination o the chemical for of 129I (throughout the emission-deposition- uptake chain) is a key factor in reducing uncertainties, allowing more precise predictive modelling of its behaviour. This study aims to determine the effect of 129I speciation on its deposition characteristics and uptake by food crops; and to review existing models of iodine behaviour, leading to better mechanistic predictive capabilities. A separate project, funded by BNFL, aims to determine the speciation of 129I at the point of emission from both the Magnox and thermal oxide reprocessing plants. By entering into these studies, and thereby effectively forming a co-contractual relationship, both MAFF and BNFL will gain added value from sharing access to a common pool of information larger than either project alone can offer.
The environmental modelling of 129I through the important food pathways is sensitive to the deposition velocity, which itself depends on the speciation of the iodine. The speciation of current discharges from the MAGNOX reprocessing and effluent treatment plants at Sellafield are poorly understood; and changes may occur with the commissioning and operation of THORP. Moreover, the detection of 129I in environmental samples is time consuming and expensive at present, generally restricting its widespread monitoring and placing greater reliance on modelling studies. A greater understanding of the speciation of 129I at the point of emission, at the point of deposition (in atmosphere and in wet and dry deposition collectors), and its subsequent environmental transfer through food chains would considerably reduce many current uncertainties in the modelling process. In order to resolve these various areas, three essential steps are required: *improving the sampling, detection, speciation and reliability of 129I measurement; *further investigating stack emissions; *conducting and environmental monitoring programme linking emissions to uptake and transfer through the food chain. Westlakes Research Institute is nearing the end of a project which has successfully developed techniques for 129I measurement and speciation, and BNFL are about to initiate a long-term study on stack emissions. The study proposed her relates to the environmental programme, but would clearly be linked to the stack emissions investigations. The particular objectives of this study are: 1.Refinement and demonstration of suitability of measurement techniques. 2.To define the forms of the airborne 129I and deposition velocities at locations relevant to milk production, using both wet and dry deposition collectors. 3.Determine the uptake to leafy green vegetables and root crops, and the transfer coefficient from grass to milk for the same locations and time period. 4.Establishing the processes determining the chemical form of iodine release and changes within the environment. 5.Link results, where possible, to those arising from the project on speciation of 129I at the stack emission points, to determine initial rates of change and subsequent stability of chemical form as a means of predicting food change impact. 6.Use derived parameters to improve existing models of 129I behaviour. The outcome of the project is of considerable importance in the modelling of foodchain doses from this important isotope.
Time-Scale and Cost
From: 1996

To: 1998

Cost: £112,220
Contractor / Funded Organisations
Westlakes Research (Trading) Ltd