Investigations of iodine-129 in the natural environment: results and implications

The distribution of iodine-129 in the terrestrial environment in west Cumbria is dominated by the continuous discharges from the Sellafield plant to atmosphere. Although most of the iodine-129 discharged goes to sea, there is no evidence that sea-to-land transfer is significant for this radionuclide. An assessment of activity concentrations in foodstuffs confirms that consumption of cows' milk is the most important contributor to intake and shows the resultant committed effective dose equivalent is less than 0.5% of the present international dose limit, which for continuous exposure over a lifetime is 1 mSv y-1. Transfer from feed to cows's milk has been followed during open pasture grazing in the summer and during the winter when the cows consume locally-produced silage. In both cases, transfer coefficients from feed to milk were lower than the consensus value currently employed for modelling purposes, but consistent with values derived more recently under a variety of field conditions. (author)