Transfer of caesium from silage to cows milk: observations and models

The transfer of radiocaesium from silage, contaminated as a result of the Chernobyl accident, to cows milk was studied in a feeding trial at Trawsgoed Experimental Husbandry Farm. Feed and milk samples obtained from the cows were analysed for Cs-137 and Cs-134 during the 124 days cows were fed contaminated silage and for a further 32 days when cows were fed uncontaminated material. The trial also involved the addition of bentonite supplements to the diet of some animals. Supplements of 250 g and 500 g sodium bentonite per day reduced Cs-137 transfer factors to milk to 0.63 and 0.33 of the control values respectively after 78 days of feeding. No effects of bentonite supplementation on the absorption of other elements were detected. The results for animals not given a supplement are compared with the cow-milk model implemented in the SPADE suite of codes, using both default parameters and input data and parameters derived from the feeding trial. The model overpredicted observed milk concentrations by a factor of 2 to 3. Reasons for this and alternative models are discussed. The sensitivity of the model to parameter selection is also emphasized.