Longterm reduction of caesium concentration in milk after nuclear fallout.

Time trends in activity concentrations in milk as observed in Austria after the Chernobyl accident are presented. Both the short term decrease immediately after fallout and the medium term decline in the years following the event are very important for estimating the total exposure to be expected from a given deposition. In order to avoid artifacts due to local fallout, plant variations, or differences in the metabolism of single animals, large areas of production were used for the observations. This was achieved by observations of activity concentrations in milk powder, produced in large milk powder plants in Austria. After an initial decay with an approximate half-life of 34 days for the period of May to August 1986, a slower decrease in activity was observed during the following years. Observed half-lives are in the range of 1.5-2.0 a. Differences in the decrease observed between the different producing areas are discussed. The radiocesium contamination of milk and milk products depends directly on its presence in grass or hay and therefore, time trends observed in milk correspond closely to the time trend in these fodders. Other foodstuffs which are also produced on grass and hay, such as beef or lamb, should therefore display similar decay patterns, except for the early period after fallout when the biological half-lives in the animals influences the decrease.