CS-137 MOBILITY IN SOILS AND ITS LONG-TERM EFFECT ON THE EXTERNAL RADIATION EXPOSURE

To predict the external gamma-dose rate of Chernobyl-derived Cs-137 fo r a period of about 100 years after its deposition, the vertical distr ibution of radiocesium in several meadow soils in the Chernobyl area a nd in Germany was determined, and the corresponding residence half-tim es of this radionuclide in the various soil layers were evaluated usin g a compartment model. The resulting residence half-times were subsequ ently used to calculate the vertical distribution of Cs-137 in the soi l as a function of time and finally to predict the external gamma-dose rates in air for these sites at various times. A regression analysis of the data obtained showed that the time dependence of the relative g amma-dose rate in air D (t) at the Chernobyl sites can be described by an exponential equation D (t) = a + b.exp(-t/c), where t is the time after deposition. For the ten German sites the best fit was obtained u sing the two-exponential equation D (t) = a.exp(-t/b) + c.exp(-t/d). T he gamma-dose rate of Cs-137 at the Chernobyl sites decreases signific antly more slowly with time than ar the German sites. This means that after e.g. 30 years the mean relative gamma-dose rate at the German si tes will have decreased from 100% (corresponding to an infinite plane source on a smooth surface) to 9% (95% confidence interval 8%-10%), wh ile at the sites in the Chernobyl area it will have decreased only to 21% (20%-23%), This difference is the result of the longer residence h alf-times of Cs-137 in the soils at the Chernobyl sites. All results a re compared with estimates from earlier studies.