Trockene Deposition von Aerosolen aus einer simulierten Reaktor-Kern-Schmelze und Eintritt von darin enthaltenem Cs-137 und Sr-090 in die Nahrungskette

Although the pressurised water reactors (PWR) in Western Europe differ considerably from the "Chernobyl type" nuclear power plants that blew up in the Ukraine on April 26$^{th}$ 1986, a melt down of their core could lead to the accidental release of radioactive aerosols. Such metal particles of a mean aerodynamic diameter of about 1 $\mu$m were artificially generated in the European RESSAC Programme to investigate the radionuclide transfer into the food chain. The objectives of the studies in large lysimeters (3,2 m$^{2}$ ; 1,5 m depth) were improved predictions and the testing of potential countermeasures under field-like conditions. The process of sedimentation accounted for the greatest proportion of the deposition on kohlrabi ($\textit{Brassica oleracea}$ var. gongylodes L.) and wheat ($\textit{Triticum aestivum}$ L.) in the (windless) greenhouse. Thermophoresis and diffusiophoresis probably also played a part in deposition on the soll (an orthic luvisol from Jülich-Merzenhausen). With increasing density of vegetation, deposition on the soll decreased, whereas total deposition and the percentage intercepted by the plants increased. This percentage, as described by an absorption coefficient of 12 m$^{2}$/kg dry weight, appeared to be 4 times the literature value derived from field studies. Foliar absorption of $^{137}$Cs from the particles was investigated by stripping epicuticular waxes with celluloseacetate film. Five days after contamination, 13-14 % of the $^{137}$Cs was found inside the leaves of wheat, and twice as much inside the leaves of kohlrabi. The first irrigation (9 mm, 5 days after contamination) removed less than 1/3 of the $^{90}$Sr, but 2/3 of the $^{137}$Cs from wheat and 2/3 of both radionuclides from kohlrabi. This suggests that the processes involved depend on the nuclide and the crop. After the rapid initial decrease of $^{137}$Cs, its weathering half-life from wheat for the following fortnight was 14 days, from then till harvest it was approximately 27 days. Hence, decrease was not exponential. After contamination ofwheat at tillering (EC 21), 0.11 % of the $^{137}$Cs deposited on the leaves was found in the grain, after contamination at shooting (EC 31) 0.25 % was found there. Unlike the easlly translocated $^{137}$Cs, the major pathway of $^{90}$Sr to the grain was the (simulated) combine harvesting with its directly contaminated straw. This often neglected secondary contamination at harvest accounted for 90 % of the $^{90}$Sr, but only 3 % of the $^{137}$Cs transfer. It could be considerably decreased in case of areal accident by setting the combine head to a greater cutting height. After harvest, tillage comparable to "normal ploughing" scattered the radionuclides irregularlyat a depth of 15-30 cm. By "double-depth ploughing", most of the radioactivity was brought into the 30-35 cm layer, but a new autoradiographic method showed that fragments of the directly contaminated soll surface were located all over the tilled profile. Regardless of the tillage, roots of winter barley absorbed $^{137}$Cs mainly in January and the transfer factor to the grain TFd(Cs), standardised to the surface area, amounted to 4$\ast$10$^{-6}$ m$^{2}$/kg fresh weight. Thus the "double depth ploughing" could not reduce the transfer