The authors thank the Bundesministerium für Umwelt;
Naturschutz und Reaktorsicherheit for supporting this work in the frame of research project 106 02 070. Furthermore the help of Dr. A. Dallos (Veszprem/Hungary) and Umweltbundesamt in Berlin (Prof. Dr. Neidhard;
Dr;
P;
Greiner;
Dr. G. Holdt) is gratefully acknowledged;