1. Department of Radiochemistry, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands, Laboratory for Ecological Risk Assessment, National Institute for Public Health and the Environment (RIVM), P.O. Box 1, NL-3720 BA Bilthoven, The Netherlands, and Toxicology Group, Wageningen University, P.O. Box 8000, NL-6700 EA Wageningen, The Netherlands