Model studies of the transfer of radionuclides in the Finnish environment: Dissertation

Environmental modelling of the effects of radionuclide releases was studied. A computer model DETRA was developed for evaluation of radionuclide transfer. The flexible structure of the model made it suitable for a wide range of applications. The model was employed to evaluate the biospheric transfer of radionuclides in two situations: 1) release cases from planned waste repositories in Finland, and 2) deposition due to the Chernobyl accident. The Chernobyl accident provided a good opportunity to test the DETRA model, originally constructed for waste disposal applications. A study was made of some biospheric local, regional and global scale scenarios, to which releases from repositories were assumed. Drinking water from a local well and consumption of fish were the most important dose pathways. The transfer of the Chernobyl deposition in some important Finnish transfer environments was evaluated. Especially the transfer of Cs-137 in the Kymijoki watercourse, including dynamic transfer to nonpredatory and predatory fish species was studied. The calculated behaviour was compared with the measured behaviour. Nuclear weapon testing fallout was also considered in the comparison studies. On the basis of validation studies it was concluded that part of the Cs-137 deposition was in insoluble form. These model assumptions give that deposition consisting of soluble Cs- 137 could cause fourfold contamination of fish per unit deposition than deposition due to the Chernobyl accident in the modelled case. One of the most important factors in the modelling proved to be the assumptions concerning the transfer to radionuclide sinks. Especially transfer to sediment sinks was a significant term. The assumptions concerning dilution volumes also affected the results. Assumptions concerning the solubility of radionuclides were important in the model: poor solubility may cause relatively rapid removal to sinks in the aquatic environments but rather slow removal in the case of terrestrial environments. Concentration of the radionuclides in food products often impact a cumulative effect to these transfer properties. Model validation was also performed within international projects, in particular by participating in the BIOMOVS project. This participation will be continued in the VAMP program. In the summary part of the thesis the main attention is paid to modelling questions, whereas in the accompanying publications the results are presented in greater detail.