Chronic radiation doses to aquatic biota
Student thesis: Doctoral Thesis
On 26 April 1986, the worst nuclear accident in the history of the nuclear industry occurred at Unit 4 of the Chernobyl nuclear power plant (ChNPP) in the Ukraine (at that time a Republic of the Soviet Union) resulting a large amount of radioactive nuclides released into the environment in Russia, Ukraine, Belarus and other European countries. The chronic radiation doses from radioactive nuclides released from the accident are still significant in the freshwater aquatic environment of the 30 km Exclusion Zone around Chernobyl. In this study, current methods of estimating radiation doses have been further developed by model inter-comparison and testing against empirical data. This research also supports previous work on the effects of radiation on aquatic biota by accurately measuring the external radiation dose in the littoral zone of Svyatoye and Perstok lakes in Belarus. The testing and critical analysis of five dose assessment models (RESRAD, FASSET, ERICA, R&D128 and the D-Max model) was carried out in order to understand the key factors which influence predictions of both internal and external doses. For the internal dose, studies of the relationship between modelled dose and organism mass showed that mass is a more important factor than organism shape. The predictions of external dose were generally more variable than those for internal dose. The most important factor causing variation in external dose was the assumed habitat of the organism and the occupancy factor. The Monte Carlo method was used to estimate the uncertainty in internal dose rate caused by the variability of fish mass and fish concentration in real environments. It was found that the variability in Cs-137 activity concentration in different fish was the most important factor contributing to the uncertainty of predictions of internal dose rate. Organism size at different life stages also has a large influence on dose. For young, small, fish, the internal dose is lower than the range of internal doses estimated by the different models, whilst the external dose could be greater than that estimated by models, especially in cases where part of the life cycle is spent on the sediment surface. Using measured values of tissue activity concentration, the model predictions of doses in Svyatoye and Perstok lakes, show good agreement with each other. In cases when tissue activity concentration data are not available, these have to be estimated from the tissue-water Concentration Ratio (CR). In this case, the predicted internal dose rates (using radionuclide concentration in the water) are more uncertain because of the variation of CR in the models. The model where CR is estimated depending on water chemistry gives the best prediction in this case. Measurements were made of the beta- and gamma- dose rate in three lakes (Perstok, Svyatoye and Dvorishche) giving profiles of dose as a function of depth. The method for gamma dose rate measurement worked well and a simple model for external gamma dose rate with depth above and below the sediment surface was found to give reasonable agreement with measured values. The beta in situ measurement was not successful and the method would need further investigation. The modelling studies carried out showed that internal dose rates to fish in Svyatoye and Perstok lakes are lower than or close to the 40 μGy hr-1 recommended limit for possible impacts from radiation .The external dose rate to insect larvae and benthic fish is much higher than to pelagic fish because the former live in or on the sediment which has a much higher activity concentration than the water. None of the estimated external dose rates in these lakes was higher than 10 μGy hr-1. It is concluded that the external dose rates to benthic biota and large fish in these closed lakes are still significant at this long time after the Chernobyl accident. But, radiation effects on these organisms may not be clearly seen, since the dose rates are below or close to guideline limits.
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