Methods for the determination of low-level actinide concentrations and their behaviour in the aquatic environment

Intentional and accidental releases have contaminated the environment with radionuclides, posing a potential health hazard to populations of the polluted regions. Low concentrations of the actinides in the environment and complex sample matrices have made their determination a time consuming and complicated task. Separation methods based on anion exchange and extraction chromatography were developed, and subsequently modified, for analysis of different sample matrices in this work. These methods were used for the investigations of the behaviour of actinides in the environment. Chemical properties play an important role in the phenomena affecting the migration of radionuclides. The method based on anion exchange was used to study the behaviour of U in a small U-Th deposit and also the behaviour of Pu, Am and Cm in a lake system after the Chernobyl accident. The speciation of U and Pu in natural waters has also been investigated. A trend of higher 234U/238U activity ratios with lower 238U concentrations was seen in the ground waters in the Palmottu analogue study site in southern Finland. This indicates chemical leaching of U(VI) in oxidising conditions and preferable dissolution of 234U due to the recoil effects of the alpha decay in reducing conditions. The factors affecting the distribution of U concentrations and the 234U/238U activity ratios in filtered ground water and the particulate fraction in the Palmottu are also discussed. The concentrations of Pu, Am and Cm in filtered water, particulate and surface sediment samples in Lake Paeijaenne in southern Finland have been determined. Pu, Am and Cm fallout from the Chernobyl accident was minor compared to global fallout from atmospheric nuclear weapon tests. Based on the 238Pu/239,240Pu isotopic ratio, only 10 % of the Pu in the surface layer of the bottom sediment derived from the Chernobyl accident. Three months after the accident, 73 % of the total 239,240Pu concentration was in the surface layer of the bottom sediment (0-1 cm), 24 % in filtered water and 3 % in the particulate fraction (diameter > 0.45 mm). The redox equilibrium of Pu in lake and sea waters studied here is dependent on the amount of organic substances and the presence of H2S in these waters. In sea water, Pu(III+IV) dominates only when H2S is present in reducing conditions whilst in lake waters, 73-82 % of the Pu exists in the lower oxidation states due to the stabilisation of Pu(III+IV) with organic substances. In this work the main property affecting the speciation of U in ground waters is the salinity. U is present as U(IV) only in highly saline ground waters where reducing conditions prevail. In low salinity waters, even in reducing conditions with low oxygen concentrations, 83-98 % of U was in the oxidised form. The method based on extraction chromatography was successfully applied during the certification process of Th, U, Pu and Am concentrations in bone ash samples, prepared by the National Institute of Standards and Technology. This rapid and simple method, when compared to the anion exchange procedure, produces dramatically less waste during the separation and purification of each actinide element. Pure spectra, coupled with high and reproducible recoveries, have been achieved for Th, U, Pu, Am and Cm. The mean recoveries for five replicate analyses of bone ash were 89 ± 3%, 86 ± 5%, 81 ± 5% and 87 ± 4% for Th, U, Pu and Am, respectively. (orig.)