Environmental impact of noble metal radionuclides and their use in nuclear forensics

Abstract

This dissertation focuses on the development of analytical protocols for the measurement of difficult-to-measure noble metal radionuclides and the detection of minute amounts of these nuclides. The noble metal radionuclides of interest were radiosilver Ag-108m and radiopalladium Pd-107, which have a considerably long half-life of 438 a and 6.5∙10^6 a, respectively. For these studies, a variety of analytical separation and enrichment procedures in combination with radiometric and mass spectrometric methods were employed. The results of these studies are published in three publications. During previous work, low activity concentrations of the radiosilver isotopes 108mAg and Ag-110m were found in seafood samples from a close proximity of the Fukushima Daiichi NPP. Increased detection limits due to concomitant gamma emitters such as Cs-134 and Cs-137 made the low-level measurements of radiosilver difficult. Nevertheless, the determination of radiosilver after a nuclear accident is of interest as it possibly can give information on the condition of control rods of PWRs or other core components. For this reason, the first publication deals with the development of a fast and precise analytical separation protocol of minute amounts of Ag-108m in the presence of magnitudes higher concentrations of other gamma emitters. The autodeposition of radiosilver on a copper plate has proven as simple and easily adaptable for almost all laboratories, for aqueous and organic samples. The autodeposition was combined with a subsequent gamma spectrometric measurement with a significantly reduced Compton background. As a further aspect, the fate of freshly deposited radiosilver and –cesium was investigated in various soil matrices. Radiosilver proved to bind strongly with the residual fraction, however, organic compounds can elevate silver’s mobility and increasing its bioavailability. In the second publication, a case study of the bioavailability of cesium and silver in edible Shiitake mushrooms was performed. Radiocesium showed a bioaccumulation in the shiitake mushroom and autoradiography revealed that the main Cs-137 activity was concentrated in the hymenium. In contrast, radiosilver was not actively taken up by the fruit body and showed a depletion towards the surrounding substrate. LA-ICP-MS measurements revealed possible sorption effects of silver on the outer parts of the fruit body during its growth. Not only can radiosilver measurements be interfered by other gamma emitters, the stable silver isotope Ag-107 itself can interfere the difficult-to measure long-lived radionuclide Pd-107. The third publication’s main subject was the development of an analytical separation protocol and subsequent ultra-trace ICP-QQQ-MS measurement of scarcely studied radiopalladium isotope Pd-107. The combination of a Pd extraction with cation exchange resin and Ni Resin and the use of an ICP-QQQ-MS in propane/He gas mode allowed the measurement of sub-ng∙kg^-1 palladium isotope concentration with reduction of major interferences. With the developed method, an enhanced isotopic Pd-107/Pd-105 ratio could be determined in a Chernobyl cooling pond sediment sample for the first time.