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EU-STRA-Strålevern

Radioactive particle transformation processes

Awarded: NOK 0.32 mill.

A major fraction of refractory radionuclides such as Uranium (U) and Plutonium (Pu) released to the environment from the nuclear weapon and fuel cycles is present as particles ranging from sub microns to fragments. Such particles can carry a substantial amount of radioactivity and associated metals, and can act as point sources. Research indicates that radionuclides associated with the particles behave differently in the environment than those that are present in mobile forms such as ions. Particle characteristics such as composition, atom and elemental ratios depend on the emitting source, while particle size, structure and oxidation states are closely linked to releases scenarios. Following deposition, ecosystem transfer of particle associated radionuclides are delayed compared to mobile radionuclide species; i.e., ecosystem transfer would be delayed until particle weathering and remobilization of associated radionuclides occur. The apparent soil-water distribution coefficient (Kd) will therefore change over time, and the thermodynamic constant concept should be replaced with rate functions. This project is the NMBU contribution to the EU COMET project. The project has focused on particle characteristics, transformation rates, remobilization and prediction of ecosystem transfer of radionuclides associated with U and/or Pu containing particles originating from selected key sources such as nuclear weapon tests, safety tests, nuclear reactor accidents and NORM. Within the project, a particle database from archived nano - and micrometer sized radioactive particles released from different sources was produced. New state-of-the-art methods involving advanced micro- and nano-analytical techniques dedicated to characterize radioactive particles were developed, enabling the quantification of transformation of particles upon exposure to leaching media. Thus, it is possible to measure the transformation rates for mobilization of radionuclides associated with particles to more bioavailable forms. The results from leaching experiments show that particle weathering and leaching depend on particle properties such as the oxidation state of the carrying matrix and that particle retention in soils and sediments delay the ecosystem transfer compared to transfer for a contamination of radionuclides on ionic forms. These data are important for improving the models used to predict the impact of radioactive contamination in the environment. Because radioactive particles released to the environment can be retained in biota, new state-of-the-art methods were developed by NMBU and partners for detection and localization of low-level radioactive particles within organisms. The results show that uptake of particles in organisms such as snails and mussels occur, and particles retained in tissues act as point sources. The phenomenon of retention of radioactive particles in biota was demonstrated both in samples collected within contaminated ecosystems as well as in laboratory experiments. The novelty of the present project is the focus on particle deposition and time dependent remobilization of associated radionuclides, and that constants should be replaced with time-functions to improve their use in dose, impact and risk assessment models.

A major fraction of refractory radionuclides such as U and Pu released to the environment from the nuclear weapon and fuel cycles is present as particles ranging from submicrons to fragments. Such particles can carry a substantial amount of radioactivity (e.g., fission and activation products, transuranics) and associated metals, and can act as point sources. Furthermore, U particles (progenies, metals) are present at NORM sites. Research indicates that particle characteristics such as composition, atom a nd element ratios depend on the emitting source, while particle size, structure and oxidation states are closely linked to the release scenarios. Following deposition, ecosystem transfer of particle associated radionuclides are delayed compared to mobile radionuclide species; i.e., ecosystem transfer would be delayed until particle weathering and remobilisation of associated radionuclides occur. The apparent soil-water distribution coefficient (Kd) will therefore change over time, and the thermodynamic co nstant concept should be replaced with rate functions. Thus, the present project focuses on particle characteristics, weathering rates, remobilization and prediction of ecosystem transfer of radionuclides associated with U and/or Pu containing particles o riginating from selected key sources (nuclear weapons tests, safety tests, conventional detonation of nuclear or DU weapons, nuclear reactor accident, NORM). Utilizing advanced techniques, leaching experiments in which well characterized particles are exp osed to abiotic and biotic degradation agents will be performed under different temperature conditions of relevance for the Arctic and for temperate zones. The parameterization should reduce uncertainties in model predictions on ecosystem transfer and env ironmental impact associated with particle contaminated areas, linking particle properties to sources, and linking particle properties to weathering rates under different environmental conditions.

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EU-STRA-Strålevern