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| Home > Publications database > Retention Mechanisms for 226Ra: solid-solution formation and adsorption |
| Home > Publications database > Retention Mechanisms for 226Ra: solid-solution formation and adsorption |
| Talk (non-conference) (Invited) | FZJ-2023-03741 |
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2023
Abstract: The fate of Ra, a decay product of 232Th, 235U, and 238U, in the environment is relevant because Ra is an important source of radioactivity in technically enhanced naturally occurring radioactive materials (TENORM). Ra-containing TENORM essentially originates from mining (e.g., uranium, phosphate) and milling operations, from coal ash, and raw material production processes e.g., oil extraction, geothermal energy production. 226Ra is a critical radionuclide in nuclear waste management, originating from e.g., nuclear industry in spent nuclear fuel or other wastes from processing uranium ore, past legacies from industry and medicine. 226Ra, is safety relevant for the deep geological disposal of spent nuclear fuel (SNF) because it is a long-lived alpha emitter (t1/2 = 1600 yrs) which dominates the radioactive dose in some scenarios regarding the late stages of SNF disposal. The prediction of 226Ra migration requires a quantitative process-understanding of the relevant retention mechanisms, i.e. adsorption, and uptake of 226Ra into a solid phase. Here we present results of experimental and theoretical studies on the structural uptake of 226Ra into the ternary solid solution of (Ba,Sr,Ra)SO4 due to the recrystallization of sulfates. This mechanism is relevant due to the fact that the solid-solutions are thermodynamically more stable than the pure phase RaSO4 and their formation leads to a lower solubility of 226Ra compared to the pure phase. The results show that already at time scales observable in the laboratory, pure sulfates are able to take up 226Ra and that their uptake can be described with thermodynamic solid-solution models. In addition, we present results regarding the adsorption of 226Ra on two important clay minerals, illite and montmorillonite. Our results show distinct differences between the selectivity of 226Ra adsorption to the two clay minerals, with a higher selectivity observed on illite. Parallel experiments with Ba indicate that 226Ra behaves similar in many geochemical conditions but is adsorbed stronger than Ba.
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