Diffusive transport of uranium and americium through clay rock down to ultra-trace levels

Abstract. Clay rock is a potential host rock for the final disposal of nuclear waste in deep geological formations. In the scenario of ground water intrusion into the nuclear waste repository and subsequent corrosion of canisters and waste, possibly released actinides, such as uranium (U) and americium (Am), would be transported through the engineered barrier and clay host rock mainly by diffusion. Actinides are known to exhibit low solubility and are strongly sorbing under the reducing conditions of deep geological formations. Diffusion experiments are therefore difficult to perform due to analytical constraints. To our knowledge, the diffusion of U in clay rock has not been investigated below concentrations of 10-7molkg-1 clay (e.g., Joseph et al., 2013) and for Am, no diffusion experiments have been performed in a clay rock, considered suitable as host rock, such as Opalinus clay (OPA). This work aimed at the investigation of the diffusive behavior of U and Am down to ultra-trace concentrations (≪10-7molkg-1) in OPA. Laboratory-scale diffusion experiments were conducted with samples of OPA, obtained from the Mont Terri underground laboratory, Switzerland for up to 240 d. The OPA samples were placed in contact with synthetic pore water (I=0.22molL-1, pH=7.24), spiked with 5×10-9molL-1 of 233U and 243Am, respectively. After termination of the experiment, the OPA samples were segmented into thin layers of 20–400 µm. The obtained clay segments were analyzed for the 233U and 243Am content with accelerator mass spectrometry. Concentration profiles were determined down to 10-12molkg-1 clay for 233U and 5×10-10molkg-1 clay for 243Am. U showed a typical diffusion profile which can be interpreted by applying Fick's laws, while the Am profile was more complex, requiring further investigation of transport modes. These results will provide relevant insights into the transport behavior of U and Am in OPA over a wide range of concentrations down to ultra-trace levels.