Sorption and diffusion of neptunium in opalinus clay
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Abstract
This thesis is about the studies on the possible migration of the long-lived and hazardous
radionuclide Np from a deep geological repository for high-level nuclear waste (HLW) in
argillaceous host rock into the environment. The transuranium element 237Np is assumed to
outlive the technical barriers due to a half-life of 2.1 million years. In this regard, its retention
by the geological barrier has to be investigated intensively. For this purpose the sorption,
diffusion, and speciation of Np were studied in the naturally occurring Opalinus Clay (OPA),
from the “Felslabor Mont Terri” in Switzerland. To further investigate the specific sorption
behaviour of Np on clay mineral phases, sorption studies with kaolinite (KGa-1b) and Na-illite
(illite du Puy) were performed. These studies were funded by the Federal Ministry for Economic Affairs and Energy (BMWi)
as part of the joint research project “Retention of long-lived radionuclides in natural
argillaceous host rock and saline systems” to investigate the effects of temperature and salinity
on the sorption and diffusion of Np in OPA. Saline systems are of highest interest, as some of
the most suitable argillaceous host rock formations for a repository in Germany are located in
the north. The groundwaters in this region differ from those in southern Germany due to the
higher salinity. A further aim of this study was the investigation of the influence of the heat
dissipation of HLW on radionuclide retention. The temperature and salinity effects were
investigated in batch experiments using OPA clay powder and in-diffusion experiments using
massive OPA bore cores. Both experimental methods provided sorption data, whereby
additional diffusion parameters could be obtained by modeling the Np diffusion. Based on the redox-sensitivity of Np, knowledge of the speciation is most relevant for the
understanding of its migration and for more precise and reliable predictions thereof. Under
natural conditions the oxidation states +V and +IV are most important for Np, where Np(V)
can be seen as the mobile Np species and Np(IV), due to a lower solubility and stronger
sorption, as the immobile one. For this reason, the determination of the oxidation state during
the diffusion is of highest importance. The related speciation investigation was successfully
done with diffusion profiles of Np in OPA using spatially-resolved synchrotron radiation for
X-ray absorption near edge structure spectroscopy at the Swiss Light Source (PSI, Switzerland).
The combination of this data with the related chemical images of the diffusion profiles by Xray
fluorescence and X-ray diffraction patterns for analysis of specific crystal phases of interest
increased the understanding about occurring redox reactions during diffusion.