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SOC degradation. These uranium(IV) minerals have been stable and immobile over geological time
frames.
Geochemical calculations with GWB [14] and revised NEA TDB [15] confirmed that U(IV) is the
preferential oxidation state in the clay/lignite layers. These calculations also indicate that the redox
conditions in the clay/lignite horizon are controlled by the SO4 2- /S 2- couple. Uranium concentrations
in the clay/lignite layer observed today are determined by amorphous UO2 and ningyoite. One important
issue which could not be clarified is, whether the more accessible U fraction found by
U(IV)/U(VI) separation really exists in the hexavalent state or has been oxidised after sampling.
4. Conclusions
This natural analogue study contributed to the Safety Case for the far-field transport in sedimentary
layers in different ways. A very important part was the aspect of method development and testing,
e.g. colloid sampling under undisturbed conditions, first application to natural samples and further
development of μ-XRF and μ-XANES as well as application of modern isotope analyses like � 34 S
signatures to identify relevant processes in the field. All these methods are important for characterisation
of a potential repository site including lab and field experiments.
Although Ruprechtov site turned out to be rather complex with regard to hydrogeology and geological
evolution, the mechanisms for immobilisation of uranium have been identified. By application
of a set of different microscopic and macroscopic analytical methods distinct immobile uranium
phases have been characterised and their long-term stability was shown. The results further
indicate that DOC does not contribute to mobilisation of U because of the relatively low DOC concentration
in the clay/lignite layer. DOC is formed by microbial degradation of SOC in the
clay/lignite layers but only a very small fraction of SOC seems to be accessible.
In general it was shown that sedimentary layers can provide a strong barrier function for uranium,
when specific prerequisites are fulfilled. Under the strongly reducing conditions in the clay/lignite
layers at Ruprechtov site, there are no indications for significant uranium release during the last
million years. The low uranium concentrations in the groundwater of app. 10 -9 mol/l are determined
by amorphous UO2 and ningyoite.
5. Acknowledgements
This project has been co-funded by the European Commission and performed as part of the sixth
Euratom Framework Programme for nuclear research and training activities (2002-2006) under contract
FI6W-CT-2004-516514, by the German Federal Ministry of Economics and Technology
(BMWi) under contract no 02E9995, and by RAWRA and Czech Ministry of Trade and Industry
(Pokrok 1H-PK25).
References
[1] Noseck, U., Brasser, Th., Rajlich, P., Laciok, A., Hercik, M., (2004). Mobility of uranium in
Tertiary argillaceous sediments - a natural analogue study. Radiochim. Acta 92, 797-803.
[2] Noseck, U., Rozanski, K., Dulinski, M., Havlova, V., Sracek, O., Brasser, Th., Hercik, M.,
Buckau, G., (2008). Characterisation of hydrogeology and carbon chemistry by use of natural
isotopes – Ruprechtov site, Czech Republic. Appl. Geochem. (in prep.).
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