Mira Markovaara-Koivisto, and 4 co-authors (2009) report that understanding uranium retention processes is essential when assessing the safety of a spent nuclear fuel repository as uranium forms over 95% of the spent fuel. It is crucial to establish whether migration mainly occurs continuously through slow diffusion, as usually postulated, or if groundwater-mediated transport occurs episodically, for example, during periods of intense, glacial recharge. This study provides insights into the principal retention mechanisms of natural uranium in granitic bedrock at Askola in Southern Finland.
The morphology and chemical composition of uranium minerals, found on the surfaces of water-conducting fractures (’open fractures’) as well as in the matrix adjacent to fractures, were investigated in polished rock slabs and thin sections optically, by scanning electron microscopy (SEM) and by energy and wavelength dispersive spectrometers (EDS and WDS). In addition, the radiogenic isotopes were analyzed in situ by inductively coupled plasma mass spectrometry coupled to a high-performance Nd:YAG deep UV (213 nm) laser ablation system.
The predominant uranium ore mineral at Askola was identified as uraninite in studies carried out from 1963-1979, occurring as primary grains in the rock matrix. In the present study, abundant secondary uranium phases were found in micro-fractures, typically associated with goethite, pyrite and chalcopyrite. Chemical analyses indicate an assemblage of uranium (VI) silicates and hydroxides; phosphate is also present. Uranium (VI) hydroxides were found to fill micro-fractures propagating from open fractures to depths of several millimetres. If released into the bedrock, U would be expected to precipitate in such micro-fractures, preferentially in association with the various iron minerals present.
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