Abstract
Occurrence of natural U in fracture smectite (main mineral component of
bentonite) was studied as an analogue to radionuclide behaviour in the
near-field of spent nuclear fuel repository. Elevated U content (57 ppm)
was observed in fracture smectite sampled from the surface of
water-carrying fracture in granite pegmatite at a depth of 70 m. The
current groundwater conditions are oxidising at the sampled point. The
U-234/U-238 activity ratio (AR) measured in the bulk U and in its
sequentially extracted phases, displays unusually low value (around
0.30). Low AR indicates preferential loss of the U-234 isotope from the
system. Because the U-234 loss can also be seen in the Th-230/U-234
activity ratio (clearly over 1), the selective removal of the U-234
isotope must have taken place more recently than what is needed to
equilibrate Th-230/U-234 pair (i.e. 350000 a). To explain the selective
U-234 loss from the smectite we postulate that bulk U is in reduced +4
form and a considerable part of the U-234 isotope in easily leachable
oxidised +6 form. This study suggests that the long-term chemical
stability of the bulk U in the smectite is due to irreversible fixation
of U in the reduced +4 form.
Original language | English |
---|---|
Pages (from-to) | 763 - 766 |
Number of pages | 4 |
Journal | Radiochimica Acta |
Volume | 88 |
Issue number | 9-11 |
DOIs | |
Publication status | Published - 2000 |
MoE publication type | A1 Journal article-refereed |
Event | 7th International Conference on Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere - Lake Tahoe, United States Duration: 26 Sep 1999 → 1 Oct 1999 |