Fracture smectite as a long-term sink for natural radionuclides: Indications from unusual U-series disequilibria

H. Marcos (Corresponding Author), J. Suksi, H. Ervanne, Kari Rasilainen

Research output: Contribution to journalArticleScientificpeer-review

6 Citations (Scopus)

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 languageEnglish
Pages (from-to)763 - 766
Number of pages4
JournalRadiochimica Acta
Volume88
Issue number9-11
DOIs
Publication statusPublished - 2000
MoE publication typeA1 Journal article-refereed
Event7th International Conference on Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere - Lake Tahoe, United States
Duration: 26 Sep 19991 Oct 1999

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montmorillonite
sinks
Radioisotopes
radioactive isotopes
indication
Isotopes
isotopes
Bentonite
spent fuels
bentonite
nuclear fuels
Spent fuels
Chemical stability
Nuclear fuels
granite
axioms
ground water
Minerals
Groundwater
near fields

Cite this

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title = "Fracture smectite as a long-term sink for natural radionuclides: Indications from unusual U-series disequilibria",
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.",
author = "H. Marcos and J. Suksi and H. Ervanne and Kari Rasilainen",
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Fracture smectite as a long-term sink for natural radionuclides : Indications from unusual U-series disequilibria. / Marcos, H. (Corresponding Author); Suksi, J.; Ervanne, H.; Rasilainen, Kari.

In: Radiochimica Acta, Vol. 88, No. 9-11, 2000, p. 763 - 766.

Research output: Contribution to journalArticleScientificpeer-review

TY - JOUR

T1 - Fracture smectite as a long-term sink for natural radionuclides

T2 - Indications from unusual U-series disequilibria

AU - Marcos, H.

AU - Suksi, J.

AU - Ervanne, H.

AU - Rasilainen, Kari

PY - 2000

Y1 - 2000

N2 - 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.

AB - 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.

U2 - 10.1524/ract.2000.88.9-11.763

DO - 10.1524/ract.2000.88.9-11.763

M3 - Article

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JO - Radiochimica Acta

JF - Radiochimica Acta

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