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The Bentonite Barrier: Microstructural properties and the influence of γ-radiation
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , iv, 81 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:38
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-34048ISBN: 978-91-7501-001-4OAI: oai:DiVA.org:kth-34048DiVA: diva2:418881
Public defence
2011-06-15, F2, entréplan, Lindstedtsvägen 28, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110608Available from: 2011-06-08 Created: 2011-05-24 Last updated: 2011-08-10Bibliographically approved
List of papers
1. Colloid diffusion in compacted bentonite: microstructural constraints
Open this publication in new window or tab >>Colloid diffusion in compacted bentonite: microstructural constraints
2010 (English)In: Clays and clay minerals, ISSN 0009-8604, Vol. 58, no 4, 532-541 p.Article in journal (Refereed) Published
Abstract [en]

In Sweden and in many other countries, a bentonite barrier will be used in the repository for spent nuclear fuel. In the event of canister failure, colloidal diffusion is a potential, but scarcely studied mechanism of radionuclide migration through the bentonite barrier. Column and in situ experiments are vital in understanding colloid diffusion and in providing information about the microstructure of compacted bentonite and identifying cut-off limits for colloid filtration. This study examined diffusion of negatively charged 2-, 5-, and 15-nm gold colloids in 4-month diffusion experiments using MX-80 Wyoming bentonite compacted to dry densities of 0.6–2.0 g/cm3. Breakthrough of gold colloids was not observed in any of the three diffusion experiments. In a gold-concentration profile analysis, colloid diffusion was only observed for the smallest gold colloids at the lowest dry density used (estimated apparent diffusivity Da 5x10–13 m2/s). The results from a microstructure investigation using low-angle X-ray diffraction suggest that at the lowest dry density used, interlayer transport of the smallest colloids cannot be ruled out as a potential diffusion pathway, in addition to the expected interparticle transport. In all other cases, with either greater dry densities or larger gold colloids, compacted bentonite will effectively prevent diffusion of negatively charged colloids due to filtration.

Keyword
Colloid Filtration • Compacted Bentonite, Gold Colloids, Diffusion, Microstructure, Nuclear Waste Disposal
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-25247 (URN)10.1346/CCMN.2010.0580408 (DOI)000289446500008 ()2-s2.0-78149471771 (ScopusID)
Note
QC 20101013. Tidigare titel: "Colloid Permeability of Compacted Bentonite: Microstructural Constraints".Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2011-06-08Bibliographically approved
2. Free Porosity of Compacted Saturated MX-80 Bentonite and Na-montmorillonite as a Function of Dry Density
Open this publication in new window or tab >>Free Porosity of Compacted Saturated MX-80 Bentonite and Na-montmorillonite as a Function of Dry Density
2009 (English)Article in journal (Refereed) Submitted
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-25250 (URN)
Note
QS 20120315Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2012-03-15Bibliographically approved
3. Effects of gamma-irradiation on the stability of colloidal Na+-Montmorillonite dispersions
Open this publication in new window or tab >>Effects of gamma-irradiation on the stability of colloidal Na+-Montmorillonite dispersions
2009 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 43, no 1, 86-90 p.Article in journal (Refereed) Published
Abstract [en]

In many concepts for final storage of spent nuclear fuel bentonite will be used as an engineered barrier, mainly due to its inertness, plasticity and ability to retard transport of radionuclides by adsorption. In the event of water-bearing fractures making contact with the bentonite barrier, generation and transport of colloidal particles will strongly depend on groundwater composition and the surface properties of the colloidal particles. The bentonite barrier will unavoidably be exposed to ionizing radiation from the spent nuclear fuel but very little is known about effects of ionizing radiation on bentonite concerning colloidal stability. In this work we have studied the effect of gamma-radiation on the stability of dilute colloidal Na+-montmorillonite dispersions using a Cs-137 gamma-source (doses of 0-53.2 kGy). Aggregation kinetics and sedimentation experiments revealed significant radiation effects, evident as increased colloid stability. The only rationale for this is a gamma-radiation induced increase in surface potential. The effects appeared to depend on the Na+-montmorillonite concentration in the irradiated dispersions, indicating that the change in surface potential is induced by aqueous radiolysis products.

Keyword
Radiation effects, gamma-radiation, Montmorillonite, Colloid stability, Bentonite
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-18118 (URN)10.1016/j.clay.2008.07.004 (DOI)000262658000013 ()2-s2.0-57649085839 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-06-08Bibliographically approved
4. Influence of γ-radiation on the reactivity of Montmorillonite towards H2O2
Open this publication in new window or tab >>Influence of γ-radiation on the reactivity of Montmorillonite towards H2O2
2012 (English)In: Radiation Physics and Chemistry, ISSN 0969-806X, Vol. 81, no 2, 190-194 p.Article in journal (Refereed) Published
Abstract [en]

Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH•, e-(aq), H•, H202, H2, H02•, H30+), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe(III) content in montmorillonite and its reactivity towards hydrogen peroxide (H2O2) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/FeTot ratio of dispersed montmorillonite are increased from ≤ 3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/FeTot ratio and the H2O2 decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H2O2 additions, since the structural Fe(II)/FeTot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H2O2.

Keyword
Bentonite, Montmorillonite, Radiation-induced effects, Hydrogen peroxide, Nuclear waste
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-34043 (URN)10.1016/j.radphyschem.2011.10.009 (DOI)000298977700014 ()2-s2.0-81255154501 (ScopusID)
Note
Updated from submitted to published 20120327. QC 20120327Available from: 2011-06-08 Created: 2011-05-24 Last updated: 2012-03-27Bibliographically approved
5. Effect of γ-radiation on Radionuclide Retention in Compacted Bentonite
Open this publication in new window or tab >>Effect of γ-radiation on Radionuclide Retention in Compacted Bentonite
2011 (English)In: Radiation Physics and Chemistry, ISSN 0969-806X, Vol. 80, no 12, 1371-1377 p.Article in journal (Refereed) Published
Abstract [en]

Compacted bentonite is proposed as an engineered barrier in many concepts for disposal of high level nuclear waste. After the initial deposition however, the bentonite barrier will inevitably be exposed to ionizing radiation (mainly gamma) under anoxic conditions. Because of this, the effects of gamma-radiation on the apparent diffusivity values and sorption coefficients in bentonite for Cs(+) and Co(2+) were tested under different experimental conditions. Radiation induced effects on sorption were in general more noticeable for Co(2+) than for Cs(+), which generally showed no significant differences between irradiated and unirradiated clay samples. For Co(2+) however, the sorption to irradiated MX80 was significantly lower than to the unirradiated clay samples regardless of the experimental conditions. This implies that gamma-radiation may alter the surface characteristics contributing to surface complexation of Co(2+). With the experimental conditions used, however, the effect of decreasing sorption was not large enough to be reflected on the obtained D. values.

Keyword
Diffusion, Sorption, Bentonite, Humics, Radionuclides, K(d)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-34045 (URN)10.1016/j.radphyschem.2011.08.004 (DOI)000295665100013 ()2-s2.0-80052968030 (ScopusID)
Note

QC 20111107 Updated from submitted to published.

Available from: 2011-06-08 Created: 2011-05-24 Last updated: 2015-02-13Bibliographically approved
6. Effects of the injection grout Silica sol on Bentonite
Open this publication in new window or tab >>Effects of the injection grout Silica sol on Bentonite
2011 (English)In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 36, no 17/18, 1580-1589 p.Article in journal (Refereed) Published
Abstract [en]

Silica sol, i.e., colloidal SiO2, may be used as a low-pH injection grout for very fine fractures in the construction of deep geological repositories for radioactive waste in Sweden and in Finland. If the bentonite barrier encounters SiO2-colloid particles under conditions favorable for aggregation, there is concern that it will modify the bentonite barrier at the bentonite/bedrock interface. In this study qualitative experiments were performed with mixed dispersions of SiO2-colloids and bentonite or homo-ionic Na/Ca-montmorillonite. Samples were prepared at different colloid concentrations and treated under various conditions such as low and high ionic strength (0.3 M NaCl), as well as dehydration and redispersing. Free swelling and settling experiments were performed in order to qualitatively compare the conditions in which SiO2-colloids affect the bulk/macro properties of bentonite. In order to study specific SiO2-colloid/montmorillonite interactions and preferred type of initial aggregation, dilute dispersions of homo-ionic montmorillonite dispersions mixed with varying concentrations of SiO2-colloids were prepared and selected samples were characterized by PCS, SEM/EDS, AFM and PXRD. The results from this study show that bentonite and montmorillonite particles can be modified by SiO2-colloids when mixed in comparable amounts, due to dehydration or high ionic strength. Some indications for increased colloidal stability for the SiO2-colloid modified clay particles were also found. From the AFM investigation it was found that initial attachment of the SiO2-colloids in Na+ dominated samples seemed to occur on the edges of the montmorillonite layers. In Ca2+ dominated samples not subjected to excess NaCl, SiO2-colloid sorption onto the faces of the montmorillonite layers was also found. In all, contact between the bentonite barrier and ungelled Silica sol should preferably be avoided.

Keyword
Bentonite, Bentonite colloids, Silica sol, Silica colloids, Colloidal stability, Colloid interactions
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-34046 (URN)10.1016/j.pce.2011.07.026 (DOI)000299195200016 ()2-s2.0-82955187651 (ScopusID)
Note
Updated from in press to published. QC 20120214Available from: 2011-06-08 Created: 2011-05-24 Last updated: 2012-04-02Bibliographically approved

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