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Diffusion of Radionuclides in Bentonite Clay: Laboratory and in sity Studies
KTH, Superseded Departments, Chemistry.
2002 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis deals with the diffusion of ions in compactedbentonite clay. Laboratory experiments were performed toexamine in detail different processes that affect thediffusion. To demonstrate that the results obtained from thelaboratory investigations are valid under in situ conditions,two different kinds of in situ experiments were performed.

Laboratory experiments were performed to better understandthe impact of ionic strength on the diffusion of Sr2+ and Cs+ions, which sorb to mineral surfaces primarily by ion exchange.Furthermore, surface related diffusion was examined anddemonstrated to take place for Sr2+ and Cs+ but not for Co2+,which sorbs on mineral surfaces by complexation.

The diffusion of anions in bentonite clay compacted todifferent dry densities was also investigated. The resultsindicate that anion diffusion in bentonite clay consists of twoprocesses, one fast and another slower. We ascribe the fastdiffusive process to intralayer diffusion and the slow processto diffusion in interparticle water, where anions are to someextent sorbed to edge sites of the montmorillonite.

Two different types of in situ experiments were performed,CHEMLAB and LOT. CHEMLAB is a borehole laboratory, where cation(Cs+, Sr2+ and Co2+) and anion (I- and TcO4-) diffusionexperiments were performed using groundwater from a fracture inthe borehole. In the LOT experiments cylindrical bentoniteblocks surrounding a central copper rod were placed in a 4 mdeep vertical borehole. The borehole was then sealed and theblocks are left for 1, 5 or>>5 years. When the bentonitewas water saturated the central copper rod is heated tosimulate the temperature increase due to radioactive decay ofthe spent fuel. Bentonite doped with radioactive Cs and Co wasplaced in one of the lower blocks.

Interestingly, the redox-sensitive pertechnetate ion (TcO4-)which thermodynamically should be reduced and precipitate asTcO2·nH2O, travelled unreduced through the bentonite.However, at some spots in the clay, the Tc activity wasconsiderably higher. We ascribe these activity peaks toiron-containing minerals in the bentonite, by which Tc(VII) hasbeen reduced to Tc(IV) and precipitated. The cations Sr2+, Cs+and Co2+, as well as the anion I-, behaved in the CHEMLABexperiments as expected from laboratory studies.

Three experiments in the LOT series are completed. The firsttwo diffusion experiments in LOT were less successful, thefirst due to the fact that saturation of the bentonite was notobtained during the experimental period and the radionuclidesdid not move at all. In the second, the uptake of the bentoniteparcel was less successful. Water from the drilling flushedaway large pieces of the top part of the bentonite and thelower part of the test parcel was super-saturated with waterand expanded when released from the rock.

The activity distribution in the second experiment wasanalysed. The Co2+ profile looked as we had expected, while Cs+had spread more than our calculations indicated. However, thethird experiment was successful from emplacement, watersaturation and heating to uptake. The activity distribution forboth cations was as expected from laboratory studies.

Altogether the three different types of experiments give auniform picture of radionuclide diffusion in bentonite clay forthe ions investigated.

Place, publisher, year, edition, pages
Stockholm: KTH , 2002. , x, 53 p.
Series
Trita-KKE, ISSN 0349-6465 ; 0202
Keyword [en]
Bentonite, montmorillonite, cation, anion, diffusion, sorption, cobalt, caesium, strontium, iodide, pertechnetate, surface diffusion, in situ
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-3443OAI: oai:DiVA.org:kth-3443DiVA: diva2:9243
Public defence
2002-12-06, 00:00
Note
QC 20100621Available from: 2002-12-03 Created: 2002-12-03 Last updated: 2010-06-21Bibliographically approved
List of papers
1. Diffusion of I, CS, and SR2- in compacted bentonite-anion exclusion and surface diffusion
Open this publication in new window or tab >>Diffusion of I, CS, and SR2- in compacted bentonite-anion exclusion and surface diffusion
1996 (English)Report (Other academic)
Series
SKB TR, 96-16
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13481 (URN)
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-18 Last updated: 2010-07-15Bibliographically approved
2. CHEMLAB - in situ diffusion experiments using radioactive tracers
Open this publication in new window or tab >>CHEMLAB - in situ diffusion experiments using radioactive tracers
1998 (English)In: Radiochimica Acta, ISSN 0033-8230, E-ISSN 2193-3405, Vol. 82, 153-156 p.Article in journal (Refereed) Published
Abstract [en]

In situ diffusion experiments in bentonite clay have been carried out in the probe CHEMLAB, using the radioactive tracers Co-57, Sr-85 and Cs-134. The measured diffusion profiles for the radionuclides are in good agreement with predicted profiles obtained from modelling based on apparent diffusivities and sorption coefficients obtained in laboratory experiments with corresponding clay density and ground water composition.

Keyword
in situ experiment, radionuclide, diffusion, cation
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13483 (URN)000077683000025 ()
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-18 Last updated: 2017-12-12Bibliographically approved
3. Sorption effects on cation diffusion in compacted bentonite
Open this publication in new window or tab >>Sorption effects on cation diffusion in compacted bentonite
1999 (English)In: Engineering Geology, ISSN 0013-7952, E-ISSN 1872-6917, Vol. 54, no 1-2, 231-236 p.Article in journal (Refereed) Published
Abstract [en]

Diffusion of Na+. Cs+, Co2+ and Sr2+ in bentonite compacted to a dry density of 1800 kg m(-3) and saturated with groundwaters and aqueous solutions of differing ionic strength have been studied experimentally using the through diffusion technique. Sorption experiments have been carried out under a wide range of pH and concentration of supporting electrolyte. The dependence of the apparent diffusivity of Na+, Cs+ and Sr2+, mainly sorbed by ion exchange, on the sorption intensity is accommodated by a model encompassing diffusion of the sorbed cations within the electrical double layer next to the mineral surface in addition to diffusion in the pore water. The apparent diffusivity of Co2+, sorbed by surface complexation reactions, corresponds to complete immobilization on sorption.

Keyword
bentonite, caesium, cation, cobalt, diffusion, sodium, sorption, strontium
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13484 (URN)10.1016/S0013-7952(99)00078-2 (DOI)000082578000027 ()
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-18 Last updated: 2017-12-12Bibliographically approved
4. Long term test of buffer material: Final report on the pilot parcels
Open this publication in new window or tab >>Long term test of buffer material: Final report on the pilot parcels
Show others...
2000 (English)Report (Other academic)
Series
SKB Technical report, 00-22
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13485 (URN)
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-18 Last updated: 2010-06-21Bibliographically approved
5. Anion diffusion pathways in bentonite clay compacted to different dry densities
Open this publication in new window or tab >>Anion diffusion pathways in bentonite clay compacted to different dry densities
2003 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 23, no 1-4, 69-76 p.Article in journal (Refereed) Published
Abstract [en]

Diffusion of the anions Cl- and I- in MX-80 compacted bentonite has been studied at different ionic strengths (0.01, 0.1 M NaClO4) and clay density (0.4, 0.8, 1.2, 1.8 g cm(-3)) at the buffered pH of bentonite 8.2 using a through-diffusion technique with measurement of breakthrough curves and concentration profiles in the bentonite. Apparent diffusivities and capacity factors (alpha=epsilon+rhoK(d)) are obtained from diffusion simulations using the computer code ANADIFF. Two diffusion processes, both with density and ionic strength dependent apparent diffusivities and capacity factors, were observed. The diffusion processes observed are ascribed to diffusion in intralayer and interparticle water. The experimental data indicate that intralayer water constitutes the dominating part of water in bentonite compacted to dry the densities 0.4-1.8 g cm(-3) studied in this work.

Keyword
chloride, iodide, diffusion, bentonite, anion exclusion, intralayer water structure, ORGANOPHILIC BENTONITES, KAOLINITE SUSPENSIONS, FISSION-PRODUCTS, RADIONUCLIDES, ADSORBENTS, ADSORPTION, SURFACE
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13491 (URN)10.1016/S0169-1317(03)00088-7 (DOI)000186093500009 ()
Note
QC 20100621Available from: 2010-06-21 Created: 2010-06-21 Last updated: 2017-12-12Bibliographically approved
6. LOT: in situ diffusion experiments using radioactive tracers
Open this publication in new window or tab >>LOT: in situ diffusion experiments using radioactive tracers
2003 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 23, no 1-4, 77-85 p.Article in journal (Refereed) Published
Abstract [en]

An experiment series at Aspo Hard Rock Laboratory, called "Long Term Test of Buffer Material", LOT, are carried out at Aspo Hard Rock Laboratory to validate models of clay buffer performance at standard KBS-3 repository conditions and to quantify clay buffer alteration processes at adverse conditions. In conjunction with the tests of the bentonite, cation diffusion tests using caesium and cobalt are performed. Each test contains 38 cylindrical blocks of bentonite clay with a hole in the middle which are placed around a copper rod in a vertically drilled hole at a depth of 450 m. In each test, four identical cylindrical bentonite plugs, doped with 1 MBq Co-57 and Cs-134, respectively, are inserted in the fifth block from the bottom. The system was left to be saturated with ground water before heating of the central copper rod started to simulate the thermal activity of radioactive waste. The experiments continued for 14 months before the bentonite blocks were drilled out using over-core drilling technique. The lowest blocks were cut up, sparsely in the outer layer, and in cubic centimeters, big samples closer to where the activity was inserted. All samples were analyzed using a gamma spectrometer to get a three-dimensional picture of the activity distribution. The system was then fitted to a diffusion model to obtain apparent diffusivities for the two cations. The apparent diffusivity for cobalt agrees well with those obtained in other in situ experiments and in laboratory studies, while the value for caesium is lower than expected. This can be due to that the clay was not fully water saturated during the experiment.

Keyword
caesium, cobalt, diffusion, in situ, LOT
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13489 (URN)10.1016/S0169-1317(03)00089-9 (DOI)000186093500010 ()
Note
QC 20100621 NR 20140804Available from: 2010-06-21 Created: 2010-06-21 Last updated: 2017-12-12Bibliographically approved
7. Anion Diffusion Experiments Using Radiotracers Under Representative Underground Conditions
Open this publication in new window or tab >>Anion Diffusion Experiments Using Radiotracers Under Representative Underground Conditions
(English)In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009Article in journal (Other academic) Submitted
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13490 (URN)
Note
QS 20120327Available from: 2010-06-21 Created: 2010-06-21 Last updated: 2017-12-12Bibliographically approved

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