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LOT: in situ diffusion experiments using radioactive tracers
KTH, Tidigare Institutioner, Kemi. KTH, Skolan för kemivetenskap (CHE), Kemi, Kärnkemi.
KTH, Skolan för kemivetenskap (CHE), Kemi, Kärnkemi.
KTH, Tidigare Institutioner, Kemi. KTH, Skolan för kemivetenskap (CHE), Kemi, Kärnkemi.
2003 (engelsk)Inngår i: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 23, nr 1-4, s. 77-85Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
2003. Vol. 23, nr 1-4, s. 77-85
Emneord [en]
caesium, cobalt, diffusion, in situ, LOT
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-13489DOI: 10.1016/S0169-1317(03)00089-9ISI: 000186093500010Scopus ID: 2-s2.0-0041387463OAI: oai:DiVA.org:kth-13489DiVA, id: diva2:325830
Merknad
QC 20100621 NR 20140804Tilgjengelig fra: 2010-06-21 Laget: 2010-06-21 Sist oppdatert: 2020-03-10bibliografisk kontrollert
Inngår i avhandling
1. Diffusion of Radionuclides in Bentonite Clay: Laboratory and in sity Studies
Åpne denne publikasjonen i ny fane eller vindu >>Diffusion of Radionuclides in Bentonite Clay: Laboratory and in sity Studies
2002 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Stockholm: KTH, 2002. s. x, 53
Serie
Trita-KKE, ISSN 0349-6465 ; 0202
Emneord
Bentonite, montmorillonite, cation, anion, diffusion, sorption, cobalt, caesium, strontium, iodide, pertechnetate, surface diffusion, in situ
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-3443 (URN)
Disputas
2002-12-06, 00:00
Merknad
QC 20100621Tilgjengelig fra: 2002-12-03 Laget: 2002-12-03 Sist oppdatert: 2010-06-21bibliografisk kontrollert

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