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Long term redox evolution in granitic rocks: modelling the redox front propagation in the rock matrix
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
2007 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 22, no 11, 2381-2396 p.Article in journal (Refereed) Published
Abstract [en]

Redox evolution in groundwaters in granitic rocks is considered to be largely due to reactions between dissolved species in the groundwater and minerals that line the walls of fractures in the bedrock. In a long term perspective, however, the reducing fracture minerals eventually become depleted. In this situation, O-2 diffuses out from the fractures and reacts with reducing species present in the porous rock matrix. The rock matrix contains the main reservoir of reducing capacity in the form of reducing minerals such as biotite. The aim of this work is to emphasize the coupled transport and reaction processes deemed important for the redox evolution in recharge groundwaters over long times, i.e. thousands of years. Results indicate that matrix diffusion of O-2 becomes limiting for the redox reactions rapidly after the reducing capacity of the fractures is depleted. The numerical model results are verified by simplified cases solved analytically. From the analytical solutions insights also are gained into when different mechanisms dominate the overall reaction, which is more difficult if only numerical results are available.

Place, publisher, year, edition, pages
2007. Vol. 22, no 11, 2381-2396 p.
Keyword [en]
Groundwater; Minerals; Numerical methods; Redox reactions; Granitic rocks; Redox evolution; Redox front propagation; Rock matrix; Rocks; biotite; granite; groundwater; numerical model; oxygen; recharge; redox conditions; transport process
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-7488DOI: 10.1016/j.apgeochem.2007.05.007ISI: 000251201900009Scopus ID: 2-s2.0-35348958339OAI: oai:DiVA.org:kth-7488DiVA: diva2:12527
Note
QC 20100818. Uppdaterad från In press till Published 20100818.Available from: 2007-09-19 Created: 2007-09-19 Last updated: 2010-08-18Bibliographically approved
In thesis
1. Modelling long-term redox processes and oxygen scavenging in fractured crystalline rocks
Open this publication in new window or tab >>Modelling long-term redox processes and oxygen scavenging in fractured crystalline rocks
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [sv]

Under de senaste decennierna har i många länder, inkluderat Sverige, långt gångna planer utvecklats för ett djupförvar av högaktivt kärnavfall. Som en del i det svenska förvarskonceptet skall avfallet inneslutas i kopparkapslar som omslutes av återfyllnadsmaterial med mycket låg genomsläpplighet av vatten. Dessutom skall kapslarna förläggas på ca 500 meters djup i granitberggrunden, vilken fungerar som en naturlig barriär för transport av radionuklider till markytan. Dessa naturliga och konstruerade barriärer väljs och utformas så att förvarets funktion kan säkras under hundra tusentals år. En viktig fråga för säkerhetsanalysen för ett sådant förvar är utvecklingen av redoxförhållandena över långa tider. Korrosionen av kopparkapslarna går fortare under oxiderande förhållanden och rörligheten hos frisläppta radionuklider ökar.

I den första delen av avhandlingen studerades förmågan hos berget att upprätthålla reducerande förhållanden på förvarsdjupet över långa tider. En modellstruktur har utvecklats med målet att ta hänsyn till alla processer som bedömts viktiga för utarmning av inträngande syre från markytan över långa tider. Förenklingar introducerades så att transparenta analytiska lösningar kunde erhållas som förenklar utvärdering av resultat och tillåter identifiering av osäkra parametrar. Komplexa system löstes numeriskt för fall då analytiska lösningar ej kunde erhållas, samt för att validera förenklingar som ligger till grund för de analytiska lösningarna. Resultat redovisades för rådande förhållanden samt för förhållanden som bedömdes rimliga under smältfasen av en glaciationsperiod. Det visade sig att de hydrauliska egenskaperna har stor påverkan på inträngningsdjupet av syre längs flödesvägar i berget. Kvoten mellan den flödesvätta ytan och flödet visade sig vara en viktig parameter som bestämmer omfattningen av interaktionen mellan löst syre i grundvattnet och reducerande mineral i berget.

Resultaten visar att för korta tider, beroende på mängd reducerande mineral och reaktionshastighet, kan kemisk reaktionskinetik bestämma förbrukningshastigheten av syre. För längre tider begränsas förbrukningen av intern diffusion i stora partiklar och bergmatrisen. Det visade sig att hänsyn måste tas till många osäkerheter för att kunna göra tillförlitliga kvantitativa uppskattningar av omfattningen av syreinträngningen.

I den andra delen av avhandlingen undersöktes konsekvenserna av inträngande syre på korrosion av kopparkapslarna. En mekanism föreslogs också för bildande av sulfid nära kapseln. Sulfid är en annan korrodent som kan bildas mikrobiellt i reducerande miljöer från sulfat och en organisk reduktant såsom metan. Beräkningarna visar att mer än 50 kg av koppar inte är sannolikt att korrodera över en miljon år.

Abstract [en]

Advanced plans for the construction of a deep geological repository for highly radioactive wastes from nuclear power plants have evolved during the past decades in many countries including Sweden. As part of the Swedish concept, the waste is to be encapsulated in canisters surrounded by low permeability backfill material. The copper canisters will be deposited at around 500 metres depth in granitic rock, which acts as a natural barrier for the transport of radionuclides to the ground surface. These natural and engineered barriers are chosen and designed to ensure the safety of the repository over hundred of thousands of years. One issue of interest for the safety assessment of such a repository is the redox evolution over long times. An oxidising environment would enhance the corrosion of the copper canisters, and increases the mobility of any released radionuclides.

In the first part of the present thesis, the ability of the host rock to ensure a reducing environment at repository depth over long times was studied. A model framework was developed with the aim to capture all processes that are deemed to be important for the scavenging of intruding oxygen from the ground surface over long times. Simplifications allowing for analytical solutions were introduced for transparency reasons so that evaluation of results is straight-forward, and so that uncertain parameter values easily can be adjusted. More complex systems were solved numerically for cases when the analytical simplifications are not applicable, and to validate the simplifications underlying the analytical solutions. Results were presented for prevailing present day conditions as well as for conditions deemed to be likely during the melting phase of a period of glaciation. It was shown that the hydraulic properties have a great influence on the oxygen intrusion length downstream along flow-paths in the rock. An important parameter that determines the extent of interaction between the dissolved oxygen and the reducing minerals in the rock was shown to be the flow-wetted surface to flow-rate ratio.

The results show that for an initial period of time, depending on the amount of reducing minerals and reaction rates, chemical reaction kinetics may control the rate of the overall depletion of oxygen. For longer times, internal diffusion resistance in large particles or in the rock matrix become rate limiting for the overall process. It was found that there are many uncertainties that have to be considered in order to make reliable quantitative predictions on the extent of oxygen intrusion.

In the second part of the thesis, the impact of intruding oxygen on the corrosion of the copper canisters was explored. Also, a mechanism for the production of sulphide close to the deposition holes was studied. Sulphide is another corroding agent that may be produced microbially in a reducing environment from sulphate in the presence of organic reductants such as methane. From calculation results it was found that corrosion of more than 50 kg of copper is not likely over a period of one million years

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. viii, 81 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:60
Keyword
Redox chemistry, hydrogeological model, groundwater chemistry, mineral interaction
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-4491 (URN)978-91-7178-756-9 (ISBN)
Public defence
2007-10-15, F3, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100818Available from: 2007-09-19 Created: 2007-09-19 Last updated: 2010-08-18Bibliographically approved

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