Migration of barium in crystalline rock : interpretation of in situ experiments

Käytetyn ydinpolttoaineen loppusijoituksen turvallisuusanalyysin sorptio- ja diffuusioparametrit on pääasiassa määritetty laboratorio-olosuhteissa. Ilmiöiden fysikaalinen käyttäytyminen voi kuitenkin olla erilaista eri mittakaavoissa, esimerkiksi laboratorio-olosuhteissa verrattuna luonnonolosuhteisiin. Tämä tunnetaan skaalaefektinä. Skaalaefektit voivat olla merkittäviä ja niillä voi olla merkitystä suuren mittakaavan mallinnuksissa, jotka käyttävät laboratoriokokeissa määritettyjä tietoja. Tämän väitöskirjan tavoitteena oli tutkia, kuinka laboratoriossa määritetyt sorptio- ja diffuusiotulokset voidaan skaalata in situ -olosuhteisiin. Tämä tehtiin määrittämällä 133Ba:n sorptio- ja diffuusioparametrit laboratoriokokeissa ja vertaamalla tuloksia Sveitsin Grimselissä toteutetun pitkäaikaisen in situ -diffuusiokokeen tuloksiin. Bariumin sorptiokäyttäytymistä tutkittiin tässä väitöskirjassa laboratoriossa tehdyissä eräsorptiokokeissa ja ohuthiesorptiokokeissa. Bariumin diffuusiota tutkittiin myös laboratoriodiffuusiokokeissa kivikuutioilla. Laboratoriokokeet tehtiin Grimselin ja Olkiluodon pääkivityypeille sekä pohjavesisimulanteissa, jotka tehtiin muistuttamaan Grimselin ja Olkiluodon pohjavesiolosuhteita. Bariumin sorptiotuloksia mallinnettiin PhreeqC-ohjelmalla ja diffuusiotulokset mallinnettiin PhreeqC-ohjelmalla sekä COMSOL Multiphysics -ohjelmalla. Lisäksi työssä kehitettiin uutta kvantitatiivista mittausmenetelmää bariumin aktiivisuuden jakautumisen mittaamiseen kivikuutioissa sekä ohuthieissä uudella elektronisella autografiamenetelmällä, BeaQuantTM:lla. Lopuksi 133Ba:n diffuusioprofiilit Grimselin in situ -diffuusiokokeesta määritettiin gammamittauksella ja autoradiografialla, ja tulokset mallinnettiin COMSOL Multiphysics -ohjelmalla. Tässä työssä selvitettiin, että bariumin laboratorio-olosuhteissa määritetyt sorptioparametrit ovat noin kaksikymmentäkertaiset verrattuna tuloksiin in situ -olosuhteissa. Tämä on tärkeää tietoa käytetyn ydinpolttoaineen loppusijoittamisen turvallisuusanalyysille, kun muunnetaan laboratoriokokeissa määritettyjä arvoja vastaamaan in situ- olosuhteita. Lisäksi työssä kehitettiin uutta elektronista autoradiografiamenetelmää, jotta sillä voitiin mitata 133Ba:n aktiivisuuden jakautumista graniittisissa kivissä kvantitatiivisesti.Käytetty ydinpolttoaine tullaan Suomessa loppusijoittamaan kallioperään rakennettuun loppusijoitustilaan. Loppusijoituksen turvallisuusanalyysissä käytetyt parametrit, kuten radioaktiivisten aineiden kulkeutumista kuvaavat sorptio- ja diffuusioparametrit, on pääasiassa määritetty laboratorio-olosuhteissa. Ilmiöiden fysikaalinen käyttäytyminen voi kuitenkin olla erilaista eri mittakaavoissa, esimerkiksi laboratorio-olosuhteissa verrattuna luonnonolosuhteisiin. Tämä ilmiö tunnetaan skaalaefektinä. Skaalaefektit voivat olla merkittäviä ja niillä voi olla merkitystä suuren mittakaavan mallinnuksissa, jotka käyttävät laboratoriokokeissa määritettyjä tietoja. Tämän väitöskirjan tavoitteena oli tutkia, kuinka laboratoriossa määritetyt sorptio- ja diffuusiotulokset voidaan skaalata in situ -olosuhteisiin. Tämä tehtiin määrittämällä 133Ba:n sorptio- ja diffuusioparametrit laboratoriokokeissa ja vertaamalla tuloksia Sveitsin Grimselissä toteutetun pitkäaikaisen in situ -diffuusiokokeen tuloksiin. Tämän työn tuloksena saatiin selvitettyä, että bariumin laboratorio-olosuhteissa määritetyt sorptioparametrit ovat noin kaksikymmentäkertaiset verrattuna tuloksiin in situ -olosuhteisissa. Tämä on todella tärkeää tietoa käytetyn ydinpolttoaineen loppusijoittamisen turvallisuusanalyysille, kun muunnetaan laboratoriokokeissa määritettyjä arvoja vastaamaan in situ- olosuhteita. Lisäksi työssä kehitettiin uutta mittausmenetelmää, jotta sillä voitiin mitata tietyn radioaktiivisen aineen, 133Ba:n, aktiivisuuden jakautumista graniittisissa kivissä kvantitatiivisesti

The sorption and diffusion of 133Ba in granitic rocks

The distribution coefficients of barium in the Olkiluoto pegmatite, veined gneiss, Grimsel granodiorite and their main minerals (quartz, plagioclase, potassium feldspar and biotite) were obtained by batch sorption experiments carried out as a function of the concentration of barium. The distribution coefficients were modelled with the PHREEQC calculation code. The results of different rocks and minerals were compared with each other and the sorption mechanisms onto different mineral surfaces were evaluated. The sorption results of barium were also compared with sorption results obtained from a previous study for caesium. In addition, the diffusion of barium into the rock cubes was also studied both experimentally and computationally. Furthermore, the rock cubes from the diffusion experiments were studied with autoradiography and scanning electron microscopy. The results showed that the distribution coefficients of barium were largest on biotite and the sorption as a function of the concentration of barium on all the minerals followed the same trend. The distribution coefficient results of veined gneiss, pegmatite and granodiorite followed the results of their main minerals. The distribution coefficients were largest on granodiorite which suggests ion exchange between barium and calcium, which is a common element in plagioclase occurring abundantly in granitic rock. It was also discovered that the distribution coefficients in saline water were considerably smaller than the results obtained from previous studies in low salinity water. This suggests that competing ions play a significant role in the sorption of barium. Finally, the concentration decrease of barium in the diffusion experiments was largest in granodiorite which can be explained with both the porous structure of all the minerals of granodiorite and with the sorption properties of barium. It was discovered with autoradiography that the barium was mainly sorbed in the dark minerals of the rocks, but, additionally, barium sorption on plagioclase was also concluded to be significant. In addition, the sorbed barium could be seen with scanning electron microscopy on the biotite veins in the fracture zones of the rock confirming that the barium was sorbed

Sorption of inorganic radiocarbon on iron oxides

The sorption of inorganic radiocarbon on goethite, hematite and magnetite was studied as a function of carbon concentration, pH and ionic strength. It was discovered that the sorption of radiocarbon on magnetite was negligible in all studied conditions. The distribution coefficients of radiocarbon on hematite and goethite decreased with increasing pH whereas the ionic strength had only a slight decreasing effect on radiocarbon sorption. The sorption on goethite and hematite was modelled with PhreeqC using a generalized double-layer surface complexation model.Peer reviewe

Modelling of solute transport and microbial activity in diffusion cells simulating a bentonite barrier of a spent nuclear fuel repository

Microbial sulfate reduction possesses a potential risk for the long-term safety of spent nuclear fuel repositories because under expected repository conditions sulfide is the main corroding agent for copper and copper-coated steel canisters foreseen in the Scandinavian disposal concepts. It is thus essential to understand and quantify the processes and factors impacting microbial sulfide production within and around compacted bentonite, which is planned to be used as a buffer material in many repository concepts. In the present study reactive transport modelling was applied to increase the understanding of diffusion cell experiments, which brought sand layers with or without inoculated microorganisms in contact with compacted and saturated bentonites of different mineralogy. Model results obtained for a sodium bentonite from Wyoming and a calcium bentonite from Bulgaria gave strong evidence for the activation of bentonite indigenous microorganisms, at least in zones of a reduced density close to the bentonite/sand interface. For all experiments, the calculations indicated that after an initial phase of favourable conditions, microbial activity was limited by the (bio-)availability of organic carbon. In the Bulgarian bentonite, characterized by a very low gypsum content, the model furthermore suggested some in-termediate control of microbial sulfate reduction by sulfate availability. The present study thus demonstrated the rapid evolution of a transport limited system in settings where zones of microbial activity are in contact with highly compacted microbially-inactive bentonite. Gypsum dissolution calculated and determined experimentally for the Wyoming bentonite indicated significant gypsum dissolution in the first 2 cm from the interface during 450 days. The reactive transport model applied successfully in this study for the description of an experimental system followed the conceptual models for microbial sulfate reduction in repository settings. The results obtained offer insights regarding the mechanism and magnitude of biogeochemical reactions that might occur in the vi-cinity of the bentonite buffer surrounding the waste canister and in so doing, may be relevant for the near field of HLW repositories

Modelling transport of reactive tracers in a heterogeneous crystalline rock matrix

A numerical reactive transport model for crystalline rocks is developed and evaluated. The model is based on mineral maps generated by X-ray micro computed tomography (X-μCT); the maps used have a resolution of approximately 30 μm and the rock samples are on the cm scale. A computational grid for the intergranular space is generated and a micro-DFN (Discrete Fracture Network) model governs the grid properties. A particle tracking method (Time Domain Random Walk) is used for transport simulations. The basic concept of the model can now be formulated as follows; “when a particle is close to a reactive mineral surface it has a certain probability to get sorbed during a certain time span. Once sorbed it will remain so a certain time”. The model requires a number of input parameters that represent the sorption properties of the reactive minerals. Attempts are made to relate the parameters to traditional distribution parameters. The model is evaluated by comparisons with recent laboratory experimental data. These experiments consider two rock types (veined gneiss and pegmatitic granite) and two radionuclides (cesium and barium). It is concluded that the new reactive transport model can simulate the experimental data in a consistent and realistic way.Peer reviewe

The sorption and diffusion of 133Ba in crushed and intact granitic rocks from the Olkiluoto and Grimsel in-situ test sites

Peer reviewe

Electronic autoradiography of Ba-133 particle emissions; diffusion profiles in granitic rocks

The spatial distribution of barium activity in granitic rocks was measured with two autoradiography techniques; digital autoradiography using phosphor imaging plate technique (Fuji 5100) and filmless electronic autoradiography (i.e. The BeaQuant (TM)), which is based on a gas detector incorporated in a micromesh Parallel Ionization Multiplier (PIM). Rock cubes taken from a diffusion experiment that were in contact with Ba-133 tracer were measured to determine diffusion profiles. In addition, the spatial distribution of Ba-133 in the samples was determined. Polymethyl methacrylate standards for Ba-133 were developed to determine the counting efficiency for electronic autoradiography. Good visual correlation between the two autoradiography methods were obtained in this study. The results of the experiments presented here can be utilized in future studies on the diffusion behavior of barium in granitic rocks.Peer reviewe

Compacted bentonite as a source of substrates for sulfate-reducing microorganisms in a simulated excavation-damaged zone of a spent nuclear fuel repository

Sulfide formed by sulfate-reducing microorganisms (SRM) is a potential safety risk in the geological disposal of spent nuclear fuel (SNF) enclosed in copper canisters because it can corrode copper. The canisters will be isolated from the environment by surrounding them with compacted bentonite. This study shows experimentally that the organic matter naturally present in compacted bentonites can become dissolved and sustain biological sulfate reduction. The experiment was conducted in cell systems consisting of an interface of compacted bentonite (at dry density of 1314–1368 kg m−3) and a low-porosity sand layer representing an excavation-damaged zone of the host rock. Some cells were inoculated with SRM and groundwater microorganisms and some were not. Varying concentrations of organic matter and sulfate in the sand layer solution resulted from partial dissolution of the studied bentonites (Wyoming, Indian, and Bulgarian). The dissolved organic matter promoted biological sulfate reduction, as demonstrated by the decrease in sulfate concentration in the sand layer solution and the formation of sulfide iron precipitates in the inoculated cells relative to the uninoculated cells. Other anaerobic microorganisms (e.g., methanogens) also became active in the cells and they along with the SRM were found to grow within the sand and/or bentonite layers of the cells. The findings of this study show that bentonites can sustain biological sulfate reduction in areas with lower density and immobilize possibly formed sulfides. However, the extent of these capabilities seems to be affected by the mineralogy of bentonites in the studied density range.acceptedVersionPeer reviewe