| 1. |
- Bruggeman, C., et al.
(författare)
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Redox-active phases and radionuclide equilibrium valence state in subsurface environments - New insights from 6th EC FP IP FUNMIG
- 2012
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Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 27:2, s. 404-413
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Tidskriftsartikel (refereegranskat)abstract
- Within the 6th EC FP Integrated Project "Fundamental Processes of Radionuclide Migration'' (FUNMIG), progress has been made to improve knowledge about the phases and reaction mechanisms involved in complex reduction processes of radionuclide contaminants in natural subsurface environments. This review paper gives an overview of the achievements made by the research groups involved in this project, and puts the scope and results of the studies in a more global context. Firstly, both thermodynamic and experimental evidence show that green rust is present and reactive in subsurface groundwater with a composition that spans the Fe(II)/Fe(III) redox boundary. Green rust has been shown to reduce Np(V), Se(VI) and Se(IV), but the pathways for the redox processes and the reaction products that result are complicated, and change as a function of the reaction parameters. Secondly, considerable evidence has emerged that Se(IV) is reduced on Fe(II)-bearing minerals which are ubiquitous in subsurface environments. The stable Se valence state in the presence of FeS(2) has been shown to be Se(0). Also, natural dissolved humic substances that contain sufficient electron donating capacity are capable of interacting with, and possibly reducing, Se(IV) to lower valence states. Thirdly, the influence of HCO(3)(-) and organic ligands on the uptake and reduction of U(VI) on Fe(II)-bearing minerals was investigated. While it appeared that HCO(3)(-) decreased the extent of U(VI) uptake by the reducing surface, the fraction of reduced U(IV) in the solid phase increased with increasing HCO(3)(-) concentration. In contrast with the observations for HCO(3)(-), organic ligands decreased both the extent of U uptake, as well as the fraction of U(IV) found in the solid phase. The studies performed within FUNMIG show that investigating reduction-oxidation mechanisms require (1) a detailed control over reaction conditions (anoxic atmosphere, purification of solid phases, initial radionuclide speciation), (2) a rigorous follow-up of reaction products (both solution chemistry and spectroscopic methods), and (3) the consideration of slow kinetics in the setting up of an experiment. These requirements make the study and assessment of redox processes one of the most demanding scientific challenges for geochemists who are asked to make predictions for radionuclide transport behaviour in the environment.
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| 2. |
- Fachinger, J, et al.
(författare)
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Behaviour of spent HTR fuel elements in aquatic phases of repository host rock formations
- 2006
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Ingår i: Nuclear Engineering and Design. - 0029-5493. ; 236:5-6, s. 543-554
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Tidskriftsartikel (refereegranskat)abstract
- One back-end option for spent HTR fuel elements proposed for future HTR fuel cycles in the EC is an open fuel cycle with direct disposal ofconditioned or non-conditioned fuel elements. This option has already been chosen in Germany due to the political decision to terminate the useof HTR technology. First integral leaching investigations at Research Centre Juelich on the behaviour of spent HTR fuel in salt brines, typical ofaccident scenarios in a repository in salt, proved that the main part of the radionuclide inventory cannot be mobilised as long as the coated particlesdo not fail. However, such experiments will not lead to a useful model for performance assessment calculations, because a failure of the coatingsby corrosion will not occur during experimental times of a few years. In order to get a robust and realistic model for the long-term behaviour inaqueous phases of host rock systems, it is necessary to understand the barrier function of the different parts of an HTR fuel element, i.e. the matrixgraphite, the different coating materials, and the fuel kernel.Therefore, our attention is focused on understanding and modelling the barrier performance of the different parts of an HTR fuel element withrespect to their barrier function, and on the development of an overall model for performance assessment. In order to understand this behaviour,it is necessary to start with investigations of unirradiated material, and to proceed with experiments with external gamma irradiation to determinethe effects of oxidising radiolysis species. Further experiments with irradiated material have to be performed to investigate the influence of theirradiation damage, and finally an investigation has to be made of the irradiated material plus additional gamma irradiation. Experimental data arenow available for the diffusive transport of radionuclides in the water-saturated graphite pore system, the corrosion rates of unirradiated graphitewith and without external gamma irradiation and unirradiated and irradiated silicon carbide, and for the dissolution rates of UO2 and (Th,U)O2 fuelkernels with and without external gamma irradiation. All investigations were performed in aquatic phases from salt, granite, and clay host rock.
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