title: Characterisation of water penetration into polycrystalline UO2
creator: Marchetti, Ilaria
subject: ddc-540
subject: 540 Chemistry and allied sciences
description: In the event of exposure of spent nuclear fuel to groundwater in a final repository, the preferential  dissolution of grain boundaries rather than matrix dissolution would cause a rapid increase of the  surface area exposed to groundwater, with effects on the mobilisation of radionuclides and on the  overall mechanical stability of the spent fuel pellet. In this respect, a research project has been  launched at the Institute for Transuranium Elements (ITU, Karlsruhe, Germany), with the goal of  gaining understanding of the mechanisms of the penetration of water into polycrystalline UO2 under  conditions relevant for final disposal in a geological repository.  As a first approach to this issue, the study has been initially focused on natural UO2 with well defined  grain size and morphology. The experiments have then been extended to SIMFUEL, an inactive  analogue of UO2 fuel containing elements simulating a spent fuel, to assess potential effects  associated to the presence of fission products. The experiments consisted of static leaching tests on  UO2 fuel pellets using 18O-labelled water. Unlike most of the diffusion studies on UO2, the experiments  in this study were conducted at low temperature (≈ 25°C and 60°C), in order to reproduce the  temperature range expected in the geological disposal scenario temperatures. To ensure sufficient  contact time between the solid phase and the solution for penetration to be detectable experiments  lasted 3-9 months. Experiments were carried out initially in oxidising conditions (in air or in N2  glovebox) and thereafter under 10 bar H2 atmosphere, in order to approach conditions more similar to  the anoxic and reducing environment expected in the final repository. Pre- and post-corrosion surface  characterisation was performed by means of scanning electron microscopy (SEM), atomic force  microscopy (AFM) and X-ray photoelectron spectrometry (XPS), while solution analysis was  performed by inductively coupled plasma mass spectrometry (ICP-MS). The penetration of water was  assessed by depth profiling of the tracer 18O using Secondary Ion Mass Spectrometry (SIMS)  microscopy. Well-known analytical models were used to fit of the profiles and determine the relevant  diffusion coefficients.  For the first time in this study, the chemical diffusion coefficient of oxygen in different UO2+x phases  was experimentally determined at temperatures below 100°C. The good agreement with extrapolated  literature data obtained from high-temperature diffusion experiments verifies the viability of the SIMS  depth profiling method for the determination of diffusion coefficients, even with a diffusion length of  tens of nm. The study has also reported for the first time evidence of the presence of oxygen/water  grain-boundary diffusivity in polycrystalline UO2 and an estimate of the diffusion coefficient at 25°C is  here presented. The study has also shown that the possibility to observe grain boundary diffusion by  SIMS depth profiling might depend to a large degree on the grade of surface oxidation of UO2. In  general, all these experimental findings have given an important starting point for the study of water  penetration in the spent fuel.  Zusammenfassung  ii  Charakterisierung der Penetration von Wassermolekülen in polykristallines UO2  Stichwörter: Urandioxid; SIMFUEL; Sekundärionen-Massenspektrometrie; Tiefenprofil; Korrosion;  Auslaugen; chemische Diffusion; Korngrenzendiffusion; Kristallgitter Diffusion;  Sauerstoff-Diffusion; Endlagerung; abgebrannte Brennelemente
date: 2013
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/14979/1/Thesis_final.pdf
identifier: DOI:10.11588/heidok.00014979
identifier: urn:nbn:de:bsz:16-heidok-149796
identifier:   Marchetti, Ilaria  (2013) Characterisation of water penetration into polycrystalline UO2.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/14979/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng