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@INPROCEEDINGS{Brandt:826244,
      author       = {Brandt, Felix and Weber, Juliane and Klinkenberg, Martina
                      and Breuer, Uwe and Barthel, Juri and Povstugar, Ivan and
                      Bosbach, Dirk},
      title        = {{R}adium retention by {B}ax{R}a1-x{SO}4 solid solution
                      formation: an electron microscopy and atom probetomography
                      investigation},
      reportid     = {FZJ-2017-00490},
      year         = {2016},
      abstract     = {The migration of radionuclides in the geosphere is to a
                      large extent controlled by sorption processesonto minerals
                      and colloids. On a molecular level, sorption phenomena
                      involve surface complexation, ion exchange aswell as solid
                      solution formation. The formation of solid solutions leads
                      to the structural incorporation of radionuclides ina host
                      structure. Such solid solutions are ubiquitous in natural
                      systems – most minerals in nature are atomisticmixtures of
                      elements rather than pure compounds because their formation
                      leads to a thermodynamically more stablesituation compared
                      to the formation of pure compounds.In some scenarios
                      describing the evolution of a geological waste repository
                      system for spent nuclear fuel in crystallinerocks 226Ra
                      dominates the radiological impact to the environment
                      associated with the potential release ofradionuclides from
                      the repository in the future. The solubility of Ra in
                      equilibrium with a BaxRa1-xSO4 solid solution ismuch lower
                      than the one calculated with RaSO4 as solubility limiting
                      phase. Due to the expected conditions in therepository near
                      field, a likely scenario will be a release of Ra from the
                      spent fuel matrix into a solution in equilibriumwith
                      pre-existing barite. Batch-type laboratory experiments
                      mimicking this scenario were carried out and indicate
                      theuptake of Ra, leading to a reduction of more than $99\%$
                      of the initial Ra concentration. The grain size and
                      morphologyof the barite grains are very similar before and
                      after the Ra uptake, although ToF-SIMS analyses indicate
                      that Ra wastaken up into the particle volumes. In order to
                      follow the uptake of Ra into barite and identify the
                      possible pathwaysinto the particle volume, we applied a new
                      approach for the detailed characterization of Ra-barites
                      obtained atdifferent stages of the recrystallization
                      experiments utilizing a combination of electron microscopy
                      and atom probetomography (APT). The preparation of the
                      barite samples was done by adapting focused ion beam milling
                      proceduresto the material.A layered structure caused by size
                      and density variations of nano-scaled pores was observed by
                      electronmicroscopy in Ra-free reference samples. The APT
                      reconstruction showed chemical inhomogeneities of H2O, Na
                      andCl present in layers of similar length scale. In
                      conclusion, both findings indicated that the layered
                      structure consisted ofnano-scaled pores filled with
                      NaCl-bearing fluids, providing a fast pathway for Ra into
                      the barite particles.Subsequently, elemental maps of
                      Ra-barites were obtained with energy-dispersive X-ray
                      spectroscopy (EDX)enabling the analysis of the evolution of
                      the Ra distribution within the solid with time. The maps
                      showed anintermediate heterogeneous Ra distribution which
                      becomes homogeneous at equilibrium state.},
      month         = {Nov},
      date          = {2016-11-27},
      organization  = {MRS Fall Meeting 2016, Boston (USA),
                       27 Nov 2016 - 2 Dec 2016},
      subtyp        = {Other},
      cin          = {IEK-6 / ZEA-3 / ER-C-2 / PGI-5},
      cid          = {I:(DE-Juel1)IEK-6-20101013 / I:(DE-Juel1)ZEA-3-20090406 /
                      I:(DE-Juel1)ER-C-2-20170209 / I:(DE-Juel1)PGI-5-20110106},
      pnm          = {161 - Nuclear Waste Management (POF3-161) / HITEC -
                      Helmholtz Interdisciplinary Doctoral Training in Energy and
                      Climate Research (HITEC) (HITEC-20170406)},
      pid          = {G:(DE-HGF)POF3-161 / G:(DE-Juel1)HITEC-20170406},
      typ          = {PUB:(DE-HGF)24},
      url          = {https://juser.fz-juelich.de/record/826244},
}