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@PHDTHESIS{Rudin:1021034,
author = {Rudin, Stefan},
othercontributors = {Bosbach, Dirk Adolf and Zobel, Mirijam},
title = {{K}inetik des ²²⁶{R}a {E}inbaus in das ternäre
{M}ischkristallsystem ({B}a,{S}r,{R}a){SO}₄ - {H}₂{O}},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-09412},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2026; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2025},
abstract = {Deep geological repositories currently represent the safest
and most suitable option for the disposal of spent nuclear
fuel. In realistic scenarios, it is necessary to consider
the influence of groundwater in the development of a
disposal system over geological time periods. A possible
consequence is the failure of the engineered barriers and
the release of radionuclides. For example, the Swedish waste
management organization SKB calculated scenarios in which
the dose after 100,000 years is dominated by the nuclide
²²⁶Ra. The formation of (Ba,Sr,Ra)SO₄ solid solutions
is one possible mechanism for ²²⁶Ra retention.
Thermodynamic modeling postulates the most effective
²²⁶Ra retention in the case of uptake in (Ba,Sr)SO₄
solid solutions with an Sr content of about 10 $mol-\%.$
However, the mechanisms of the ²²⁶Ra uptake processes
into the barite structure on the atomic scale were not yet
sufficiently understood. The temporal evolution of ²²⁶Ra
uptake by Ba-rich solid solutions in the
(Ba,Sr,Ra)SO₄-H₂O system was still unknown. The aim of
this study was to develop an understanding of both the
processes relevant to ²²⁶Ra uptake into the barite
structure and the kinetics of ²²⁶Ra uptake in Ba-rich
(Ba,Sr,Ra)SO₄-H₂O systems. A theoretical approach from
atomistic simulation methods was selected to describe the
ion uptake processes that take place during the
crystallization processes on the atomic scale. In addition,
the ²²⁶Ra uptake in the (Ba,Sr,Ra)SO₄-H₂O system was
considered on the basis of existing long-term experiments by
analysis using microscopic methods and geochemical modeling.
A description of the processes at the barite-water
interface, which could be important for the kinetics of
(Ba,Ra)SO₄ solid solution formation, required the
resolution of the differences between the uptake processes
of Ba2+ and Ra2+ ions into the barite structure. An accurate
simulation of the chemical bonds and water, while keeping
the computational effort reasonable, was essential. A hybrid
simulation approach was tested and verified, which allowed a
quantum-mechanical description of the barite and the ion
uptake processes using density functional theory (DFT). A
stepwise approach was chosen so that suitable models of the
barite crystal structure, the barite (001) surface, and
different steps on this surface could be developed and
tested. The effects of water were included in the
simulation, whereby both hydrated ions and the barite-water
interfaces could be successfully simulated. This study thus
made it possible for the first time to describe complete
Ba2+ and Ra2+ uptake processes, starting from the ion
completely dissolved in water to its complete uptake into
the barite structure at several positions, using ab
initio-based methods. The hybrid approach allowed a
quantum-mechanical representation of the chemical bonds and
the effects of the aqueous solution for all intermediate
steps. At the same time, an acceptable computational cost
could be maintained. The Ba2+ uptake process was mainly
determined by the dehydration of the attaching ions and the
formation of chemical bonds from these ions to the remaining
barite surface. In addition, intermediate steps in ion
uptake were determined that have not been previously
described in the literature. The role of Ba2+ and Ra2+
kink-site nucleation for anisotropic barite-(001) growth and
(Ba,Ra)SO₄ solid solution formation was investigated using
the hybrid DFT-continuum approach, supported by classical
force field-based molecular dynamics simulations. The Ba2+
uptake at the barite-(001)-water interface required higher
activation energies at the low positions compared to the
high positions on both the acute and obtuse ⟨120⟩-steps.
However, a higher relevance of the low-site positions for
barite crystal growth could be assumed due to their higher
stability. The Ba2+ uptake preferentially occurred at the
obtuse step due to the more open step geometry since less
dehydration was required there compared to the Ba2+ uptake
at the acute step. The simulations of step growth processes
by the uptake of Ba2+ ions indicate a slow and uniform
growth of the acute step and a fast anisotropic growth of
the obtuse step. The uptake of Ra2+ at the
barite-(001)-water interface requires higher activation
energies than the uptake of Ba2+ due to the lattice
distortion. The rate-limiting steps in the formation of
⟨120⟩-steps thus occur primarily through the uptake of
Ba2+. The presence of Ra2+ ions has only a minor influence
on the kinetics of (Ba,Ra)SO₄ crystal growth. However, the
energies of the lattice distortion are partially compensated
by the easier dehydration of the Ra2+ ions compared to the
Ba2+ ions. A kinetically induced preferential uptake of Ra2+
ions over Ba2+ ions during the non-rate-limiting
⟨120⟩-step growth, whose processes required a higher
degree of dehydration, is thus likely. The role of the
presence of ²²⁶Ra and the Sr content on the
equilibration of the Ba-rich ternary (Ba,Sr,(Ra))SO₄-solid
solution-H₂O system was investigated using long-term
experiments. The approach to thermodynamic equilibrium took
place through several parallel dissolution and
recrystallization steps of the (Ba,Sr,(Ra))SO₄ solid
solution. Despite the overall undersaturation during almost
the entire experimental period of the Ra-free reference
experiments, recrystallization could take place due to
locally varying supersaturations. In the Ra-free reference
experiments, however, the equilibration processes were so
slow that complete thermodynamic equilibrium was not
achieved within 664 or 1967 days. The net-fraction of solid
material to be dissolved to reach thermodynamic equilibrium
determined the particle morphologies that occurred during
equilibration through kinetic processes: a high Sr content
and a low ratio between solid and aqueous solution caused
large-scale dissolution, resulting in the formation of large
cavities and frame-like structures. A low Sr content and a
high solid/liquid ratio led to recrystallization within
small cavities and formation of porous grains. Higher
temperatures and ²²⁶Ra presence accelerate precipitation
and recrystallization but produce similar morphologies. The
time courses of all ²²⁶Ra-containing experiments
followed the same trend: Initially, ²²⁶Ra-containing
compositions were supersaturated. Due to the partial
dissolution of the original solid solutions and the
recrystallization of Ba-rich solid solutions, the ²²⁶Ra
was taken up into the solid solution structures. Higher
²²⁶Ra concentration in terms of the amount of solid
increased the dissolution and recrystallization rate,
causing a temporary decrease in the ²²⁶Ra concentration
in the solution below the thermodynamic equilibrium due to
Ra entrapment in the solid. Once the ²²⁶Ra ion
distribution had approached equilibrium between the solid
and the solution, or a ²²⁶Ra entrapment was established,
further dissolution and recrystallization steps were slow.
Further dissolution and re-precipitation steps resulted in
homogeneous ²²⁶Ra distribution in the solid solutions
and in an approach towards thermodynamic equilibrium. The
particle morphologies that emerged changed only slightly
after 98 days. In summary, effective ²²⁶Ra retention can
be expected by its uptake it into existing Ba-containing
(Ba,Sr)SO₄ solid solutions in contact with Ra-containing
solutions.},
cin = {513410 / 510000},
ddc = {620},
cid = {$I:(DE-82)513410_20140620$ / $I:(DE-82)510000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-09412},
url = {https://publications.rwth-aachen.de/record/1021034},
}
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