Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure Page: 4 of 44
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Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure
ii September 2012
Theory
As for our previous study of model waste form of SrTiO3 for fission product 90Sr, we pursue a combined
theoretical/experimental approach to study of decay processes of a 137Cs-containing aluminosilicate
pollucite waste form. The theoretical portion of this report focuses on the atomistic simulation of pure and
Ba-doped pollucite single crystals. Where possible, we direct the theoretical effort so as to be consistent
with the ion-implantation experiments. Atomic configurations and total energies were determined for
stoichiometric and defected pollucite unit cells using density functional theory. Reference energies for all
the system constituents, Ba and Cs metal, cubic BaO and rhombohedral Cs20, 02 were also calculated.
Spin-orbit and finite-temperature effects were not included.
The primitive unit cell for the pollucite structure has 160 atoms, 16 formula units of the ideal CsAlSi2O6
composition, for a full formula of Cs16Al16Si32096. The Si and Al atoms are distributed randomly
throughout the pollucite framework. We investigated the effect of the Al distribution by first identifying
several structural subsets of the silicate network in the analogous pure Si02 zeolite framework, and then
distributing Al atom in this network according to these structural subsets. We identified 5 distinct Al
distributions with this approach. The energy differences among these five models are quite small. We
conclude that the precise location of the Al ions has only a minor effect on the system energetics, and we
therefore proceeded with a single configuration for the rest of the calculations.
Calculations were performed with 1 to 16 Ba ions substituted for Cs. Ba was either left uncompensated,
or was compensated by either excess oxygen or Cs vacancies. The results show that with the decay of Cs
to Ba, the removal (or escape) of Cs from the pollucite crystal is energetically unfavorable. The lowest
equilibrium energy is achieved by precipitating half the Ba (as metal, or as BaO if oxygen is supplied by
environment) so the remaining Ba is compensated by Cs vacancies. This means that as the waste form
decays, remaining Cs should stay trapped. However, information about ion mobilities is still needed.
Finally, we find that at equilibrium, Ba and vacancies tend to segregate into Ba-rich regions. This is
consistent with the segregation of Ba that is seen in the analysis of the Ba-doped pollucite sample
provided by Sandia Laboratory. This raises questions about the effects of Ba precipitation in the decaying
waste form.
Further investigation that includes explicit inclusion of additional excess oxygen, the presence of fluorine
ions, and consideration of aluminosilicate backbone defects is needed for a complete correlation to the
experimental data. In particular, a description of defect formation and evolution in crystal network is
needed to explore the effects of irradiation-induced amorphization. As stated above, theoretical results
show that Cs release is unlikely for intact crystalline pollucite. Based on these calculations and the
experimental data, we hypothesize that the amorphization process is critical to the migration and release
of Cs in the F+ implanted samples.
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Jiang, Weilin & Van Ginhoven, Renee M. Chemical and Charge Imbalance Induced by Radionuclide Decay: Effects on Waste Form Structure, report, September 28, 2012; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc844272/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.