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Effects of ionizing radiation on the hollandite structure-type: Ba_0.85Cs_0.26Al_1.35Fe_0.77Ti_5.90O₁₆

Abdesselam Abdelouas^1,*, Satoshi Utsunomiya^2,, Tomo Suzuki¹, Bernd Grambow¹, Thierry Advocat³, Florence Bart³ and Rodney C. Ewing²

¹ SUBATECH, EMN-CNRS/IN2P3-Université de Nantes, BP 20722, 44307 Nantes, France
² Department of Geological Sciences, University of Michigan, 1100 N. University Avenue, Ann Arbor, Michigan 48109-1005, U.S.A.
³ CEA Valrho Marcoule, 30207 Bagnols-sur-Cèze cedex, France

Correspondence: ^* E-mail: Abdesselam.Abdelouas{at}subatech.in2p3.fr

The hollandite structure-type has received considerable attention as a nuclear waste form for the incorporation of radioactive ¹³⁵Cs and ¹³⁷Cs, both of which are important fission product radionuclides in the high-level nuclear waste generated by the reprocessing of used nuclear fuel. A critical concern has been the effects of high doses of ionizing radiation from incorporated Cs on the long-term structural stability of the hollandite structure. Optimization of the synthesis conditions has resulted in the hollandite stoichiometry of Ba_0.85Cs_0.26Al_1.35Fe_0.77Ti_5.90O₁₆. To evaluate the effect of Cs-beta-decay on this stoichiometry, we have simulated the ionizing radiation using 200 kV electron beam using transmission electron microscopy (TEM) at 298 and 573 K. Complete amorphization was achieved at doses of 1.1 x 10¹⁴ and 1.8 x 10¹⁴ Gy at temperatures of 298 and 573 K, respectively. Electron energy-loss spectroscopy (EELS) of the Cs M-edge revealed the selective loss of Cs at the maximum doses. Hollandite irradiated using gamma rays, ~10⁶ Gy, which has defects associated with the formation of Ti³⁺ and O₂^– had a dissolution rate similar to that of the pristine hollandite, suggesting that the initial stage of defect formation does not influence chemical durability. Because the accumulated dose in the hollandite with 5 wt% of radioactive ¹³⁷Cs₂O is estimated to be ~2.0 x 10¹⁰ Gy after 500 years, the hollandite structure should be stable under the conditions anticipated for geologic disposal.

Key Words: Hollandite • ionizing radiation • TEM • cesium • beta-decay