Effects of ionizing radiation on the hollandite structure-type: Ba0.85Cs0.26Al1.35Fe0.77Ti5.90O16

Abstract

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 Ba0.85Cs0.26Al1.35Fe0.77Ti5.90O16. 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 × 1014 and 1.8 × 1014 Gy at temperatures of 298 and 573 K, respectively. Electron energyloss 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 O2̅ 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 × 1010 Gy after 500 years, the hollandite structure should be stable under the conditions anticipated for geologic disposal.