Abstract

The susceptibility of uranophane, a uranyl sheet silicate, ideally Ca(UO2)2(SiO3OH)2(H2O)5, to ionizing irradiation has been evaluated by systematic irradiations with 200 keV electrons over the temperature range 94 to 573 K. High-resolution transmission electron microscopy revealed that amorphous domains formed locally, concurrently with a gradual disordering of the entire structure. Amorphization doses at room temperature were 1.1 × 1010 Gy for uranophane, 1.3 × 1010 Gy for Sr-substituted uranophane, and 1.9 × 1010 Gy for Eu-substituted uranophane; thus, there was an increase in amorphization dose with increasing average atomic mass. At 573 K, the amorphization dose of uranophane was 2.0 × 1011 Gy. The temperature dependence of the amorphization dose of uranophane has two stages; ď≤413 K and >413 K. Based on a defect accumulation model, the effective activation energies for amorphization at each stage are 0.0440 eV and 0.869 eV, respectively. This suggests that the presence of H2O (and OH) reduce the energy deposition required to cause–amorphization. Above 413 K, the amorphization dose increased due to the absence of H2O and OH and the absence of radiolytic decomposition of H2O and OH.

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