EFFECT OF IONIZING RADIATION ON URANOPHANE

Uranophane, Ca(UO₂)₂(SiO₃OH)₂(H₂O)₅, occurs as an alteration product of uraninite and the UO₂ is used nuclear fuels when they are exposed to moist, oxidizing conditions. The secondary alteration products of spent nuclear fuel are important because they may retard the release of long-lived radionuclides, such as ²³⁷Np, by solid-state incorporation mechanisms. However, the alteration phases on the surface of corroded spent fuel will be subjected to high doses of ionizing radiation (20 Gy/h 100 years after discharge from the reactor); thus, we have investigated the effects of ionizing radiation on uranophane. Radiation effects were followed by in situ measurement using SAED and EELS in a TEM. The sample was tilted at 20^o to avoid electron channeling effects and irradiated by 200 keV e^- from 94 to 673 K. Dose rate was between 5.0 x 10⁷ and 42 x 10⁷ e^-/cm²/sec. At 298 K, the SAED from the [100] zone revealed a gradual decrease of intensity of all diffraction spots, but 0k0 (k=2i) and 0lm (l, m=2i+1) were more gresistanth to loss of intensity than the other diffraction maxima. The oxygen K-edge EELS spectrum revealed a small peak at 6 eV lower energy than the main peak at 540 eV. This peak disappeared gradually during amorphization. Temperature dependence of dose required for amorphization showed a gradual increase from 2.9 x 10⁹ e^-/cm² (94 K) to 7.3 x 10⁹ e^-/cm² (413 K), and the amorphization dose (8.8 x 10¹⁰ e^-/cm²) increased drastically at > 413 K. Amorphization did not occur at > 573 K. Because the stopping power of uranophane can be calculated as 42 MeV for 200 keV, the uranophane remains crystalline even at a high dose of 2.3 x 10² Gy at a temperature > 573 K.