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Stability of uranium (VI) peroxide hydrates under ionizing radiation

¹ Department of Chemical Engineering, Universitat Politecnica de Catalunya (UPC), 08028 Barcelona, Spain
² Department of Geological Sciences, University of Michigan, Michigan 48109-1005, U.S.A.
³ Centre Tecnològic, Manresa (CTM), 08242 Manresa, Barcelona, Spain

The uranyl peroxide, studtite (UO₄·4H₂O, C2/c, Z = 4), is expected to form as a consequence of alpha radiolysis of water in contact with spent nuclear fuel (SNF) in a geologic repository. Investigation of its stability is, therefore, of critical importance because secondary U(VI) phases may incorporate trace amounts of radionuclides and thus retard their mobility away from a repository site. To examine the effect of ionizing radiation on uranyl peroxides, electron-beam irradiation experiments have been conducted on two synthetic uranyl peroxides: studtite and metastudtite (UO₄·2H₂O, Immm, Z = 2). All experiments were done using a transmission electron microscope (TEM) with an acceleration voltage of 200 kV at room temperature. The fluence required to completely amorphize studtite was 0.51–1.54 x 10¹⁷ e/cm², which is equivalent to an absorbed dose of 0.73–1.43 x 10⁷ Gy. Metastudtite becomes amorphous at a higher absorbed dose (1.31 x 10⁷ Gy) than studtite, most likely because it contains fewer water molecules in its structure. These uranyl peroxides partially amorphize at doses that are one-tenth of the dose required for complete amorphization. With continued irradiation, uraninite nanocrystals form that are a few nanometers in diameter, at 4–20 x 10¹⁰ Gy. In a geologic repository, for spent nuclear fuel, the estimated absorbed doses due to ionizing radiation may be as high as 10⁸–10¹¹ Gy after 10⁶ years. This is well in excess of doses in the laboratory experiments that caused the uranyl peroxides to become amorphous and decompose.

Key Words: Ionizing radiation • TEM • uranyl peroxide • amorphization