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The uranyl peroxide, studtite (UO 4 ⋅4H 2 O, 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 (UO 4 ⋅2H 2 O, 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 × 10 17 e/cm 2 , which is equivalent to an absorbed dose of 0.73-1.43 × 10 7 Gy. Metastudtite becomes amorphous at a higher absorbed dose (1.31 × 10 7 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 × 10 10 Gy. In a geologic repository, for spent nuclear fuel, the estimated absorbed doses due to ionizing radiation may be as high as 10 8 -10 11 Gy after 10 6 years. This is well in excess of doses in the laboratory experiments that caused the uranyl peroxides to become amorphous and decompose.

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