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 × 1017 e/cm2, which is equivalent to an absorbed dose of 0.73-1.43 × 107 Gy. Metastudtite becomes amorphous at a higher absorbed dose (1.31 × 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 × 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.
Sorption of actinides, particularly plutonium, onto submicrometer-sized colloids increases their mobility, but these plutonium colloids are difficult to detect in the far-field. We identified actinides on colloids in the groundwater from the Mayak Production Association, Urals, Russia; at the source, the plutonium activity is approximately 1000 becquerels per liter. Plutonium activities are still 0.16 becquerels per liter at a distance of 3 kilometers, where 70 to 90 mole percent of the plutonium is sorbed onto colloids, confirming that colloids are responsible for the long-distance transport of plutonium. Nano-secondary ion mass spectrometry elemental maps reveal that amorphous iron oxide colloids adsorb Pu(IV) hydroxides or carbonates along with uranium carbonates.
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