• journal_title：American Mineralogist
• Contributor：Alexandra Rey ; Satoshi Utsunomiya ; Javier Giménez ; Ignasi Casas ; Joan de Pablo ; Rodney C. Ewing
• Publisher：Mineralogical Society of America
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2009.2908
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：94
• issue：2-3
• firstpage：229
• section：Articles

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.