Dry oxidation or hydration, and mechanical fracture

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Dry-air oxidation or humid air hydration of SNF in air or in the presence of limited amounts of groundwater may play an important role in radionu­clide releases (Ahn and Mohanty, 2008; Ahn, 1996b). The UO2 matrix will fracture (or crack) upon oxidation or hydration by volume change. Lower oxidized oxides such as UO24 will contract, whereas higher oxides such as U3O8 or UO3 hydrates will expand. Lower and higher oxides are defined here as oxides with a (O/U) ratio smaller and larger than 2.4, respectively. Fractions (e. g., 10-6-10-3) of oxidized or hydrated phases are likely to be respirable aerosol less than 10 micrometer (3.9 microinch) in size. The aerosol will increase the radionuclide release in air. The oxidized or hydro­lysed phases also increase the area of SNF surface exposed to groundwater. This increase of the exposed surface area is in turn expected to increase radionuclide release in groundwater. Similarly, mechanical impacts such as those caused by seismic events can also fragment the SNF into particles.

Figure 7.10 shows the fraction of respirable particles, depending on the impact energy absorbed. The fine-grained and porous rim structure near cladding of high burnup (above about 60 GWd/MTU) UO2 may also affect the magnitude of the radionuclide release fraction (NRC, 2007 ).

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