The Behavior or Light Water Reactor Fuel after the Cladding is Breached under Unsaturated Test Conditions
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The Behavior or Light Water Reactor Fuel after the Cladding is Breached under Unsaturated Test Conditions J. C. Cunnane, J. A. Fortner and R. J. Finch Chemical Technology Division, Argonne National Laboratory, Argonne, IL60439 ABSTRACT Experiments were conducted to examine the corrosion behavior of fuel and cladding under test conditions selected to determine how fuel with breached cladding may behave under unsaturated repository conditions. The results discussed here were obtained from two test samples, each consisting of ~3.5-in. segments of ATM103, a moderate-burnup (~30 GWd/MtU) PWR fuel, exposed to humid air at 175°C. Visual examination of the samples after 540 days revealed that each had developed an axial crack that passed through a drilled hole in the cladding and ran the full length of the sample. Destructive examination of the fuel and cladding showed that the cladding had experienced extensive fuel-side corrosion. Metallographic examination of the cladding shows the hydride distribution near the drilled hole, the fracture surfaces, and the fuel-side corrosion products. Electron microscopy (SEM, TEM) and electron diffraction analyses were used to characterize the corrosion products. The results indicate that the fuel-side corrosion of the cladding and the specific volume increase associated with the formation of the corrosion products caused the hoop stresses that resulted in the observed axial splitting. The implications of these results for the expected evolution of the spent fuel cladding after the cladding is initially breached in an unsaturated repository are discussed. INTRODUCTION The Yucca Mountain Project is evaluating the long-term performance of spent fuel from commercial reactors under the range of unsaturated conditions to which it may be exposed in a potential repository located in volcanic tuff at Yucca Mountain, Nevada. After the fuel cladding is eventually breached, the fuel in the breached rods will be exposed to humid air and perhaps silicate-rich groundwater. The subsequent radionuclide release rates from such breached fuel rods are uncertain, in part because the evolution of the state of fuel rods, once breached, remains uncertain. The experiments described here were designed to establish plausible scenarios for the evolution of the state of fuel rods with breached cladding. Alteration of spent CANDU fuel with breached cladding has been investigated in long-term tests conducted in moisture-saturated air [1,2]. Destructive examination of the fuel rods from these tests showed that the grain boundaries in the fuel were oxidized to form UO3 hydrates near the fuel/cladding interface along the full length of the fuel rods. These observations were attributed to oxidizing species produced by radiolysis of condensate water films, and to nitric acid produced by radiolysis of the moist air in the gap region of the fuel rods. In studies conducted for the Yucca Mountain project the predominant fuel alteration products that formed upon contact with humid air were found to be dehydrated scho
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