Effects of Air Oxidation on the Dissolution Rate of LWR Spent Fuel
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EFFECTS OF AIR OXIDATION ON THE DISSOLUTION RATE OF LWR SPENT FUEL W. J. Gray, L. E. Thomas, and R. E. Einziger Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352 ABSTRACT Dissolution rates for air-oxidized spent fuel were measured in flowthrough tests where U concentrations were kept well below the solubility limit. Results from two types of specimens, separated grains and coarse particles, both in oxidized (U 409+,) and unoxidized (U0 2) conditions indicated only minor effects of oxidation on the surface-area-normalized rates. Similar results were obtained for unirradiated specimens in three different oxidation states (U0 2, U30 7, and U30s). These observations have important practical implications for disposal of spent fuel in a geologic repository as well as implications regarding the oxidative dissolution mechanism of U0 2 fuel. INTRODUCTION Spent U0 2 fuel from light-water reactors (LWRs) is being evaluated as a waste form for disposal in a geologic repository. The rate of dissolution of the UO 2 matrix is a major factor controlling the long-term rate of release of soluble radionuclides such as 135Cs and 99Tc, [1, 2]. Therefore, it is important to determine how the rate of dissolution is affected by water chemistry and by the physical and chemical conditions of the fuel. One fuel condition that must be investigated is a higher oxidation state resulting from exposure to air at mildly elevated temperatures. Fuel oxidation could occur in a potential repository site at Yucca Mountain, Nevada, which is in the unsaturated zone above the water table and would be relatively dry. Liquid water would not be expected to contact the fuel during the first several hundred years after disposal while waste package temperatures exceed 950 C. If the waste container and fuel cladding both fail during that time period, the fuel could be exposed to air and become oxidized. Later, after the repository has cooled, the fuel could be contacted by water. Oxidation of spent fuel could increase its dissolution rate by increasing the chemical reaction rate of the fuel with water(a) or by increasing the surface area of the fuel as a result of oxidationinduced cracking of grain boundaries [3]. To explore the first possibility, oxidized and unoxidized spent fuel specimens consisting of separated grains were tested. These specimens eliminate the uncertainty in effective surface area that would be associated with partially exposed grain boundaries of larger particles, and thus allow intrinsic reaction rates to be determined. The second possibility was explored by testing specimens consisting of coarse particles, each containing thousands of grains. If oxidation increased the effective surface area by cracking open the grain boundaries, the coarse particle specimens of oxidized and unoxidized spent fuel would dissolve at different rates. The oxidized spent fuel used in these tests had the cubic structure of U409 and a stoichiometry of -U0 24. U40 9 of this stoichiometry, hereafter designated U409+x, is the normal product of oxidizin
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