Testing of Uranium Dioxide Containing Different Levels of Alpha Activity Under Anaerobic and Reducing Conditions
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1104-NN03-09
Testing of Uranium Dioxide Containing Different Levels of Alpha Activity Under Anaerobic and Reducing Conditions Arvid Ödegaard-Jensen1, and Virginia Oversby2 1 Nuclear Chemistry, Chalmers University of Technology, Götegorg, SE-412 96, Sweden 2 owner/operator, VMO Konsult, Karlavaegen 70, Stockholm, SE-114 59, Sweden ABSTRACT Samples of UO2 containing 5% 235U and 0, 5, or 10% 233U were tested in a dilute simulated groundwater using N2 or H2 gas as the atmosphere. Some tests included metallic iron in the solution. Each test condition used samples run in triplicate for at least 21 days and each test condition was run for two consecutive test periods. The results show that the dissolution behavior of the samples is the same in both N2 and H2 atmospheres. The amount of U dissolved under these conditions clearly showed the enhancement of dissolution due to oxidation of the sample surface by radiolysis products for conditions without metallic Fe. When a piece of iron was included in the bottom of the test vessel, the amount of dissolution decreased dramatically indicating that the Fe(II) ions released from the corroding iron were able to react with most of the radiolysis products before they could oxidize the UO2 surface. INTRODUCTION Sweden plans to dispose of spent nuclear reactor fuel in a deep geologic repository in granitic rock. The conditions in the repository in the long term will be reducing and water is not expected to contact the fuel until after more than 1000 years. At that time, most of the beta- and gamma-active nuclides will have decayed away and the radiation will be dominated by alpha radiation. In order to simulate the radiolysis field for dissolution of spent fuel with an age of more that 1000 years, we have used uranium dioxide containing 5% 235U and 0, 5, or 10% 233U. The 10% 233U sample gives an alpha activity similar to that which spent fuel will have 3000 years after disposal. Earlier work [1, 2] had shown that dissolution of UO2 samples in dilute synthetic groundwater in the presence of actively corroding iron was insensitive to the presence of alpha radiolysis products. The oxidants produced by radiolysis of water appeared to be consumed effectively by either the Fe(II) ions in the water or the H2 produced during the corrosion of the Fe under anaerobic conditions. The series of tests reported here was designed to determine which of the two possible reducing agents was the controlling factor in consuming the oxidizing agents and suppressing oxidation of the UO2 surface under the test conditions. EXPERIMENTAL DETAILS The tests were run in a dilute synthetic groundwater that had been equilibrated with pure metallic iron. This produces an initial [Fe2+] in solution of about 0.1 ppm [3]. The composition of the water is given in table I. The tests were run in steel pressure vessels that contained an inner compartment made of PEEK (polyether ether ketone), a nonreactive material. The UO2
samples were contained in small PEEK saucers. For tests that contained Fe, a small piece of pure Fe
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