Radiation-induced decomposition of U(VI) alteration phases of UO 2
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Radiation-induced decomposition of U(VI) alteration phases of UO2 Satoshi Utsunomiya and Rodney C. Ewing Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA ABSTRACT U6+-phases are common alteration products of spent nuclear fuel under oxidizing conditions, and they may potentially incorporate actinides, such as long-lived 239Pu and 237Np, delaying their transport to the biosphere. In order to evaluate the ballistic effects of α-decay events on the stability of the U6+-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr2+) for six different structures of U6+-phases: uranophane, kasolite, boltwoodite, saleeite, carnotite, and liebigite. The target uranyl-minerals were characterized by powder X-ray diffraction and identification confirmed by SAED (selected area electron diffraction) in TEM (transmission electron microscopy). The TEM observation revealed no initial contamination of uraninite in these U6+ phases. All of the samples were irradiated with in situ TEM observation using 1.0 MeV Kr2+ in the IVEM (intermediate-voltage electron microscope) at the IVEMTandem Facility of Argonne National Laboratory. The ion flux was 6.3 x 1011 ions/cm2/sec. The specimen temperatures during irradiation were 298 and 673 K, respectively. The Kr2+-irradiation decomposed the U6+-phases to nanocrystals of UO2 at doses as low as 0.006 dpa. The cumulative doses for the pure U6+-phases, e.g., uranophane, at 0.1 and 1 million years (m.y.) are calculated to be 0.009 and 0.09 dpa using SRIM2003. However, with the incorporation of 1 wt.% 239Pu, the calculated doses reach 0.27 and ~1.00 dpa in ten thousand and one hundred thousand years, respectively. Under oxidizing conditions, multiple cycles of radiation-induced decomposition to UO2 followed by alteration to U6+-phases should be further investigated to determine the fate of trace elements that may have been incorporated in the U6+-phases. INTRODUCTION Spent nuclear fuel consists primarily of UO2 (> 95 wt%), and the mass balance, depending on the burn-up is ~ 1% Pu, 2-3% fission product elements and small amounts of other transuranium elements, such as 237Np (a result of α-decay of 241Am). Under oxidizing conditions, the UO2 will alter in the presence of water to an assemblage of U6+-phases [1-3]. During the alteration of UO2, radionuclides are released, but some, particularly actinides, such as long-lived 239Pu (half-life = 24,100 years) and 237Np (half-life = 2.1 million years), may be re-incorporated into the U6+phases [4]. Thus, these secondary, U6+ phases become “sinks” for actinides, delaying their transport to the biosphere and lowering their contribution to the calculated doses in performance assessments of geologic repositories [5,6]. Recently, there has been an increased effort to understand the paragenesis of the U6+alteration phases of UO2 [7] and their stabilities [8] in order to better understand their role in controlling the mobility of radionuclides released during the corrosion of UO2[9]. The major focu
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