Radiation-induced decomposition of U(VI) alteration phases of UO₂

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 a-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 IVEM-Tandem Facility of Argonne National Laboratory. The ion flux was 6.3 × 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.