Precipitation of Crystalline N p O 2 During Oxidative Corrosion of Neptunium-Bearing Uranium Oxides
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PRECIPITATION OF CRYSTALLINE NpO2 DURING OXIDATIVE CORROSION OF NEPTUNIUM-BEARING URANIUM OXIDES Robert J. Finch Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439 USA ABSTRACT We report crystallization of NpO2 during aqueous corrosion of unirradiated Np-bearing U3O8 under nominally oxidizing conditions. Powders of Np0.33U2.67O8 (Np:U = 1:8) were reacted in humid air at 90° and 150°C for several weeks in sealed stainless-steel vessels. Reacted solids were examined by scanning and transmission electron microscopies (SEM and TEM) and X-ray powder diffraction (XRD). Dehydrated schoepite, (UO2)O0.25–z(OH)1.5+2z (0 ≤ z ≤ 0.15), is the predominant U(VI) compound formed in these experiments and is a minor sink for Np (containing 2 wt.% Np, maximum). The primary sink for Np during corrosion of Np0.33U2.67O8 at 150° C is crystalline NpO2, which crystallized within 2 weeks of reaction in humid air. Corrosion of Np0.33U2.67O8 at 90° C resulted in crystallization of approximately equal proportions of crystalline NpO2 and Np2O5 during 16 weeks of reaction in humid air. INTRODUCTION The long half life (2.14 Ma) and potential mobility of Neptunium-237 (237Np) in oxidizing groundwaters make it an important isotope for evaluating long-term performance of a geologic repository for high-level nuclear waste. Current performance-assessment (PA) models for a potential repository at Yucca Mountain, Nevada, indicate that the dose contribution from 237Np may become significant after about 50,000 years [1]. This dose estimate is founded in part on the assumption that dissolved concentrations of Np in groundwaters emanating from a breached waste package containing commercial spent nuclear fuel will be limited by the precipitation of Np2O5 [1-3]. To date, no “pure” Np compounds, including crystalline Np2O5, have been identified as alteration products of corroded spent fuel in laboratory experiments on fuel dissolution. Oxidative dissolution of spent nuclear fuels under hydrologically unsaturated conditions, as examined in “drip” experiments conducted at Argonne National Laboratory [4], releases Np to solution congruently with U; however, a substantial amount of U derived from the dissolving fuel matrix has re-precipitated on the surface of the corroded fuel [4,5], implying that a comparable fraction of Np is also being retained in precipitated solids. Furthermore, the concentrations of Np released to solution in drip experiments are well below Np concentrations that might be expected if Np2O5 was limiting the dissolved concentration of Np. In fact, if concentrations of Np in solutions recovered from ANL drip tests are representative of concentrations likely to emanate from a breached waste container with commercial spent nuclear fuel, Np-related radiation doses to individuals near Yucca Mountain might be several orders of magnitude lower than current estimates. Currently available thermodynamic data predict that NpO2 is the predominant stable solid at redox and pH conditions expected in groundwaters at the repository
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