Neptunium Incorporation into Uranium(VI) Compounds formed During Aqueous Corrosion of Neptunium-Bearing Uranium Oxides
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NEPTUNIUM INCORPORATION INTO URANIUM(VI) COMPOUNDS FORMED DURING AQUEOUS CORROSION OF NEPTUNIUM-BEARING URANIUM OXIDES Robert J. Finch, Jeffery A. Fortner, Edgar C. Buck1, Stephen F. Wolf2 Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439 USA 1 Present address: Battelle Pacific Northwest National Laboratory, Richland, WA 99352 USA 2 Present address: Indiana State University, Terre Haute, IN, 47809, USA ABSTRACT We report results of experimental studies on the behavior of Np during aqueous corrosion of unirradiated Np-bearing U oxides. Np-doped U oxides were reacted in humid air at 90°C and 150°C for several weeks within sealed stainless-steel vessels. Reacted solids were examined by scanning and transmission electron microscopies (SEM and TEM), electron energy-loss spectroscopy (EELS), 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. Preliminary EELS analysis on crushed grains verify that dehydrated schoepite formed at 150°C contains up to approximately 2 wt.% Np, corresponding to a maximum Np:U molar ratio of approximately 1:40. These are maximum values because the degree to which surface-sorbed Np is present on the grains analyzed is not yet known. Crystalline NpO2 also precipitated during these experiments, and the concentration of Np in dehydrated schoepite may represent the maximum amount of Np that can be incorporated into dehydrated schoepite under the experimental conditions. INTRODUCTION Because if its long half life (2.14 Ma), radiotoxicity and potential mobility, neptunium-237 ( Np) is an isotope relevant to evaluating long-term performance of a geologic repository for high-level nuclear waste. Current performance-assessment (PA) models for the potential repository at Yucca Mountain, Nevada, indicate that the dose contribution from 237Np may become significant beyond 10,000 years [1]. Because Np is potentially mobile in oxygenated groundwaters [2], it is important to understand processes that may affect migration of Np in groundwaters that have interacted with Np-bearing waste forms. Understanding such processes provides a scientific basis for conceptual models used to estimate Np concentrations in groundwaters emanating from a breached waste package containing commercial spent nuclear fuel. Dissolution of spent nuclear fuels under hydrologically unsaturated conditions, as examined in “drip” experiments conducted at Argonne National Laboratory [3], 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], implying that a comparable fraction of Np is also being retained in precipitated solids. Crystal chemical similarities between oxysalts of Np(V) and U(VI) indicate that Np(V) may substitute into some U(VI) corrosion products [5]; however, Np(V) can substitute for U(VI) only if (1) dissolved Np and U co-exist in solution such that they can co-precipi
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