Publications

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Understanding the corrosion of spent nuclear fuel (SNF) and the subsequent mobilization of released radionuclides, particularly under oxidizing conditions, is one of the key issues in evaluating the long-term performance of a nuclear waste repository. However, the large amounts of iron in the metal waste package may create locally reducing conditions that would lower corrosion rates for the SNF, as well as reduce the solubility of some key radionuclides, e.g., Tc and Np. In order to investigate the interactions among SNF-waste package-fluids, four smallscale (∼1:40 by length) waste package mockups were constructed using metals similar to those proposed for use in waste packages at the proposed repository at Yucca Mountain. Each mockup experiment differed with respect to water input, exposure to the atmosphere, and temperature. Simulated Yucca Mountain process water (YMPW) was injected into three of the mockups at a rate of 200 μL per day for five days a week using a calibrated needle syringe. The YMPW was prepared by equilibrating 50 mg/L silica as sodium metasilicate with air, and adding enough HC1 to lower the pH to 7.6 in contact with an excess of powdered calcite. X-ray powder diffraction and scanning electron microscopy confirm that, where corrosion occurred, the dominant corrosion product in all cases was magnetite. In the high temperature (60°C) experiment, hematite and a fibrous, Fe-O-Cl phase were also identified. The Fe(II)/Fe(III) ratios measured in the corrosion products using a wet chemistry technique indicate extremely low oxygen fugacities (10-36 bar). Experiments are in progress in which 0.1g powdered UO2 was included in the mock-up in order to investigate the relative kinetics of Fe and U oxidation and to identify the U corrosion products formed under these conditions. © 2008 Materials Research Society.

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Apatite, Ca{sub 5}(PO{sub 4}){sub 3}(F,OH,Cl)(P6{sub 3}/m, Z=2), is the most abundant phosphate mineral on Earth. The end-member hydroxyapatite, Ca{sub 5}(PO{sub 4}){sub 3}OH(P2{sub 1}/b), is the primary mineral component in bones and teeth and tends to scavenge and sequester heavy metals in the human body. Hydroxyapatite has also been shown to be effective at sequestering radionuclides and heavy metals in certain natural systems (Dybowska et al., 2004). Hydroxyapatite has been the focus of many laboratory studies and is utilized for environmental remediation of contaminated sites (Moore et al., 2002). The crystal structure of apatite tolerates a great deal of distortion caused by extensive chemical substitutions. Metal cations (e.g. REE, actinides, K, Na, Mn, Ni, Cu, Co, Zn, Sr, Ba, Pb, Cd, Fe) substitute for Ca, and oxyanions (e.g. AsO{sub 4}{sup 3-}, SO{sub 4}{sup 2-}, CO{sub 3}{sup 2-}, SiO{sub 4}{sup 4-}, CrO{sub 4}{sup 2-}) replace PO{sub 4}{sup 3-} through a series of coupled substitutions that preserve electroneutrality. Owing to the ability of apatite to incorporate 'impurities'(including actinides) gives rise to its proposed use as a waste form for radionuclides. Recent work at Sandia National Laboratory demonstrated that hydroxyapatite has a strong affinity for U, Pu, Np, Sr and Tc reduced from pertechnetate (TcO{sub 4}{sup -}) by SnCl{sub 2} (Moore et al., 2002). Based on these earlier promising results, an investigation was initiated into the use of apatite-type materials doped with aliovalent cations including Fe, Cu and Sn as Tc-scavengers. Synthetic Fe and Cu-doped hydroxyapatite samples were prepared by precipitation of Ca, from Ca-acetate, and P, from ammonium phosphate. The Fe and Cu were introduced as chlorides into the Ca-acetate solution. Stannous chloride was used as a reducing agent and was apparently incorporated into the crystal structures of the hydroxyapatite samples in small, as yet undetermined quantities.

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