XPS Valence State Determination of Np And Pu in Multicomponent Borosilicate Glass and Application to Leached 76-68 Waste
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XPS VALENCE STATE DETERMINATION OF Np AND Pu IN MULTICOMPONENT BOROSILICATE GLASS AND APPLICATION TO LEACHED 76-68 WASTE GLASS SURFACES
D. P. KARIM, D. J. LAM, H. DIAMOND, A. M. FRIEDMAN Argonne National Laboratory, Argonne, Il 60439 D. G. COLES, F. BAZAN Lawrence Livermore Laboratory, Livermore, Ca 94720 AND G. L. McVAY Battelle Pacific Northwest Laboratory, Richland, Wa 99352
INTRODUCTION Current plans for high-level radioactive waste disposal call for long term emplacement of canisters of a suitably leach resistant waste form in deep underground repositories. Proposed regulations [11 call for canisters with a designed lifetime of 1000 years. After this initial period, most of the remaining activity in the waste will be associated with the actinide elements. Thus, the leach behavior of the actinides takes on special importance. In particular, the leaching mechanisms must be understood if we are to attach any confidence to long term predictive extrapolation of short term leach tests. The actinides, unlike the rare earths, are not particularly similar in chemical behavior nor is their behavior reliably modeled by that of other elements. Thus, more studies directed toward the behavior of the actinides in candidate waste forms are important. The present study involves the use of x-ray photoemission spectroscopy (XPS) to examine the behavior of actinides at the surface of glasses. XPS is an inherently surface sensitive technique, probing the outermost 20-30A of a sample. The technique is routinely used to provide quantitative compositional information by monitoring the intensity of photoemitted electrons from core levels characteristic of specific elements as a sample is illuminated with monochromatic x-rays. In some cases, small changes in the binding energies of these core levels (chemical shifts) can be used to get specific information on the environments and chemical states of ions in a solid. Prior to this study, little work has been done to establish procedures for determining Np or Pu oxidation states in silicate glasses from XPS spectra. Based on our experience with U+4 and q+6 in glasses [21, we hoped that chemical shifts of the Np and Pu 4f core lines would provide convenient signatures for the respective actinide oxidation states. Determining the actinide oxidation states present in the bulk glass and providing the means to monitor changes in oxidation state at leached surfaces should play a major role in establishing the mechanism by which the actinides are released into solution. The actinides, in general, exhibit multiple oxidations states (especially Pu which is known to exist in +3, +4, +5, and +6 states) having greatly different solubilities which in turn influence whether the elements are retained in enhanced concentration in leached glass surface layers. By studying the buildup of these layers, surface studies can provide complementary information to that obtained from solution studies of actinides released into a leachate.
68 TABLE I Glass Composition (wt%)
76-101 Si0 2 B20 3 Na 2 0
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