Interactions of Uranium and Neptunium With Cementitious Materials Studied by XAFS
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INTERACTIONS OF URANIUM AND NEPTUNIUM WITH CEMENTITIOUS MATERIALS STUDIED BY XAFS E. R. SYLWESTER', P. G. ALLEN', P. ZHAO', B. E. VIANI 2 'Glenn T. Seaborg Institute for Transactinium Science, Lawrence Livermore National Laboratory, P.O. Box 808, MS L-231, Livermore, CA 94551, sylwesterl @llnl.gov 2 Geosciences & Environmental Technologies Division, Lawrence Livermore National Laboratory ABSTRACT We have investigated the interaction of U(VI) and Np(V) actinide ions with cementitious materials relevant to nuclear waste repositories using X-Ray Absorption Fine Structure (XAFS) Spectroscopy. The actinide ions were individually loaded onto untreated as well as hydrothermally treated cements. The mixtures were then equilibrated at varying pH's for periods of 1 month and 6 months. In all cases uranium was observed to remain in the initial UO22+ form in the Near Edge (XANES) spectra. The uranium samples show evidence of inner-sphere interactions on both treated and untreated cements at all pH's, with the uranyl complexing with the mineral surface via sharing of equatorial oxygens. On treated cement near-neighbor U-U interactions are also observed, indicating the formation of oligomeric surface complexes or surface precipitates. Neptunium was observed to undergo a reduction from the initial NpO 2' to Np 4+ Calculated % reduction showed ca. 15% of Np(V) is reduced to Np(IV) after a 1 month equilibration time. After 6 months higher % reduction of between 40% and 65% was observed. No Np-Np interactions were observed in the EXAFS spectra, which suggests that surface precipitation of NpO 2 is an unlikely mechanism for sorption. INTRODUCTION Cementitious materials comprise a large fraction of the near-field environment of US high-level nuclear waste repositories. Interactions of the waste radioisotopes with concrete can be expected to occur after primary containment is breached but before migration to the far field. Concrete has been shown to be a strong sorber of some actinides [1, 2]. However, it is expected that during storage times >1,000 years the infrastructure of these repositories will be subjected to radiative heating as well as variations in humidity. These conditions may alter the chemical or structural characteristics of the cement and the materials' ability to sorb waste radionuclides. We have used X-Ray Absorption Fine Structure (XAFS) spectroscopy to study samples of Uranium and Neptunium sorbed onto both treated and untreated portland cement in order to determine changes in the speciation (oxidation state and structure) of the radionuclides as a function of cement treatment, sample pH, and time. EXPERIMENT Sample Preparation Samples of crushed (
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