Plutonium Contamination Valence State Determination Using X-ray Absorption Fine Structure Permits Concrete Recycle

This paper describes the determination of the speciation of plutonium contamination present on concrete surfaces at the Rocky Flats Environmental Technology Site (RFETS). At RFETS, the plutonium processing facilities have been contaminated during multiple events over their 50 year operating history. Contamination has resulted from plutonium fire smoke, plutonium fire fighting water, milling and lathe operation aerosols, furnace operations vapors and plutonium "dust" diffusion. Samples considered representative of the contamination processes were removed, and shipped to Los Alamos National Laboratory for preparation as targets for X-ray Absorption Fine Structure measurements. The samples were taken to the Stanford Synchrotron Radiation Laboratory (SSRL) and bombarded with Xrays produced under dedicated synchrotron x-ray production conditions (3.0 GeV e, 45-100 mA). Standards containing Pu(+4) and Pu(+5), blanks spiked with Pu(+4) and Pu(+5), and the field samples and duplicates were irradiated. Evaluation of the data determined that Pu(+4) and Pu(+5) standards were readily identifiable, that the "spiked samples" were consistent with the standards, that the duplicate samples were comparable and that all of the Pu contamination samples were Pu(+4). These concrete samples represent about half of the plutonium samples ever analyzed by XAFS. This methodology is applicable to surface contamination plutonium and does not alter the chemistry of the sampled material. The contamination levels are relatively low and no other method of speciation is available for this concentration of plutonium. The use of this methodology should be considered when speciation determination is needed of relative low concentrations of plutonium contamination. The decommissioning of RFETS includes the removal of several multistory structures and the subsequent return to grade of the basements. Current estimates are 170,000 cubic meters of structural fill will be required. The Decontamination & Decommissioning Program has proposed to recycle as structural fill the bulk of the 100,000 cubic meters of above-grade concrete on the site. Successful recycle would reduce the environmental impact of the site cleanup by 19,000 offsite transportation round trips totaling over 1,700,000 km. A significant portion of the concrete would be from potentially plutonium contaminated buildings so the long term environmental transport of plutonium contamination will impact the recycling method. The plutonium speciation was determined to be Pu(+4). This means the plutonium is present as PuO2 which is essentially insoluble in ground water. Therefore, transport of the Pu from contamination remaining on recycled concrete does not occur until bulk transport of the concrete occurs. This confirms that the recycle method poses essentially no threat to the health and safety of the public and the environment. WM’02 Conference, February 24-28, 2002, Tucson, AZ