Evaluation of a conceptual model for the subsurface transport of plutonium involving surface mediated reduction of PuV to PuIV.

A conceptual model is proposed to explain the transport behavior of plutonium in laboratory columns packed with a sandy coastal soil from the U.S. Department of Energy (DOE)'s Savannah River Site. The column transport experiments involved the introduction of a finite step input of plutonium, predominately in the +5 oxidation state, into the columns followed by elution with a low-carbonate solution of 0.02 M NaClO4 at pH 3, 5, and 8. Total plutonium concentrations were measured in the effluent as a function of time. These elution profiles suggest at least two distinct physical/chemical forms of plutonium, each with a different mobility. To explain the observed behavior, the following conceptual model was evaluated: [1] equilibrium partitioning of plutonium (V) and plutonium (IV) between the aqueous and sorbed phases as defined by pH-dependent, oxidation-state specific distribution coefficients and [2] kinetic reduction of plutonium (V) to plutonium (IV) in the sorbed phase. The conceptual model was applied to the column experiments through a one-dimensional advective/dispersive mathematical model, and predictions of the mathematical model were compared with the experimental data. Overall, the model was successful in predicting some of the major features observed in the experiments. It also yielded quantitative estimates of the rate constant for surface mediated reduction of plutonium (V) to plutonium (IV) that were of the same order (10(-4) to 10(-5) s(-1)) as those calculated from batch data both for this soil and for goethite.

The use of extraction chromatography resins to concentrate actinides and strontium from soil for radiochromatographic analyses.

An analytical technique utilizing selective extractant resins to concentrate strontium and actinides from soil followed by separation with radiochromatography was evaluated. The technique was tested using uncontaminated soil samples spiked with a radionuclide tracer solution that were either microwave-aided acid digested or leached with a strong acid. Extraction of the strontium and actinides from the acidified solution was accomplished using a serial arrangement of Sr-Resin and TRU-Resin columns. The combined eluate solutions from the extraction resins were treated with HNO(3) and H(2)O(2) to oxidize residual extractant and eluates prior to separation and analysis of the radionuclides by radiochromatography. Chromatograms obtained with larger soil mass loadings resulted in either incomplete peak resolution of the tracers or had highly variable peak elution times, indicative of an ionic interfering constituent(s). Better separations (e.g., chromatograms that resolved all radioactive constituents) were obtained when the sample mass loading was decreased, but with a concurrent decreased sensitivity for the radionuclides. Elemental analyses of the soil were conducted to provide data on the ionic constituents in unprocessed soil and post-processed soil samples. These results identified aluminum as an interfering contributor to the poor performance exhibited by the radiochromatographic separations.

A comparison of four aqueous-miscible liquid scintillation cocktails with an alpha/beta discriminating Wallac 1415 liquid scintillation counter.

This work was directed towards an assessment of alpha/beta separation, alpha detection efficiency, energy resolution, quench sensitivity, and background count rate for four commercially available liquid scintillation cocktails using a Wallac 1415 liquid scintillation counter. The four cocktails were Wallac Oy OptiPhase HiSafe 3 and HiSafe 2, and Packard Ultima Gold AB and Ultima Gold XR. Aqueous radioactive solutions were prepared in 0.1 N HCl and contained 241Am (5.49 MeV alpha) and 90Sr/90Y (0.46/2.28 MeV beta-). The radioactive solutions were loaded into the cocktails at a ratio of 1 mL of aqueous solution to 10 mL of cocktail in 20 mL high-density polyethylene (HDPE) liquid scintillation vials. The effect of three levels of chemical quench on the various properties of interest was assessed for each of the LS cocktails. Alpha/beta discrimination was quantified by the fraction of beta interactions that "spillover" into the alpha window and vice versa, at the working discriminator setting. The working discriminator setting was defined as the point where the spillover of beta interactions into the alpha window and alpha interactions into the beta window were equal. For the low-quench samples, Ultima Gold AB had the lowest spillover (0.25%) and Ultima Gold XR had the highest spillover (0.8%). For the high-quench samples, the spillover ranged from 4% (HiSafe 3) to 10.5% (Ultima Gold XR). The detection efficiency for 241Am was nearly 100% and decreased with increased quench because of loss from spillover. HiSafe 2 gave the best energy resolution, 500 keV (FWHM), for the low-quench sample. The lowest background count rate was achieved with Ultima Gold AB, 0.056 cpm in the 241Am window, for a low-quench blank.

Potential effects of surface water components on actinide determinations conducted by ion chromatography.

An elution program for separating actinides (thorium, uranium, neptunium, plutonium, americium, and curium) on low hydrophobicity ion exchange columns was evaluated for solutions spiked with actinides and common surface water components. Potential interferences from dissolved ions (Na+, K+, Ca2+, Cl-, and SO(4)2-), humic acid, and radium were investigated. Sulfate levels greater than 0.25 mumol interfered with separation of americium, curium, and plutonium. Humic acid levels above 100 micrograms produced distinct widening of actinide peaks and reduced actinide recoveries. These interferences limit the range of useful sample volumes and create a need for sample pretreatment procedures. No interferences were produced by 0.025 to 2.5 mumol Ca2+, 0.045 to 4.5 mumol Na+, 0.015 to 1.5 mumol K+, and 0.025 to 4.5 mumol Cl-. In the absence of interferences, the program effectively separated radium from the actinides.

Field test of electret ion chambers for environmental monitoring.

A field test of electret ion chambers was performed to evaluate their performance in making environmental exposure measurements at nuclear facilities. The objectives of the study were to determine electret ion chamber variability and to perform comparisons with thermoluminescent dosimeter and high-pressure ion chamber measurements. Three electret ion chambers were placed at each of 40 monitoring locations in the vicinity of a commercial nuclear power station during four consecutive quarters. The electret ion chamber measurements were compared to thermoluminescent dosimeter measurements made by the Nuclear Regulatory Commission and the South Carolina Department of Health and Environmental Control. Two types of comparison were made with the high-pressure ion chamber. One used yearly average electret ion chamber measurement and instantaneous high-pressure ion chamber measurements at 15 of the monitoring locations. The other involved the simultaneous exposure of five electret ion chambers and the high-pressure ion chamber for 15 d at a single location. The mean ratios of electret ion chamber measurements to thermoluminescent dosimeter measurements was 1.06. The mean ratio of electret ion chamber measurements to instantaneous and simultaneous high-pressure ion chamber measurements were 1.06 and 1.07, respectively. Electret variability, defined here as the ratio of the standard deviation to the mean, was determined for each set of three detectors. The average variability for the 160 sets of quarterly measurements was approximately 7%. Among the 450 individual electret measurements, there were six outliers. Based on the results of this study, electret ion chambers appear to yield accurate measurements of environmental exposure provided that measures are taken to either minimize or correct for radon interferences and care is taken to prevent spurious discharges during handling.

A simple approximation for estimating centerline gamma absorbed dose rates due to a continuous Gaussian plume.

A simple approximation for estimating the centerline gamma absorbed dose rates due to a continuous Gaussian plume was developed. To simplify the integration of the dose integral, this approach makes use of the Gaussian cloud concentration distribution. The solution is expressed in terms of the I1 and I2 integrals which were developed for estimating long-term dose due to a sector-averaged Gaussian plume. Estimates of tissue absorbed dose rates for the new approach and for the uniform cloud model were compared to numerical integration of the dose integral over a Gaussian plume distribution.

An exact solution to the Gaussian cloud approximation for gamma absorbed dose due to a ground-level release.

An exact solution is presented for the classical Gaussian cloud approximation for long-term averaged estimates of gamma absorbed dose due to a ground-level release of radioactive gases and particles. The solution is in the form of relatively simple functions and is much easier to implement than the traditional solution. Also, it is shown that the Gaussian cloud approximation predicts lower absorbed dose than the uniform cloud approximation. The difference is significant when the product of gamma attenuation coefficient and vertical dispersion coefficient is small. Since the Gaussian cloud approximation is based on a more realistic concentration distribution than the uniform cloud approximation, the calculations using the former should give more accurate estimates of the long-term averaged dose for the sector-averaged plume.