Robert A. Fjeld

The effect of particle charge on penetration in an electret filter

Experiments were performed to identify collection mechanisms for 0.5 mu m diameter particles in electret filter media and to determine the effect of particle charge on penetration. Highly monodisperse polystyrene particles were charged to various levels, and their penetration through charged to various levels, and their penetration through charged and discharged electret filters was measured with an optical particle counter. Particle penetration through charged filters was significantly lower than through discharged filters. Also, in charged filters large decreases in penetration were observed with increasing particle charge, while in discharged filters much smaller decreases occurred. Based on these results it is concluded that electrophoresis played a dominant role in the collection of charged particles, dielectrophoresis was important only at very low charge levels, and mechanical collection processes were relatively unimportant. The experimental data were compared to theoretical penetration predictions of R.C. Brown (1981). Although measured values of penetration were found to be significantly higher than those calculated from theory, the data show a systematic dependence on a single dimensionless parameter that is predicted by theory. >

The impact of mineralogy in the U(VI)—Ca—PO4 system on the environmental availability of uranium

Kinetic dissolution studies were conducted on four prominent U-Ca-PO4 minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1 mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the mineralsexamined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1 mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium.

Continuum field-diffusion theory for bipolar charging of aerosols

Abstract Continuum regime field-diffusion theory for the acquisition of charge by particles exposed to bipolar ions is developed. An approximate numerical solution to the governing equations is described which, in the limit as the external field approaches zero, yields results within 1% of exact solutions. The numerical method is used to calculate particle charge as a function of time, external electric field strength, ratio of positive to negative ion conductivity, particle radius and particle dielectric constant. With increasing time, charge is predicted to approach a steady state value asymptotically. For a given conductivity ratio, steady state charge is predicted to increase almost linearly with increasing external electric field. For a given charging time and field strength, charge asymptotically approaches unipolar levels with increasing conductivity ratio. Predictions based on theory generally show good agreement with experiment, typically being within ± 10–15%. The exception to this is for intermediate conductivity ratios, where theory exceeds experiment by as much as 30°.

Evaluation of a conceptual model for the subsurface transport of plutonium involving surface mediated reduction of Pu(V) to Pu(IV)

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.

THE EFFECT OF SAMPLE MATRIX QUENCHING ON THE MEASUREMENT OF TRACE URANIUM CONCENTRATIONS IN AQUEOUS SOLUTIONS USING KINETIC PHOSPHORIMETRY

Laser-induced kinetic phosphorimetry is an accurate, sensitive and rapid alternative to radiometric determination of natural and depleted uranium in aqueous solutions. This method offers detection limits below 10 ng/l U (2.5·10−4 Bq/l natural U) and a broad analytical range to 5 mg/l U (130 Bq/l natural U). For many samples, dilution is the only sample preparation required. However, because this technique infers uranyl concentrations from time-resolved phosphorescence intensities, results are dependent upon sample matrix constituents that affect the phosphorescence of the uranyl cation. This study examines the influence of cations, anions and ligands common to natural water, process and bioassay samples on the quenching of uranyl phosphorescence and the consequences for lower limits of detection and accuracy of measurements.

Pu(V) transport through Savannah River Site soils - an evaluation of a conceptual model of surface- mediated reduction to Pu (IV).

Over the last fifteen years the Savannah River Site (SRS) in South Carolina, USA, was selected as the site of three new plutonium facilities: the Mixed Oxide Fuel Fabrication Facility, Pit Disassembly and Conversion Facility, and the Pu Immobilization Plant. In order to assess the potential human and environmental risk associated with these recent initiatives, improved understanding of the fate and transport of Pu in the SRS subsurface environment is necessary. The hypothesis of this study was that the more mobile forms of Pu, Pu(V) and Pu(VI), would be reduced to the less mobile Pu(III/IV) oxidation states under ambient SRS subsurface conditions. Laboratory-scale dynamic flow experiments (i.e., column studies) indicated that Pu(V) was very mobile in SRS sediments. At higher pH values the mobility of Pu decreased and the fraction of Pu that became irreversibly sorbed to the sediment increased, albeit, only slightly. Conversely, these column experiments showed that Pu(IV) was essentially immobile and was largely irreversibly sorbed to the sediment. More than 100 batch sorption experiments were also conducted with four end-member sediments, i.e., sediments that include the chemical, textural, and mineralogical properties likely to exist in the SRS. These tests were conducted as a function of initial Pu oxidation state, pH, and contact time and consistently demonstrated that although Pu(V) sorbed initially quite weakly to sediments, it slowly, over the course of <33 days, sorbed very strongly to sediments, to approximately the same degree as Pu(IV). This is consistent with our hypothesis that Pu(V) is reduced to the more strongly sorbing form of Pu, Pu(IV). These studies provide important experimental support for a conceptual geochemical model for dissolved Pu in a highly weathered subsurface environment. That is that, irrespective of the initial oxidation state of the dissolved Pu introduced into a SRS sediment system, Pu(IV) controls the environmental transport within a couple weeks and Pu strongly binds to the sediment, limiting its mobility.

Characterization of the mobilities of selected actinides and fission/activation products in laboratory columns containing subsurface material from the Snake River Plain

Abstract Laboratory column tests were performed to characterize the mobilities of 60Co, 90Sr, 137Cs, 233U, 239Pu, and 241Am in a basalt sample and a composite of sedimentary interbed from the Snake River Plain at the Idaho National Engineering and Environmental Laboratory. The radionuclides were spiked into a synthetic groundwater (pH 8, ionic strength = 0.004 M) and introduced into the columns (D = 2.6 cm, L = 15.2 cm) as finite steps with a width of 1 pore volume followed by unspiked synthetic groundwater. The effluent concentrations were measured continuously for up to 200 pore volumes. Hydrogen-3 was used as a nonreactive tracer in all of the experiments to monitor for channeling. In the basalt sample, the behavior of 90Sr, 137Cs, and 233U was quite different from that of 60Co, 239Pu, and 241Am. The column effluent curves for the former were characterized by single peaks containing, within the limits of experimental uncertainty, all of the activity in the spike. The mobilities were ordered as follows: 233U ([overbar]R = 5.6) > 90Sr ([overbar]R = 29) > 137Cs ([overbar]R = 79). The curves for the other radionuclides were characterized by two or three fractions, each having a distinctly different mobility. Cobalt-60 had high- ([overbar]R = < 3), intermediate- ([overbar]R = 34), and low- (R > 200) mobility fractions. Although a majority of the 239Pu and 241Am had low mobility (R > 200), there were high-mobility (R < 3) fractions of each (17 to 29% for 239Pu and 7 to 12% for 241Am). In sedimentary interbed, mobilities were generally much lower than in basalt. Uranium-233 was the only radionuclide with 100% recovery within 200 displaced pore volumes, and it had a retardation factor of 30. However, high-mobility fractions were observed for 60Co (1 to 4%) and 239Pu (1.1 to 2.4%). These results could have important implications with respect to transport modeling. If the multiple-mobility fractions observed here are also present in the field, transport predictions based on classical modeling approaches that incorporate mobilities from batch sorption experiments are likely to be in error.

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.

Evaluation of a direct extraction/liquid scintillation counting technique for the measurement of uranium in water

An extraction procedure utilizing alpha liquid scintillation was evaluated for the rapid determination of uranium in aqueous environmental samples. The extraction efficiency of the system was measured under varying chemical conditions including pH. The procedure was evaluated against a traditional radiochemical technique using both laboratory prepared control samples and actual groundwater. Finally, the possibility of obtaining isotopic information from the liquid scintillation spectra was also investigated using a curve fitting routine.