25 research outputs found
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Development of Controlled Pitch Nano Arrays for Application in Nano Scale Based Proportional Counters
Proportional counters (PCs) are a type of gas-filled radiation detection device capable of distinguishing between a wide range of radiation types and energies. In this application, however, these devices are limited by high power consumption and high bias potentials required to operate in the proportional detection regime. Previous work performed with a single carbon nanotube (CNT) anode has shown that nanoscale-based PCs can operate at bias potentials of 10V rather than the 1000V range required for PCs. ''Proof of concept'' experiments with a single CNT as the anode exhibit a small detection volume and consequently required long count times (24 hrs). To make this a practical detector technology (i.e., decrease the count time), the effective detection volume has to be increased. Experimental data and electric field modeling show that if the pitch (spacing between individual nanotubes) of the arrays is too small, the electric field of the individual nanostructure will collapse and the nanoscale array will behaved as a single macro-scale field with the associated high bias potential required to reach the proportional region. Electric-field modeling of the affect of nanostructure pitch on the electric field distribution of these arrays predicted that a pitch of about two-and-a-times the height of the nanostructure was required to retain the nanoscale electric field. In this work, we report on the fabrication and electrical property testing of nanoscale arrays with a range of controlled pitches
In-Situ Kd Values nad Geochemical Behavior for Inorganic and Organic Constituents of Concern at the TNX Outfall Delta REC.EIVED
in the bulk utter unncaling in forming gas but not after high tempemture ( 1100"C) anneaIs in Ar. The presence of hydrogen dmmatical[y incrcascs the broad PL band centered in the near-infmred after tinncaling Lit1100"C but htis almost no effect on the PL spectral distribution. Hydrogen is found to sclccti~cly trap in the region where Si nanocrystais are formed, consistent with a model of H ptissivatin: surfucc states tit the Si/SiO: interface that leads to enhanced PL. The thermal stabiIity of the tmppul H and the PL yield observed ufter a high temperature anneal have been studied. The hydrogen concentration and PL yield are unchanged for subsequent anneals up to 400"C. However, ubovc -IO(YCthe PL decreases tind a more complicated H chemistry is evident. Similar concentmtions of H or D ure tmpped after annealing in HJ or Dj forming gas; however, no differences in the PL yield or spcctrtil distribution tire observed, indicating that the electronic transitions resulting in luminescence are not dependent on the moss of the hydrogen species
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Additional Information for E-Area Vault Performance Assessment, Appendix I `Suspect Soil Performance` - Results of Modeling the Effects of Organic Matter on the Mobility of Radionuclides as it Relates to the Disposal of Wood Products in E-Area Slit Trenches
Numerous laboratory and field studies have shown that the chemical form (i.e., speciation) of many metals and radionuclides is affected by the presence of naturally occurring organic matter (OM) and its degradation products. The effects of OM (e.g., wood products) on the speciation and, therefore, the mobility of Am, Bk, Cf, Cm, Cs, Ni, NpO{sub 2}, Rb, Sr. UO{sub 2}, and Zr were estimated through use of geochemical and groundwater flow modeling. Due to the complex mixture nature of naturally occurring OM, the OM system was simplified through use of surrogate compounds (citric acid and ethylenedinitrilotetraacetic acid (EDTA)) to estimate effects of OM on radionuclide mobility. Using this approach, OM was found to have no effect on the inventory limits for Cs, NpO{sub 2}, Rb and Zr. The inventory limits for the isotopes of Am, Bk, Cf, Cm, Ni, Pd, PuO{sub 2}, Sr, and UO{sub 2} calculated in the presence of OM decreased over a range of 26 percent for U-233 to 48 percent for Pu-240. The information in this report will be included in the next revision of the E-Area Vaults Performance Assessment
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Uranium partitioning under acidic conditions in a sandy soil aquifer
The partitioning of uranium in an aquifer down gradient of two large mixed waste sites was examined with respect to the solution and soil chemistry (e.g., pH redox potential and contaminant concentration) and aqueous-phase chemical speciation. This involved generation of field-derived, batch sorption, and reactive mineral surface sorption data. Field-derived distribution coefficients for uranium at these waste sites were found to vary between 0.40 and 15,000. Based on thermodynamic speciation modeling and a comparison of field and laboratory data, gibbsite is a potential reactive mineral surface present in modified soils at the sites. Uranium partitioning data are presented from field samples and laboratory studies of background soil and the mineral surface gibbsite. Mechanistic and empirical sorption models fit to the field-derived uranium partitioning data show an improvement of over two orders of magnitude, as measured by the normalized sum of errors squared, when compared with the single K{sub d} model used in previous risk work. Models fit to batch sorption data provided a better fit of sorbed uranium than do models fit to the field-derived data
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SALTSTONE AND RADIONUCLIDE INTERACTIONS: RADIONUCLIDE SORPTION AND DESORPTION, AND SALTSTONE REDUCTION CAPACITY
The overall objective of this study was to measure a number of key input parameters quantifying geochemical processes in the subsurface environment of the Savannah River Site's (SRS's) Saltstone Facility. For the first time, sorption (K{sub d}) values of numerous radionuclides were measured with Saltstone and Vault 2 concrete. Particular attention was directed at understanding how Tc adsorbs and desorbs from these cementitious materials with the intent to demonstrate that desorption occurs at a much slower rate than adsorption, thus permitting the use of kinetic terms instead of (or along with) the steady state K{sub d} term. Another very important parameter measured was the reduction capacity of these materials. This parameter is used to estimate the duration that the Saltstone facility remains in a reduced chemical state, a condition that maintains several otherwise mobile radionuclides in an immobile form. Key findings of this study follow. K{sub d} values for Am, Cd, Ce, Co, Cs, Hg, I, Np, Pa, Pu, Se, Sn, Tc, U, and Y for Saltstone and Vault 2 concrete were measured under oxidized and reduced conditions. Precipitation of several of the higher valence state radionuclides was observed. There was little evidence that the Vault 2 and Saltstone K{sub d} values differed from previous SRS K{sub d} values measured with reducing grout (Kaplan and Coates 2007). These values also supported a previous finding that K{sub d} values of slag-containing cementitious materials, tend to be greater for cations and about the same for anions, than regular cementitious materials without slag. Based on these new findings, it was suggested that all previous reducing concrete K{sub d} values be used in future PAs, except Np(V) and Pu(IV) K{sub d} values, which should be increased, and I values, which should be slightly decreased in all three stages of concrete aging. The reduction capacity of Saltstone, consisting of 23 wt-% blast furnace slag, was 821.8 microequivalents per gram ({micro}eq/g). This value was approximately the same value as the one measured for 100% blast furnace slag. The cause for this approximately four-fold greater reduction capacity than anticipated is not known, but may be the result of the higher pH of Saltstone (pH {approx}11) compared to blast furnace slag (pH {approx}8), the presence of reducing minerals in the fly ash used to make the Saltstone, or to the Saltstone possibly having semi-conductor properties. These reduction capacity values will result in a near four-fold increase in the estimated duration that the Saltstone facility will remain in a reduced chemical state. The implication of this result is that oxidation-state-sensitive contaminants, such as Pu, Np, and Tc, will remain for a longer duration in a much less mobile form than previously believed. The reduction capacity of vault concrete, which consisted of 10 wt-% blast furnace slag, was 240 {micro}eq/g. Essentially all Am, Cd, Ce, Co, Cs, Hg, Sr, and Y was (ad)sorbed within four hours, whereas <3% of the adsorbed metals desorbed from these solids after 90 hours of continuous leaching. In particular, desorption of Tc (under oxidizing conditions) was >10{sup 3} fold slower than (ad)sorption (under reducing conditions). An important implication of this finding is that if groundwater by-passes or short-circuits the reduction capacity of the Saltstone by flowing along a crack, the ability of the oxygenated water to promote Tc desorption is appreciably less than that predicted based on the K{sub d} value. Relatively low Tc K{sub d} values, 6 to 91 mL/g, were measured in these studies indicating that little if any of the Tc(VII) introduced into the Saltstone or Vault 2 concrete suspensions was reduced to Tc(IV). Such a reduction results in apparent K{sub d} values on the order of 10{sup 4} mL/g. As such, these Tc sorption/desorption experiments need additional investigation to fully represent Saltstone environmental conditions. It is important to understand the limits of these data. They do not provide insight into how radionuclides cured and immobilized in Saltstone will leach from the Saltstone. However they do provide insight into how radionuclides once released into porewater will interact with Saltstone or vault concrete. The use of these site-specific data would greatly improve the pedigree of the input data for the Saltstone performance assessment. Additionally, these studies provided important guidance and technical justification for the conceptual geochemical model to be used in the Saltstone performance assessment
Acidic Functional Groups of Suwannee River Natural Organic Matter, Humic Acids, and Fulvic Acids
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Evaluation of Natural and in-Situ Remediation Technologies for a Coal-Related Metals Plume
Metals contamination exceeding drinking water standards (MCLs) is associated with acidic leachate generated from a coal pile runoff basin at the Savannah River Site (SRS) in Aiken, South Carolina. The metals plume extends over 100 acres with its' distal boundary about onehalf mile from the Savannah River. Based on the large plume extent and high dissolved iron and aluminum concentrations, conventional treatment technologies are likely to be ineffective and cost prohibitive. In-situ bioremediation using existing groundwater microbes is being evaluated as a promising alternative technology for effective treatment, along with consideration of natural attenuation of the lower concentration portions of the plume to meet remedial goals. Treatment of the high concentration portion of the groundwater plume by sulfate-reducing bacteria (SRB) is being evaluated through laboratory microcosm testing and a field-scale demonstration. Organic substrates are added to promote SRB growth. These bacteria use dissolved sulfate as an electron acceptor and ultimately precipitate dissolved metals as metal sulfides. Laboratory microcosm testing indicate SRB are present in groundwater despite low pH conditions, and that their growth can be stimulated by soybean oil and sodium lactate. The field demonstration consists of substrate injection into a 30-foot deep by 240-foot long permeable trench. Microbial activity is demonstrated by an increase in pH from 3 to 6 within the trench. Downgradient monitoring will be used to evaluate the effectiveness of SRB in reducing metal concentrations. Natural attenuation (NA) is being evaluated for the low concentration portion of the plume. A decrease in metal mobility can occur through a variety of abiotically and/or biotically mediated mechanisms. Quantification of these mechanisms is necessary to more accurately predict contaminant attenuation using groundwater transport models that have historically relied on simplified conservative assumptions. Result s from matched soil/porewater samples indicate higher soil/water partition coefficients (Kds) with increasing distance from the source. In addition, site-specific metals availability is being assessed using sequential extraction techniques, which more accurately represent environmental conditions as compared to default EPA extraction methods. Due to elevated sulfate levels in the plume, SRB are most likely to be the dominant biotic contributor to NA processes