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Effect of Reducing Groundwater on the Retardation of Redox-Sensitive Radionuclides

Description: Laboratory batch sorption experiments were used to investigate variations in the retardation behavior of redox-sensitive radionuclides. Water-rock compositions used during these experiments were designed to simulate subsurface conditions at the Nevada Test Site (NTS), where a suite of radionuclides were deposited as a result of underground nuclear testing. Experimental redox conditions were controlled by varying the oxygen content inside an enclosed glove box and by adding reductants into the testing solutions. Under atmospheric (oxidizing) conditions, the radionuclide distribution coefficients varied with the mineralogical composition of the sorbent and the water chemistry. Under reducing conditions, distribution coefficients showed marked increases for {sup 99}Tc and {sup 237}Np in devitrified tuff, but much smaller variations in alluvium, carbonate rock, and zeolitic tuff. This effect was particularly important for {sup 99}Tc, which tends to be mobile under oxidizing conditions. Unlike other redox-sensitive radionuclides, iodine sorption may decrease under reducing conditions when I{sup -} is the predominant species. Overall, sorption of U to alluvium, devitrified tuff, and zeolitic tuff under atmospheric conditions was less than in the glove-box tests. However, the mildly reducing conditions achieved here were not likely to result in substantial U(VI) reduction to U(IV). Sorption of Pu was not affected by the decreasing redox conditions achieved in this study, as the predominant sorbed Pu species in all conditions was expected to be the low-solubility and strongly sorbing Pu(OH){sub 4}. Depending on the aquifer lithology, the occurrence of reducing conditions along a groundwater flowpath could potentially contribute to the retardation of redox-sensitive radionuclides {sup 99}Tc and {sup 237}Np, which are commonly identified as long-term dose contributors in the risk assessment in various nuclear facilities.
Date: April 21, 2008
Creator: Hu, Q; Zavarin, M & Rose, T P
Partner: UNT Libraries Government Documents Department

Distribution of 99Tc and 129I in the Vicinity of Underground Nuclear Tests at the Nevada Test Site

Description: {sup 99}Tc and {sup 129}I are important contributors to risk assessment due to their long half-lives and high mobility as aqueous anionic species. We analyzed {sup 99}Tc and {sup 129}I in groundwater samples in and near 11 underground nuclear tests and in melt glass and rock samples retrieved from the Chancellor test cavity, Nevada Test Site. The {sup 129}I/{sup 127}I ratio ranges from 10{sup -3} to 10{sup -6} in cavity water and 10{sup -4} to 10{sup -9} in satellite wells. The {sup 99}Tc concentration ranges from 3 to 10{sup -4} Bq/L in cavity waters and from 0.3 to 10{sup -4} Bq/L in satellite wells. Downstream migration is apparent for both radionuclides. However, it is affected by both retardation and initial distribution. In-situ {sup 99}Tc and {sup 129}I K{sub d}s calculated using rubble and water concentrations are 3 to 22 mL/g and 0 to 0.12 mL/g, respectively and are suggestive of mildly reducing conditions. {sup 129}I distribution in the melt glass, rubble and groundwater of the Chancellor test cavity is 28%, 24% and 48%, respectively; for {sup 99}Tc, it is 65%, 35% and 0.3%, respectively. Our partitioning estimates differ from those of underground tests in French Polynesia, implying that fission product distribution may vary from test to test. Factors that may influence this distribution include geologic conditions (e.g. lithology, water and CO{sub 2} content) and the cooling history of the test cavity.
Date: March 17, 2006
Creator: Zhao, P.; Hu, Q.; Rose, T. P.; Nimz, G. J. & Zavarin, M.
Partner: UNT Libraries Government Documents Department

Sorption Behavior and Morphology of Plutonium in the Presence of Goethite at 25 and 80C

Description: In this study, we examined the sorption behavior of Pu at elevated temperatures in the presence of one relevant mineral, goethite ({alpha}-FeOOH), over a range of concentrations that span solubility-controlled to adsorption-controlled concentrations. We focused on the sorptive behavior of two common forms of Pu: aqueous Pu(IV) and intrinsic Pu(IV) nano-colloids at 25 and 80 C in a dilute pH 8 NaCl/NaHCO{sub 3} solution. The morphology of Pu sorbed to goethite was characterized using transmission electron microscopy (TEM). We examined the relative stability of PuO{sub 2} precipitates, PuO{sub 2} nano-colloids, Pu{sub 4}O{sub 7} surface precipitates, and monomeric sorbed Pu as a function of temperature and over a time scale of months.
Date: June 11, 2012
Creator: Zavarin, M.; Zhao, P.; Dai, Z.; Carroll, S. A. & Kersting, A. B.
Partner: UNT Libraries Government Documents Department

Development of a Composite Non-Electrostatic Surface Complexation Model Describing Plutonium Sorption to Aluminosilicates

Description: Due to their ubiquity in nature and chemical reactivity, aluminosilicate minerals play an important role in retarding actinide subsurface migration. However, very few studies have examined Pu interaction with clay minerals in sufficient detail to produce a credible mechanistic model of its behavior. In this work, Pu(IV) and Pu(V) interactions with silica, gibbsite (Aloxide), and Na-montmorillonite (smectite clay) were examined as a function of time and pH. Sorption of Pu(IV) and Pu(V) to gibbsite and silica increased with pH (4 to 10). The Pu(V) sorption edge shifted to lower pH values over time and approached that of Pu(IV). This behavior is apparently due to surface mediated reduction of Pu(V) to Pu(IV). Surface complexation constants describing Pu(IV)/Pu(V) sorption to aluminol and silanol groups were developed from the silica and gibbsite sorption experiments and applied to the montmorillonite dataset. The model provided an acceptable fit to the montmorillonite sorption data for Pu(V). In order to accurately predict Pu(IV) sorption to montmorillonite, the model required inclusion of ion exchange. The objective of this work is to measure the sorption of Pu(IV) and Pu(V) to silica, gibbsite, and smectite (montmorillonite). Aluminosilicate minerals are ubiquitous at the Nevada National Security Site and improving our understanding of Pu sorption to aluminosilicates (smectite clays in particular) is essential to the accurate prediction of Pu transport rates. These data will improve the mechanistic approach for modeling the hydrologic source term (HST) and provide sorption Kd parameters for use in CAU models. In both alluvium and tuff, aluminosilicates have been found to play a dominant role in the radionuclide retardation because their abundance is typically more than an order of magnitude greater than other potential sorbing minerals such as iron and manganese oxides (e.g. Vaniman et al., 1996). The sorption database used in recent HST models (Carle et al., ...
Date: October 28, 2008
Creator: Powell, B A; Kersting, A; Zavarin, M & Zhao, P
Partner: UNT Libraries Government Documents Department

Np and Pu Sorption to Manganese Oxide Minerals

Description: Manganese oxide minerals are a significant component of the fracture lining mineralogy at Yucca Mountain (Carlos et al., 1993) and within the tuff-confining unit at Yucca Flat (Prothro, 1998), Pahute Mesa (Drellack et al., 1997), and other locations at the Nevada Test Site (NTS). Radionuclide sorption to manganese oxide minerals was not included in recent Lawrence Livermore National Laboratory (LLNL) hydrologic source term (HST) models which attempt to predict the migration behavior of radionuclides away from underground nuclear tests. However, experiments performed for the Yucca Mountain Program suggest that these minerals may control much of the retardation of certain radionuclides, particularly Np and Pu (Triay et al., 1991; Duff et al., 1999). As a result, recent HST model results may significantly overpredict radionuclide transport away from underground nuclear tests. The sorption model used in HST calculations performed at LLNL includes sorption to iron oxide, calcite, zeolite, smectite, and mica minerals (Zavarin and Bruton 2004a; 2004b). For the majority of radiologic source term (RST) radionuclides, we believe that this accounts for the dominant sorption processes controlling transport. However, for the case of Np, sorption is rather weak to all but the iron and manganese oxides (Figure 1). Thus, we can expect to significantly reduce predicted Np transport by accounting for Np sorption to manganese oxides. Similarly, Pu has been shown to be predominantly associated with manganese oxides in Yucca Mountain fractured tuffs (Duff et al., 1999). Recent results on colloid-facilitated Pu transport (Kersting and Reimus, 2003) also suggest that manganese oxide coatings on fracture surfaces may compete with colloids for Pu, thus reducing the effects of colloid-facilitated Pu transport (Figure 1b). The available data suggest that it is important to incorporate Np and Pu sorption to manganese oxides in reactive transport models. However, few data are available for inclusion in our ...
Date: August 30, 2005
Creator: Zhao, P; Johnson, M R; Roberts, S K & Zavarin, M
Partner: UNT Libraries Government Documents Department

Summary of Radionuclide Reactive Transport Experiments in Fractured Tuff and Carbonate Rocks from Yucca Flat, Nevada Test Site

Description: In the Yucca Flat basin of the Nevada Test Site (NTS), 747 shaft and tunnel nuclear detonations were conducted primarily within the tuff confining unit (TCU) or the overlying alluvium. The TCU in the Yucca Flat basin is hypothesized to inhibit radionuclide migration to the highly transmissive and regionally extensive lower carbonate aquifer (LCA) due to its wide-spread aerial extent, low permeability, and chemical reactivity. However, fast transport pathways through the TCU by way of fractures may provide a migration path for radionuclides to the LCA. Radionuclide transport in both TCU and the LCA fractures is likely to determine the location of the contaminant boundary for the Yucca Flat/Climax Mine Corrective Action Unit (CAU). Radionuclide transport through the TCU may involve both matrix and fracture flow. However, radionuclide migration over significant distances is likely to be dominated by fracture transport. Transport through the LCA will almost certainly be dominated by fracture flow, as the LCA has a very dense, low porosity matrix with very low permeability. Because of the complex nature of reactive transport in fractures, a stepwise approach to identifying mechanisms controlling radionuclide transport was used. The simplest LLNL experiments included radionuclide transport through synthetic parallel-plate fractured tuff and carbonate cores. These simplified fracture transport experiments isolated matrix diffusion and sorption effects from all other fracture transport processes (fracture lining mineral sorption, heterogeneous flow, etc.). Additional fracture transport complexity was added by performing induced fractured LCA flowthrough experiments (effect of aperture heterogeneity) or iron oxide coated parallel plate TCU flowthrough experiments (effect of fracture lining minerals). Finally naturally fractured tuff and carbonate cores were examined at LLNL and LANL. All tuff and carbonate core used in the experiments was obtained from the USGS Core Library, Mercury, Nevada. Readers are referred to the original reports ''Radionuclide Transport in Tuff and ...
Date: October 11, 2006
Creator: Zavarin, M; Roberts, S; Reimus, P & Johnson, M
Partner: UNT Libraries Government Documents Department


Description: Underground atomic weapons testing at the Nevada Test Site introduced numerous radionuclides that may be used to characterize subsurface hydrologic transport processes in arid climates. Beginning in 1975, groundwater adjacent to the CAMBRIC test, conducted beneath Frenchman Flat in 1965, was pumped steadily for 16 years to elicit experimental information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, {sup 36}Cl and {sup 85}Kr, were extensively monitored prior to their discharge into an unlined ditch flowing toward a dry lake bed over a kilometer away. We have applied a large (6km x 6km x 1km) and highly resolved (4 m) variably saturated flow model to investigate infiltration into the 220-m vadose zone underlying the ditch as well as subsequent groundwater recharge and well recirculation processes. A Lagrangian particle-tracking model has been used to compute flow pathways and estimate radionuclide travel and residence times in various parts of the system based upon the flow model. Results are consistent with rising tritium levels observed in a monitoring well since 1991. They suggest that recirculation of the ditch effluent through the vadose zone, into groundwater, and back to the test cavity and pumping well are responsible for diluted, tritium-based groundwater age dates observed in 2000 at these locations, as well as for increased tailing effects observed in the pumping well elution curves. Altogether, the models and experimental observations provide an improved basis to understand both historical and future movements of test-related radionuclides in groundwater near CAMBRIC.
Date: March 16, 2006
Creator: Maxwell, R; Tompson, A; Carle, S; Zavarin, M & Kollet, S
Partner: UNT Libraries Government Documents Department

Assessing field-scale migration of mobile radionuclides at the Nevada Test Site

Description: Numerous long-lived radionuclides, including {sup 99}Tc (technetium) and {sup 129}I (iodine), are present in groundwater at the Nevada Test Site (NTS) as a result of 828 underground nuclear weapons tests conducted between 1951 and 1992. We synthesize a body of groundwater data collected on the distribution of a number of radionuclides ({sup 3}H, {sup 14}C, {sup 36}Cl, {sup 99}Tc and {sup 129}I), which are presumably mobile in the subsurface and potentially toxic to down-gradient receptors, to assess their migration at NTS, at field scales over distances of hundreds of meters and for durations of more than thirty years. Qualitative evaluation of field-scale migration of these radionuclides in the saturated zone provides an independent approach to validating their presumably conservative transport in the performance assessment of the proposed geological repository at Yucca Mountain, which is located on the western edge of NTS. The analyses show that the interaction of {sup 3}H with a solid surface via an isotopic exchange with clay lattice hydroxyls may cause a slight delay in the transport of {sup 3}H. The transport of {sup 14}C could be retarded by its isotopic exchange with carbonate minerals, and the exchange may be more pronounced in the alluvial aquifer. In particular, {sup 99}Tc may not necessarily exist as a mobile and conservative species {sup 99}TcO{sub 4}{sup -}, as commonly assumed for NTS groundwater. This is corroborated with recent in situ redox potential measurements, both across and near Yucca Mountain, showing that groundwater at multiple locations is not oxidizing. Speciation of iodine and its associated reactivity and mobility is also complex in the groundwater at the NTS and deserves further attention. The assumption of no retardation for the transport of {sup 99}Tc (especially) and {sup 129}I, used at the performance assessment of Yucca Mountain repository, is probably overly conservative and results ...
Date: September 26, 2006
Creator: Hu, Q; Rose, T P; Smith, D K; Moran, J E & Zavarin, M
Partner: UNT Libraries Government Documents Department

Nickel and manganese interaction with calcite

Description: Many divalent metal cations sorb to calcite surfaces and incorporate into calcite to varying degrees. Since calcite may sorb trace elements in the environment, the factors controlling metal-calcite interactions are critical to understanding element cycling. The interaction of divalent metal cations with calcite can be critical to toxic metal immobilization, nutrient cycling, interpretation of past redox conditions, tracing fluid flow, for example. Sorption of Ni and Mn on calcite surfaces was studied by Zachara et al.. At any particular pH, the sorption of Mn on calcite was greater than Ni. This was attributed in part to the similarity of divalent Mn and Ca with respect to ion size. Although direct spectroscopic evidence was not available, sorption/desorption results suggested that Mn quickly forms a surface precipitate or solid solution while Ni forms a hydrated surface complex that may incorporate into calcite much more slowly via recrystallization. Because Mn(II) ionic radius is similar to that of Ca(II) (0.80 versus 1.0{angstrom}), and because MnCO{sub 3} has a structure similar to calcite, it is likely that Mn can substitute directly for Ca in the calcite structure. The ionic radius of Ni(II) is significantly smaller (0.69{angstrom}) and Ni(OH){sub 2} precipitation is likely to be favored in most systems. For Ni, direct substitution for Ca is less likely or may require more significant calcite lattice deformation.
Date: August 9, 1999
Creator: Doner, H & Zavarin, M
Partner: UNT Libraries Government Documents Department

Modeling of Groundwater Flow and Radionuclide Transport at the Climax Mine sub-CAU, Nevada Test Site

Description: The Yucca Flat-Climax Mine Corrective Action Unit (CAU) on the Nevada Test Site comprises 747 underground nuclear detonations, all but three of which were conducted in alluvial, volcanic, and carbonate rocks in Yucca Flat. The remaining three tests were conducted in the very different hydrogeologic setting of the Climax Mine granite stock located in Area 15 at the northern end of Yucca Flat. As part of the Corrective Action Investigation (CAI) for the Yucca Flat-Climax Mine CAU, models of groundwater flow and radionuclide transport will be developed for Yucca Flat. However, two aspects of these CAU-scale models require focused modeling at the northern end of Yucca Flat beyond the capability of these large models. First, boundary conditions and boundary flows along the northern reaches of the Yucca Flat-Climax Mine CAU require evaluation to a higher level of detail than the CAU-scale Yucca Flat model can efficiently provide. Second, radionuclide fluxes from the Climax tests require analysis of flow and transport in fractured granite, a unique hydrologic environment as compared to Yucca Flat proper. This report describes the Climax Mine sub-CAU modeling studies conducted to address these issues, with the results providing a direct feed into the CAI for the Yucca Flat-Climax Mine CAU. Three underground nuclear detonations were conducted for weapons effects testing in the Climax stock between 1962 and 1966: Hard Hat, Pile Driver, and Tiny Tot. Though there is uncertainty regarding the position of the water table in the stock, it is likely that all three tests were conducted in the unsaturated zone. In the early 1980s, the Spent Fuel Test-Climax (SFT-C) was constructed to evaluate the feasibility of retrievable, deep geologic storage of commercial nuclear reactor wastes. Detailed mapping of fractures and faults carried out for the SFT-C studies greatly expanded earlier data sets collected in association ...
Date: September 28, 2007
Creator: Pohlmann, K.; Ye, M.; Reeves, D.; Zavarin, M.; Decker, D. & Chapman, J.
Partner: UNT Libraries Government Documents Department

Actinide Sorption in Rainier Mesa Tunnel Waters from the Nevada Test Site

Description: The sorption behavior of americium (Am), plutonium (Pu), neptunium (Np), and uranium (U) in perched Rainier Mesa tunnel water was investigated. Both volcanic zeolitized tuff samples and groundwater samples were collected from Rainier Mesa, Nevada Test Site, NV for a series of batch sorption experiments. Sorption in groundwater with and without the presence of dissolved organic matter (DOM) was investigated. Am(III) and Pu(IV) are more soluble in groundwater that has high concentrations of DOM. The sorption K{sub d} for Am(III) and Pu(IV) on volcanic zeolitized tuff was up to two orders of magnitude lower in samples with high DOM (15 to 19 mg C/L) compared to samples with DOM removed (< 0.4 mg C/L) or samples with naturally low DOM (0.2 mg C/L). In contrast, Np(V) and U(VI) sorption to zeolitized tuff was much less affected by the presence of DOM. The Np(V) and U(VI) sorption Kds were low under all conditions. Importantly, the DOM was not found to significantly sorb to the zeolitized tuff during these experiment. The concentration of DOM in groundwater affects the transport behavior of actinides in the subsurface. The mobility of Am(III) and Pu(IV) is significantly higher in groundwater with elevated levels of DOM resulting in potentially enhanced transport. To accurately model the transport behavior of actinides in groundwater at Rainier Mesa, the low actinide Kd values measured in groundwater with high DOM concentrations must be incorporated in predictive transport models.
Date: December 17, 2007
Creator: Zhao, P.; Zavarin, M.; Leif, R.; Powell, B.; Singleton, M.; Lindvall, R. et al.
Partner: UNT Libraries Government Documents Department

Neptunium Transport Behavior in the Vicinity of Underground Nuclear Tests at the Nevada Test Site

Description: We used short lived {sup 239}Np as a yield tracer and state of the art magnetic sector ICP-MS to measure ultra low levels of {sup 237}Np in a number of 'hot wells' at the Nevada National Security Site (NNSS), formerly known as the Nevada Test Site (NTS). The results indicate that {sup 237}Np concentrations at the Almendro, Cambric, Dalhart, Cheshire and Chancellor sites, are in the range of 3 x 10{sup -5} to 7 x 10{sup -2} pCi/L and well below the MCL for alpha emitting radionuclides (15 pCi/L) (EPA, 2009). Thus, while Np transport is believed to occur at the NNSS, activities are expected to be well below the regulatory limits for alpha-emitting radionuclides. We also compared {sup 237}Np concentration data to other radionuclides, including tritium, {sup 14}C, {sup 36}Cl, {sup 99}Tc, {sup 129}I, and plutonium, to evaluate the relative {sup 237}Np transport behavior. Based on isotope ratios relative to published unclassified Radiologic Source Terms (Bowen et al., 1999) and taking into consideration radionuclide distribution between melt glass, rubble and groundwater (IAEA, 1998), {sup 237}Np appears to be substantially less mobile than tritium and other non-sorbing radionuclides, as expected. However, this analysis also suggests that {sup 237}Np mobility is surprisingly similar to that of plutonium. The similar transport behavior of Np and Pu can be explained by one of two possibilities: (1) Np(IV) and Pu(IV) oxidation states dominate under mildly reducing NNSS groundwater conditions resulting in similar transport behavior or (2) apparent Np transport is the result of transport of its parent {sup 241}Pu and {sup 241}Am isotopes and subsequent decay to {sup 237}Np. Finally, measured {sup 237}Np concentrations were compared to recent Hydrologic Source Term (HST) models. The 237Np data collected from three wells in Frenchman Flat (RNM-1, RNM-2S, and UE-5n) are in good agreement with recent HST ...
Date: December 3, 2010
Creator: Zhao, P; Tinnacher, R M; Zavarin, M; Williams, R W & Kersting, A B
Partner: UNT Libraries Government Documents Department