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Geochemical simulation of dissolution of West Valley and DWPF [Defense Waste Product Facility] glasses in J-13 water at 90{sup 0}C.

Description: Dissolution of West Valley and Defense Waste Product Facility (DWPF) glasses in J-13 water at 90{sup 0}C at the candidate Yucca Mountain, Nevada repository was simulated using the EQ3/6 computer code package. The objectives of the study were to attempt to predict the concentrations of radionuclides and other glass components in solution resulting from glass dissolution, and to identify potential precipitates that sequester glass components. Modified projected inventories of 10,000 year-old West Valley and DWPF SRL-165 frit glasses were used as starting glass compositions. J-13 water was considered to be representative of groundwater at Yucca Mountain. A total of 10 grams of each glass was assumed to dissolve congruently into a kilogram of J-13 water in a closed system. No inhibitions to precipitation, except for crystalline SiO{sub 2} polymorphs, were assumed to exist. Radiolysis and materials interactions were not considered. Simulation results predict that radionuclides and other glass components precipitate predominantly in the form of oxides and hydroxides, together with carbonates, silicates and phosphates. Precipitates appear to be effective in limiting the concentrations of radionuclides and other elements in solution. The general compositional trends in precipitates and solution chemistry are the same in the West Valley and DWPF simulations, except for variations arising from differences in glass chemistry. 20 refs., 7 figs., 3 tabs.
Date: November 1, 1987
Creator: Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Geochemical simulation of reaction between spent fuel waste form and J-13 water at 25{sup 0} and 90{sup 0}C.

Description: Geochemical simulations of the degradation of spent fuel waste form in the presence of groundwater at the candidate Yucca Mountain, Nevada repository have been carried out to attempt to predict elemental concentrations in solution and to identity potential radionuclide-bearing precipitates. Spent fuel was assumed to dissolve congruently into a static mass of J-13 groundwater at 25{sup 0}C and 90{sup 0}C. Simulation results inidcate that haiweeite, soddyite, Na{sub 2}U{sub 2}O{sub 7}(c) and schoepite are potential U-bearing precipitates. Na{sub 2}U{sub 2}O{sub 7}(c) is only predicted to occur at 90{sup 0}C. U concentrations in solution and the identity of the U-bearing precipitate depend on the activity of SiO{sub 2}(aq) in solution. U concentrations are limited to < 1 mg/kg when sufficient SiO{sub 2}(aq) exists in solution to precipitate uranyl silicates. Depletion of SiO{sub 2}(aq) in solution by the precipitation of silicates results in predicted increases of U concentrations to 87 and 619 mg/kg at 25{sup 0}C and 90{sup 0}C, respectively. Subsequent reaction and precipitation of schoepite cause U concentrations to decrease. Radionuclide other than U commonly precipitate as oxides in the simulations. The precipitation of solid phases appears to be extremely effective in limiting the concentrations of some radionuclides, such as Pu and Th, in solution. Increasing the temperature from 25{sup 0}C to 90{sup 0}C does not impact greatly the identify of precipitated phases or solution composition, except in the case of U. 7 refs., 7 figs., 2 tabs.
Date: November 1, 1987
Creator: Bruton, C.J. & Shaw, H.F.
Partner: UNT Libraries Government Documents Department

Solubility-limited concentrations and aqueous speciation fo U, Pu, Np, Am and Tc: Comparison between results of Bruno and Sellin (1992) and calculations using GEMBOCHS (version R16)

Description: Aqueous speciation and solubility-limited concentrations of U, Pu, Np, Am and Tc were calculated with EQ3/6 and version comR16 of the GEMBOCHS data base for comparison to similar calculations made by Bruno and Sellin (1992) for the SKB 91 exercise. Bruno and Sellin utilized data from the older 0288 version of the EQ3/6 data base but substituted their own data sets for U and Pu. Equilibria were computed in representative fresh and saline Finnsjoen-waters under oxidizing and reducing conditions. The comparisons showed that slight discrepancies exist for U because Bruno and Sellin used thermodynamic data from sources that pre-date the NEA data base. This NEA data base is incorporated into GEMBOCHS. Discrepancy also exists for Pu under reducing conditions because of the choice of thermodynamic data for solid Pu(OH){sub 4}. GEMBOCHS predicts Pu concentrations in solution that are about 1 to 2 orders of magnitude greater than Bruno and Sellin`s values. Np concentration in the oxidizing saline water computed with GEMBOCHS is 20 times higher than Bruno and Sellin`s value. Under reducing conditions, however, GEMBOCHS predicts an order of magnitude less Np in solution. GEMBOCHS computes Am concentrations in solution about 2--3-times larger than those of Bruno and Sellin. Bruno and Sellin`s data base lacks the aqueous species Am(CO{sub 3}){sub 2}{sup {minus}} although this species occurs only sparingly (< 10 mol%) in the fresh waters. Overall Tc data bases differ significantly because more recent critical compilations of Tc data have been incorporated into GEMBOCHS since Bruno and Sellin`s 0288 version. Nonetheless, results for Tc are broadly similar, although GEMBOCHS predicts Tc concentrations about 1.7 times higher than Bruno and Sellin`s values for the reducing waters.
Date: May 1, 1995
Creator: Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Calculations of fluid-mineral equilibria in the Aspo Hard Rock Laboratory

Description: The purpose of this report is to evaluate the utility of the EQ3/6 geochemical codes in describing mineral-fluid equilibria in the low temperature (<25{degrees}C) systems at the Aespoe Hard Rock Laboratory (HRL). Data on fluid chemistry and on fracture-filling mineralogy with depth were obtained from Smellie and Laaksoharju. Average temperatures in the HRL boreholes are generally less than 20{degrees}C. EQ3/6 was used to evaluate the extent to which equilibrium is achieved between minerals and fluids in these systems. Smellie and Laaksoharju used the PHREEQE geochemical modeling code to calculate saturation indices for fracture-lining minerals in boreholes KAS02, KAS03, KAS04 and KAS06 in order to ``support the presence or absence of the major fracture minerals``. They noted that only calcite and gypsum may be expected to attain equilibrium under the low temperature conditions Aespoe. However, they used closeness to equilibrium as an indicator of ``stable conditions, long bedrock residence/reaction times and slow to stagnant flow in the system.`` EQ3 was used to calculate mineral saturation indices for comparison, and EQ6 was used to try to predict the mineral assemblages coexisting with fluids.
Date: May 1, 1995
Creator: Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Ion sorption onto hydrous ferric oxides: Effect on major element fluid chemistry at Aspo, Sweden

Description: The observed variability of fluid chemistry at the Aespoe Hard Rock Laboratory is not fully described by conservative fluid mixing models. Ion exchange may account for some of the observed discrepancies. It is also possible that variably charged solids such as oxyhydroxides of Fe can serve as sources and sinks of anions and cations through surface complexation. Surface complexation reactions on hydrous ferric oxides involve sorption of both cations and anions. Geochemical modeling of the surface chemistry of hydrous ferric oxides (HFOs) in equilibrium with shallow HBH02 and deep KA0483A waters shows that HFOs can serve as significant, pH-sensitive sources and sinks for cations and anions. Carbonate sorption is favored especially at below-neutral pH. A greater mass of carbonate is sorbed onto HFO surfaces than is contained in the fluid when 10 g goethite, used as a proxy for HFOs, is in contact with 1 kg H{sub 2}O. The masses of sorbent required to significantly impact fluid chemistry through sorption/desorption reactions seem to be reasonable when compared to the occurrences of HFOs at Aespoe. Thus, it is possible that small changes in fluid chemistry can cause significant releases of cations or anions from HFOs into the fluid phase or, alternately, result in uptake of aqueous species onto HFO surfaces. Simulations of the mixing of shallow HBH02 and native KA0483A waters in the presence of a fixed mass of goethite show that surface complexation does not cause the concentrations of Ca, Sr, and SO{sub 4} to deviate from those that are predicted using conservative mixing models. Results for HCO{sub 3} are more difficult to interpret and cannot be addressed adequately at this time.
Date: June 1, 1996
Creator: Bruton, C.J. & Viani, B.E.
Partner: UNT Libraries Government Documents Department

Effect of cation exchange on major cation chemistry in the large scale redox experiment at Aespoe

Description: Predicting the chemical changes that result from excavating a repository below the groundwater table in granitic terrain is a major focus of the SKB geochemistry program. The modeling study presented here demonstrates that cation exchange can play a major role in controlling the fluid chemistry that results when groundwaters of differing composition mix due to flow induced by excavation of the HRL tunnel. The major goal of this study was to assess whether an equilibrium cation exchange model could explain the composition of groundwater sampled from boreholes in the HRL tunnel. Given the consistency of the cation exchange hypothesis with observations, geochemical modeling was used to assess whether the quantity of exchanger necessary to match model results and observation was physically reasonable. The impact of mineral dissolution and precipitation on fluid chemistry was also evaluated. Finally, the compositions of exchanger phases expected to be in equilibrium with various Aespoe groundwaters were predicted.
Date: October 1, 1994
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

The role of cation exchange in controlling groundwater chemistry at Aspo, Sweden

Description: Construction-induced groundwater flow has resulted in the mixing of relatively dilute shallow groundwater with more concentrated groundwater at depth in the underground Hard Rock Laboratory (HRL) at Aespoe, Sweden. The observed compositional variation of the mixed groundwater cannot be explained using a conservative mixing model. The geochemical modeling package EQ3/6, to which a cation-exchange model was added, was used to simulate mixing between the two fluids. The results of modeling simulations suggest that cation exchange between groundwater and fracture-lining clays can explain the major element fluid chemistry observed in the HRL. The quantity of exchanger required to match simulated with observed fluid chemistry is reasonable and is consistent with the observed fracture mineralogy. This preliminary study establishes cation exchange as a viable mechanism for controlling the chemical evolution of groundwaters in a fracture-dominated dynamic flow system. This modeling study also strengthens their confidence in the ability to model the potential effects of fracture-lining minerals on the transport of radionuclides in a high level nuclear waste repository.
Date: January 1, 1995
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Assessing the role of cation exchange in controlling groundwater chemistry during fluid mixing in fractured granite at Aspo, Sweden

Description: Geochemical modeling was used to simulate the mixing of dilute shallow groundwater with deeper more saline groundwater in the fractured granite of the Redox Zone at the Aespoe underground Hard Rock Laboratory (HRL). Fluid mixing simulations were designed to assess the role that cation exchange plays in controlling the composition of fluids entering the HRL via fracture flow. Mixing simulations included provision for the effects of mineral precipitation and cation exchange on fluid composition. Because the predominant clay mineral observed in fractures in the Redox Zone has been identified as illite or mixed layer illite smectite, an exchanger with the properties of illite was used to simulate cation exchange. Cation exchange on illite was modeled using three exchange sites, a planar or basal plane site with properties similar to smectite, and two edge sites that have very high affinities for K, Rb, and Cs. Each site was assumed to obey an ideal Vanselow exchange model, and exchange energies for each site were taken from the literature. The predicted behaviors of Na, Ca, and Mg during mixing were similar to those reported in a previous study in which smectite was used as the model for the exchanger. The trace elements Cs and Rb were predicted to be strongly associated with the illite exchanger, and the predicted concentrations of Cs in fracture fill were in reasonable agreement with reported chemical analyses of exchangeable Cs in fracture fill. The results of the geochemical modeling suggest that Na, Ca, and Sr concentrations in the fluid phase may be controlled by cation exchange reactions that occur during mixing, but that Mg appears to behave conservatively. There is currently not enough data to make conclusions regarding the behavior of Cs and Rb.
Date: June 1, 1996
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Effect of cation exchange of major cation chemistry in the large scale redox experiment at Aspo. Revision 1

Description: Geochemical modeling was used to test the hypothesis that cation exchange with fracture-lining clays during fluid mixing in the Aespoe Hard Rock Laboratory can significantly affect major element chemistry. Conservative mixing models do not adequately account for changes in Na, Ca and Mg concentrations during mixing. Mixing between relatively dilute shallow waters and more concentrated waters at depth along fracture zones was modeled using the EQ3/6 geochemical modeling package. A cation exchange model was added to the code to describe simultaneously aqueous speciation, mineral precipitation/dissolution, and equilibration between a fluid and a cation exchanger. Fluid chemistries predicted to result from mixing were compared with those monitored from boreholes intersecting the fracture zone. Modeling results suggest that less than 0.1 equivalent of a smectite exchanger per liter of groundwater is necessary to account for discrepancies between predictions from a conservative mixing model and measured Na and Ca concentrations. This quantity of exchanger equates to an effective fracture coating thickness of 20 {micro}m or less given a fracture aperture width of 1,000 {micro}m or less. Trends in cation ratios in the fluid cannot be used to predict trends in cation ratios on the exchanger because of the influence of ionic strength on heterovalent exchange equilibrium. It is expected that Na for Ca exchange will dominate when shallow waters such as HBHO2 are mixed with deeper waters. In contrast, Na for Mg exchange will dominate mixing between deeper waters.
Date: June 1, 1996
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Modeling ion exchange in clinoptilolite using the EQ3/6 geochemical modeling code

Description: Assessing the suitability of Yucca Mtn., NV as a potential repository for high-level nuclear waste requires the means to simulate ion-exchange behavior of zeolites. Vanselow and Gapon convention cation-exchange models have been added to geochemical modeling codes EQ3NR/EQ6, allowing exchange to be modeled for up to three exchangers or a single exchanger with three independent sites. Solid-solution models that are numerically equivalent to the ion-exchange models were derived and also implemented in the code. The Gapon model is inconsistent with experimental adsorption isotherms of trace components in clinoptilolite. A one-site Vanselow model can describe adsorption of Cs or Sr on clinoptilolite, but a two-site Vanselow exchange model is necessary to describe K contents of natural clinoptilolites.
Date: June 1, 1992
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Modeling fluid-rock interaction at Yucca Mountain, Nevada; A progress report, April 15, 1992

Description: Volcanic rocks at Yucca Mountain, Nevada aie being assessed for their suitability as a potential repository for high-level nuclear waste. Recent progress in modeling fluid-rock interactions, in particular the mineralogical and chemical changes that may accompany waste disposal at Yucca Mountain, will be reviewed in this publication. In Part 1 of this publication, ``Geochemical Modeling of Clinoptilolite-Water Interactions,`` solid-solution and cation-exchange models for the zeolite clinoptilolite are developed and compared to experimental and field observations. At Yucca Mountain, clinoptilolite which is found lining fractures and as a major component of zeolitized tuffs, is expected to play an important role in sequestering radionuclides that may escape from a potential nuclear waste repository. The solid-solution and ion-exchange models were evaluated by comparing predicted stabilities and exchangeable cation distributions of clinoptilolites with: (1) published binary exchange data; (2) compositions of coexisting clinoptilolites and formation waters at Yucca Mountain; (3) experimental sorption isotherms of Cs and Sr on zeolitized tuff, and (4) high temperature experimental data. Good agreement was found between predictions and expertmental data, especially for binary exchange and Cs and Sr sorption on clinoptilolite. Part 2 of this publication, ``Geochemical Simulation of Fluid-Rock Interactions at Yucca Mountain,`` describes preliminary numerical simulations of fluid-rock interactions at Yucca Mountain. The solid-solution model developed in the first part of the paper is used to evaluate the stability and composition of clinciptilolite and other minerals in the host rock under ambient conditions and after waste emplacement.
Date: August 1, 1992
Creator: Viani, B.E. & Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Geochemical analysis of fluid mineral relations in the Tiwi Geothermal Field, Philippines

Description: Geochemical modeling simulations are being used to examine the source of the reservoir fluids in the Tiwi geothermal field and to evaluate the chemical and physical processes responsible for producing observed vein parageneses. Such information can be used to trace the evolution of the Tiwi geothermal field through time. The React geochemical modeling code was used to simulate the effects of isothermal and isoenthalpic boiling, conductive cooling and heating, and incorporation of condensed steam, on fluids from the Matalibong area. Predicted mineral stabilities were used to identify mineral indicators for each process. Calcite and anhydrite precipitation were favored by conductive heating, while illite precipitation was favored when condensed steam was added to the reservoir fluid. Reconstructed downhole fluids from borehole Mat-25 are acidic and are consistent with the presence of illite as the latest alteration mineral in veins. The processes of isothermal and isoenthalpic boiling could be differentiated from conductive cooling by the presence of epidote and/or calcite during boiling, and illite during cooling. Both boiling and cooling favored precipitation of quartz, K-feldspar, wairakite, and pyrite. Ratios of Na, Cl, and Br in waters from the Matalibong are relative to seawater indicate a significant component of seawater in reservoir fluids.
Date: January 1, 1997
Creator: Bruton, C.J.; Moore, J.N. & Powell, T.S.
Partner: UNT Libraries Government Documents Department

Field-based tests of geochemical modeling codes using New Zealand hydrothermal systems

Description: Hydrothermal systems in the Taupo Volcanic Zone, North Island, New Zealand are being used as field-based modeling exercises for the EQ3/6 geochemical modeling code package. Comparisons of the observed state and evolution of the hydrothermal systems with predictions of fluid-solid equilibria made using geochemical modeling codes will determine how the codes can be used to predict the chemical and mineralogical response of the environment to nuclear waste emplacement. Field-based exercises allow us to test the models on time scales unattainable in the laboratory. Preliminary predictions of mineral assemblages in equilibrium with fluids sampled from wells in the Wairakei and Kawerau geothermal field suggest that affinity-temperature diagrams must be used in conjunction with EQ6 to minimize the effect of uncertainties in thermodynamic and kinetic data on code predictions.
Date: June 1, 1994
Creator: Bruton, C.J.; Glassley, W.E. & Bourcier, W.L.
Partner: UNT Libraries Government Documents Department

Testing EQ3/6 and GEMBOCHS using fluid-mineral equilibria in the wairakei geothermal system

Description: The ability of the EQ3 and EQ6 geochemical modeling codes and the GEMBOCHS thermodynamic data bases to simulate geochemical changes in the post-emplacement environment at the potential Yucca Mountain, Nevada repository is being tested using observed mineral-fluid relations in the Taupo Volcanic Zone of New Zealand. In this report, comparisons between observed equilibria and simulations of field relations in the Wairakei geothermal system are used to test the codes and data bases in high temperature systems. Analysis of trends in water and gas chemistries and well discharge characteristics with time were used to identify a set of representative water and gas analyses from zones producing at about 250{degrees}C. The most common vein minerals at this temperature are: wairakite, adularia, epidote, quartz, albite, chlorite, calcite, prehnite, and pyrite. Calculations were carried out using version 7.2a R134 of EQ3 and version 7.2a R130 of EQ6 and the SUPCRT and COM subsets of the R24 version of GEMBOCHS. Thermodynamic data bases using different data for Al aqueous species were sued to identify the data set which produced the best matches between observed and calculated equilibria. The simulations described in this paper suggest that EQ6 can be used to identify facies of minerals that will be stable in various environments, but can not be used to predict the exact phase assemblage that is in equilibrium with a given water.
Date: August 28, 1995
Creator: Bruton, C.J.
Partner: UNT Libraries Government Documents Department

Geothermal areas as analogues to chemical processes in the near-field and altered zone of the potential Yucca Mountain, Nevada repository

Description: The need to bound system performance of the potential Yucca Mountain repository for thousands of years after emplacement of high-level nuclear waste requires the use of computer codes. The use of such codes to produce reliable bounds over such long time periods must be tested using long-lived natural and historical systems as analogues. The geothermal systems of the Taupo Volcanic Zone (TVZ) in New Zealand were selected as the site most amenable to study. The rocks of the TVZ are silicic volcanics that are similar in composition to Yucca Mountain. The area has been subjected to temperatures of 25 to 300 C which have produced a variety of secondary minerals similar to those anticipated at Yucca Mountain. The availability of rocks, fluids and fabricated materials for sampling is excellent because of widespread exploitation of the systems for geothermal power. Current work has focused on testing the ability of the EQ3/6 code and thermodynamic data base to describe mineral-fluid relations at elevated temperatures. Welfare starting long-term dissolution/corrosion tests of rocks, minerals and manufactured materials in natural thermal features in order to compare laboratory rates with field-derived rates. Available field data on rates of silica precipitation from heated fluids have been analyzed and compared to laboratory rates. New sets of precipitation experiments are being planned. The microbially influenced degradation of concrete in the Broadlands-Ohaaki geothermal field is being characterized. The authors will continue to work on these projects in FY 1996 and expand to include the study of naturally occurring uranium and thorium series radionuclides, as a prelude to studying radionuclide migration in heated silicic volcanic rocks. 32 refs.
Date: February 1, 1995
Creator: Bruton, C.J.; Glassley, W.E. & Meike, A.
Partner: UNT Libraries Government Documents Department

Testing geochemical modeling codes using New Zealand hydrothermal systems

Description: Hydrothermal systems in the Taupo Volcanic Zone, North Island, New Zealand are being used as field-based modeling exercises for the EQ3/6 geochemical modeling code package. Comparisons of the observed state and evolution of selected portions of the hydrothermal systems with predictions of fluid-solid equilibria made using geochemical modeling codes will: (1) ensure that we are providing adequately for all significant processes occurring in natural systems; (2) determine the adequacy of the mathematical descriptions of the processes; (3) check the adequacy and completeness of thermodynamic data as a function of temperature for solids, aqueous species and gases; and (4) determine the sensitivity of model results to the manner in which the problem is conceptualized by the user and then translated into constraints in the code input. Preliminary predictions of mineral assemblages in equilibrium with fluids sampled from wells in the Wairakei geothermal field suggest that affinity-temperature diagrams must be used in conjunction with EQ6 to minimize the effect of uncertainties in thermodynamic and kinetic data on code predictions. The kinetics of silica precipitation in EQ6 will be tested using field data from silica-lined drain channels carrying hot water away from the Wairakei borefield.
Date: December 1, 1993
Creator: Bruton, C. J.; Glassley, W. E. & Bourcier, W. L.
Partner: UNT Libraries Government Documents Department

Radionuclide Reaction Chemistry as a Function of Temperature at the Cheshire Site

Description: The goals of this task were to evaluate the availability of published temperature-dependent thermodynamic data for radionuclides and sorbing minerals and to evaluate the applicability of published estimation methods for temperature-dependent aqueous complexation, radionuclide mineral precipitation, and sorption. This task fills a gap in the hydrologic source term (HST) modeling approach, which, with few exceptions, has neglected the effects of temperature on radionuclide aqueous complexation, using 25 C complexation data for all temperatures without evaluating the consequences of this assumption. In this task, we have compiled thermodynamic data available in the literature and evaluated the options and benefits of applying temperature-dependent radionuclide speciation to future HST modeling. We use the recent experience of HST modeling at Cheshire (Pawloski et al., 2001) to focus our evaluation. Our literature search revealed that few thermodynamic data or extrapolation methods could be used to define the temperature-dependent speciation of key HST radionuclides Np, Pu, Am, and U, particularly for the higher valence-state (e.g., 5+ and 6+), the oxidation states most pertinent to NTS groundwater conditions at Cheshire. This suggests that using 25 C data for all temperatures may be the best modeling approach currently available. We tested established estimation techniques such as the Criss-Cobble method and other correlation algorithms to calculate thermodynamic parameters needed to extrapolate aqueous complexation data to higher temperatures. For some reactions, the isocoulombic method does allow calculation of free energy data and equilibrium values at higher temperatures. Limitations in algorithms and input data for pentavalent and hexavalent cations prevent extending temperature ranges for reactions involving radionuclides in these oxidation states and their complexes. In addition, for many of the radionuclides of interest, carbonate complexes appear to be the dominant complexes formed in NTS groundwaters, and data for these types of complexes are lacking for radionuclides as well as analog species. ...
Date: October 31, 2005
Creator: Burton, E A; Bruton, C J; Johnson, M R; Rard, J & Zavarin, M
Partner: UNT Libraries Government Documents Department

Field-based tests of geochemical modeling codes: New Zealand hydrothermal systems

Description: Hydrothermal systems in the Taupo Volcanic Zone, North Island, New Zealand are being used as field-based modeling exercises for the EQ3/6 geochemical modeling code package. Comparisons of the observed state and evolution of the hydrothermal systems with predictions of fluid-solid equilibria made using geochemical modeling codes will determine how the codes can be used to predict the chemical and mineralogical response of the environment to nuclear waste emplacement. Field-based exercises allow us to test the models on time scales unattainable in the laboratory. Preliminary predictions of mineral assemblages in equilibrium with fluids sampled from wells in the Wairakei and Kawerau geothermal field suggest that affinity-temperature diagrams must be used in conjunction with EQ6 to minimize the effect of uncertainties in thermodynamic and kinetic data on code predictions.
Date: December 1, 1993
Creator: Bruton, C.J.; Glassley, W.E. & Bourcier, W.L.
Partner: UNT Libraries Government Documents Department

Geothermal chemistry/exploration investigations at Dixie Valley, Nevada

Description: Dixie Valley geothermal field has continuously produced electric power since 1988. At the request of Oxbow Geothermal Corp. and the US Department of Energy, the authors have organized an inter-agency team of investigators to examine several topics of concern regarding management and behavior of the resource. These topics include scaling of the injection system, recharge of the reservoir, geochemical monitoring of the reservoir, and development of increased fumarolic activity north of the power plant.
Date: December 1, 1998
Creator: Goff, F.; Bergfeld, D.; Counce, D.; Janik, C.J.; Bruton, C.J. & Nimz, G.
Partner: UNT Libraries Government Documents Department

Preliminary investigation of scale formation and fluid chemistry at the Dixie Valley Geothermal Field, Nevada

Description: The chemistry of geothermal, production, and injection fluids at the Dixie Valley Geothermal Field, Nevada, was characterized to address an ongoing scaling problem and to evaluate the effects of reinjection into the reservoir. Fluids generally followed mixing-dilution trends. Recharge to the Dixie Valley system apparently originates from local sources. The low-pressure brine and injection waters were saturated with respect to amorphous silica, which correlated with the ongoing scaling problem. Local shallow ground water contains about 15% geothermal brine mixed with regional recharge. The elevated Ca, Mg, and HCO{sub 3} content of this water suggests that carbonate precipitation may occur if shallow groundwater is reinjected. Downhole reservoir fluids are close to equilibrium with the latest vein mineral assemblage of wairakite-epidote-quartz-calcite. Reinjection of spent geothermal brine is predicted to affect the region near the wellbore differently than it does the region farther away.
Date: June 27, 1997
Creator: Bruton, C.J.; Counce, D.; Bergfeld, D.; Goff, F.; Johnson, S.D.; Moore, J.N. et al.
Partner: UNT Libraries Government Documents Department

The EQ3/6 software package for geochemical modeling: Current status

Description: EQ3/6 is a software package for modeling chemical and mineralogic interactions in aqueous geochemical systems. The major components of the package are EQ3NR (a speciation-solubility code), EQ6 (a reaction path code), EQLIB (a supporting library), and a supporting thermodynamic data base. EQ3NR calculates aqueous speciation and saturation indices from analytical data. It can also be used to calculate compositions of buffer solutions for use in laboratory experiments. EQ6 computes reaction path models of both equilibrium step processes and kinetic reaction processes. These models can be computed for closed systems and relatively simple open systems. EQ3/6 is useful in making purely theoretical calculations, in designing, interpreting, and extrapolating laboratory experiments, and in testing and developing submodels and supporting data used in these codes. The thermodynamic data base supports calculations over the range 0-300{degree}C. 60 refs., 2 figs.
Date: July 1, 1988
Creator: Wolery, T.J.; Jackson, K.J.; Bourcier, W.L.; Bruton, C.J.; Viani, B.E.; Knauss, K.G. et al.
Partner: UNT Libraries Government Documents Department

Cement minerals at elevated temperature: Thermodynamic and structural characteristics

Description: Large quantities of cementitious materials may be used in the construction of a potential nuclear waste repository. Temperatures in the emplacement drifts may reach over 200 C owing to decay heat from radioactive waste for various ``extended-dry`` repository scenarios. Despite its potential significance, the mineralogic response of cement to elevated temperature is not well known. The chemistry of fluid introduced to the repository from cementitious materials can also have a significant impact on repository performance. The masses of water associated with the use of cementitious materials such as shotcrete, which includes both structural and pore water, can be sizable. Pore water may be driven out by heating, and structural water may be released through phase dehydration. An experimental and modeling program has been designed to elucidate the structural and thermodynamic response of cement minerals to elevated temperature. The components of the program include: (a) synthesis of hydrated Ca-silicates; (b) structural analysis of cement phases during heating and dehydration/rehydration; (c) mechanistic and thermodynamic descriptions of the hydration/dehydration behavior of hydrated Ca-silicates as a function of temperature, pressure and relative humidity; (d) study of naturally occurring hydrated Ca-silicates; and (e) measurements of thermodynamic data for hydrated Ca-silicates.
Date: November 1, 1993
Creator: Bruton, C. J.; Phillips, B. L.; Meike, A.; Martin, S. & Viani, B. E.
Partner: UNT Libraries Government Documents Department

Evaluation of the hydrologic source term from underground nuclear tests in Frenchman Flat at the Nevada Test Site: The Cambric test

Description: The objectives of this project are to develop and apply a modeling frame- work to quantitatively evaluate the nature and extent of radionuclide migration within the immediate, near field environment about an underground nuclear test. Specifically, it will involve evaluation of ² The speciation and abundance of radionuclides that are introduced into groundwater as aqueous species or colloids, and ² The rate and extent of radionuclide movement, dilution, and reaction in groundwater surrounding the working point of a test. To be clear, interest will only be focused on processes that have occurred well after the nuclear test, as opposed to the more dynamic processes that take place during or immediately after detonation. The meaning of "near field" in this case will loosely refer to a volume of diameter 4-8 R<sub>c</sub>, centered on the working point and chimney of the test, where Rc is the radius of the blast cavity. For a given nuclear test, this information will collectively comprise the test's "hydrologic source term". This work relies on and is being supported by existing data, analyses, and interpretations that have been made at the Nevada Test Site (NTS) during the American nuclear test program and previous and ongoing studies related to radionuclide migration in the subsurface (Kersting, 1996).
Date: March 23, 1999
Creator: Bourcier, W L; Bruton, C J; Carle, S F; Kersting, A B; Pawloski, G A; Rard, J A et al.
Partner: UNT Libraries Government Documents Department

Simulation of Radionuclide Migration in Groundwater away from an Underground Nuclear Test

Description: Reactive transport simulations are being used to evaluate the nature and extent of radionuclide contamination within alluvium surrounding an underground nuclear test at the Nevada Test Site (NTS). Simulations are focused on determining the abundance and chemical nature of radionuclides that are introduced into groundwater, as well as the rate and extent of radionuclide migration and reaction in groundwater surrounding the working point of the test. Transport simulations based upon a streamline-based numerical model are used to illustrate the nature of radionuclide elution out of the near-field environment and illustrate the conceptual modeling process. The numerical approach allowed for relatively complex flow and chemical reactions to be considered in a computationally efficient manner. The results are particularly sensitive to the rate of melt glass dissolution, distribution of reactive minerals in the alluvium, and overall groundwater flow configuration. They provide a rational basis from which defensible migration assessments can proceed.
Date: December 13, 1999
Creator: Bruton, C.J.; Bourcier, W.L.; Shumaker, D.E.; Kersting, A.B.; Smith, D.K.; Carle, S.F. et al.
Partner: UNT Libraries Government Documents Department