bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Effects of Silicic Acid, Aluminate Ion Activity And Hydrosilicate Gel Development on the Dissolution Rate of a Simulated British Magnox Waste Glass
Authors: Abraitis P. K. , McGrail B. P. , Trivedi D. P. 
Published: April 2011

AbstractThe dissolution rate of a simulated Magnox waste glass has been investigated in single-pass flow-through experiments designed to investigate the role of Al and Si in the dissolution process. The results indicate that both Al and Si species suppress the rate of dissolution. These effects may be modelled using a combined Al/Si affinity term in a conventional glass dissolution rate law. Aluminium species may also play an inhibitory role when present at relatively high solution activities. In Si-rich alkaline media, the concentration of aluminium is controlled to very low levels by the development of secondary aluminosilicate phases. Removal of Al by secondary phase precipitation results in dissolved Al activities below that required to reach ‘saturation’ with respect to the glass.
Characterization of Pore Structure and Hydraulic Property Alteration in Pressurized Unsaturated Flow Tests
Authors: McGrail B. P. , Lindenmeier C. W. , Martin P. F. 
Published: April 2011

AbstractThe pressurized unsaturated flow (PUF) test is a new experimental method for the evaluation of the long-term corrosion behavior of waste forms and other engineered barrier materials. Essentially, the technique provides a means to flow water through a porous bed of test material or materials at elevated temperature and under hydraulically unsaturated conditions. Bulk volumetric content, effluent pH and electrical conductivity are monitored in real time using a computer control and data acquisition system. In previous papers, we have reported on the changes in bulk water content, effluent chemistry, and glass corrosion rates that result from the formation of alteration products during these tests. These measurements are now supplemented through the use of the ultracentrifugation apparatus (UFA) for hydraulic property measurements and highresolution, x-ray microtomography (XMT) to provide 3-D spatial and temporal imaging of water distribution and pore structure alteration during these tests. Quantitative changes in the water retention characteristic were correlated with the onset of zeolite formation in the tests. Extensive alteration of the glass resulted in cementation of the glass grains near the bottom of the column, which was observed in situ using the XMT.
Oxidative Corrosion of Spent uo2 Fuel in Vapor and Dripping Groundwater at 90°C
Authors: Finch Robert J. , Buck Edgar C. , Finn Patricia A. , Bates John K. 
Published: April 2011

AbstractOxidative dissolution of spent UO2 fuel in vapor and dripping groundwater at 90°C occurs via general corrosion at fragment surfaces. Dissolution along fuel-grain boundaries is also evident in samples contacted by the largest volumes of groundwater, and corroded grain boundaries extend at least 20 or 30 grains deep (> 200 μm), possibly throughout mm-sized fragments. Apparent dissolution of fuel along defects that intersect grain boundaries has produced 50 to 200 nm-diameter dissolution pits that penetrate 1–2 μm into each grain, giving rise to a “worm-like” texture along fuel-grain boundaries. Sub-micrometer-sized fuel shards are common between fuel grains and may contribute to the reactive surface area of fuel exposed to groundwater. Outer surfaces of reacted fuel fragments develop a fmne-grained layer of corrosion products adjacent to the fuel (5–15 μm thick). A more coarsely crystalline layer of corrosion products commonly covers the fine-grained layer, the thickness of which varies considerably among samples (from less than 5 μm to greater than 40 μm). The thickest and most porous corrosion layers develop on fuel fragments exposed to the largest volumes of groundwater. Corrosion-layer compositions depend strongly on water flux, with uranyl oxy-hydroxides predominating in vapor experiments, and alkali and alkaline earth uranyl silicates predominating in high drip-rate experiments. Low drip-rate experiments exhibit a complex assemblage of corrosion products, including phases identified in vapor and high drip-rate experiments.
Modelling UO2 and Simfuel Leaching Behaviour in Granite Groundwater Under Oxidizing Conditions
Authors: Quiñones J. , Serrano J. Garcia , Serrano J. A. , Arocas P. Diaz , Rodriguez J. L. , Blazquez J. 
Published: April 2011

AbstractThe Uranium concentration leaching behaviour for UO2 and SIMFUEL in granite groundwater under oxidizing conditions is modelled. The model contemplates two terms: a source term and a loss term. The source term represents the bulk dissolution process and its upper limit is controlled by the solubility of the thermodynamically stable solid phase. The loss term emulates the solute removed due to groundwater flow into the repository. It can forecast the uranium steady state concentration when groundwater flow is considered. The model has been tested with three different types of leaching experiments, performed with UO2 and SIMFUEL. Good agreement between experimental results and model predictions are found.
The Melt-Dilute Treatment of Al-Base Highly Enriched DOE Spent Nuclear Fuels: Principles and Practices
Authors: Adams Thad M. , Peacock Harold B. , Rhode Frederick C. , lyer Natraj C. 
Published: April 2011

AbstractThe melt-dilute treatment technology program is focused on the development and implementation of a treatment technology for diluting highly enriched (>20% 235U) aluminum spent nuclear fuel to low enriched levels (<20% 235U and qualifying the LEU Al-SNF form for geologic repository storage. In order to reduce the enrichment of these assemblies prior to ultimate geologic repository disposal, the melt-dilute technology proposes to melt these SNF assemblies and then dilute with additions of depleted uranium. The benefits accrued from this treatment process include the potential for significant volume reduction, reduced criticality potential, and the potential for enhanced SNF form characteristics. The emphasis within the development program to date has been on determining the process metallurgy and off-gas system design for the treatment of all types of Al SNF (UAIx, Al-U3O8, and AI-U3Si2). In determining the process metallurgy a wide range of alloys, representative of those expected in the Al-SNF form, have been fabricated and their product characteristics, namely microstructure, homogeneity, phase composition, and “ternary” constituent effects have been analyzed. As a result of the presence of species within the melt which will possess significant vapor pressures in the desired operating temperature range an off-gas system is necessary. Of the volatile species the one of greatest concern is 137Cs.
Alternate Source Term Models for Yucca Mountain Performance Assessment Based on Natural Analog Data and Secondary Mineral Solubility
Authors: Murphy William M. , Codell Richard B. 
Published: April 2011

AbstractPerformance assessment calculations for the proposed high level radioactive waste repository at Yucca Mountain, Nevada, were conducted using the Nuclear Regulatory Commission Total-System Performance Assessment (TPA 3.2) code to test conceptual models and parameter values for the source term based on data from the Peña Blanca, Mexico, natural analog site and based on a model for coprecipitation and solubility of secondary schoepite. In previous studies the value for the maximum constant oxidative alteration rate of uraninite at the Nopal I uranium body at Peña Blanca was estimated. Scaling this rate to the mass of uranium for the proposed Yucca Mountain repository yields an oxidative alteration rate of 22 kg yr−1, which was assumed to be an upper limit on the release rate from the proposed repository. A second model was developed assuming releases of radionuclides are based on the solubility of secondary schoepite as a function of temperature and solution chemistry. Releases of uranium are given by the product of uranium concentrations at equilibrium with schoepite and the flow of water through the waste packages. For both models, radionuclides other than uranium and those in the cladding and gap fraction were modeled to be released at a rate proportional to the uranium release rate, with additional elemental solubility limits applied. Performance assessment results using the Peña Blanca oxidation rate and schoepite solubility models for Yucca Mountain were compared to the TPA 3.2 base case model, in which release was based on laboratory studies of spent fuel dissolution, cladding and gap release, and solubility limits. Doses calculated using the release rate based on natural analog data and the schoepite solubility models were smaller than doses generated using the base case model. These results provide a degree of confidence in safety predictions using the base case model and an indication of how conservatism in the base case model may be reduced in future analyses.
Modelling of the Radionuclide Release from an Initially Defective Canister
Authors: Liu L. , Neretnieks I. 
Published: April 2011

AbstractTo investigate the effect of geochemical conditions of the repository on the release rate of radionuclides from an initially defective canister, a mathematical model is proposed. The model is based on the concept that ligands diffuse through the damage into the canister and form complexes with uranium, increasing its solubility. The complexes then diffuse out to the surroundings. Accordingly, the model combines a chemical equilibrium model for the solubility of uranium under strongly oxidizing conditions, and a transport model for the release rate of uranium. The transport model takes into account the diffusion resistance in the amage as well as in the surrounding compacted bentonite clay and the water flow in the rock. Using this model, the sensitivity of the release rate of aqueous uranium species to the concentrations of various groundwater component species, to the diffusion coefficients and to other parameters is systematically investigated. The results show that, the concentrations of carbonate, phosphate, silicate and calcium species in deep groundwaters play the most important role in determining the release rate of radionuclides. The results help identify the key diffusion coefficients that are needed to obtain more reliable estimates of the release rate.
Study of Ion Exclusion Using Artificial Reference Samples for Rocks
Authors: Valkiainen Matti , Aalto Hannu , Lehikoinen Jarmo , Olin Markus 
Published: April 2011

AbstractMatrix diffusion in rocks has been extensively studied experimentally with various methods including tracer experiments. Comparing the rock-capacity factors (α) for Cl-36 to corresponding values for H-3, an anion-exclusion tendency has been observed in many cases. This study uses a well-characterized reference material: porous silica (Geltech Inc.), with pore diameters of 2.5, 5, 7.5 and 20 nanometers. The equilibration-leaching method and tracers H-3 (H-3) and Cl-36 (Cl−) were used to determine the capacity factors of the samples in three molarities of NaCl solution: 0.0026, 0.01 and 0. 1 mol/liter. The ratio αcl/αH-3 gives the anion exclusion for each solution/pore diameter combination. A clear anion-exclusion tendency was seen both as a function of the decreasing pore diameter and liquid molarity.The modeling was based on the assumption that the silica surface acts as a proton donor according to SiOH = SiO− + H+. The pore dimensions are also included in the calculations. In the case of the silica samples, good agreement was found when comparing the experimental and calculated values of chloride exclusion. This suggests that the surface complexation model for matrix diffusion can explain the observed exclusion phenomena in rocks although the parameters needed for the application of the model are difficult to obtain in the case of rocks.
Generation and Characterisation of Colloids of the Near Field / far Field Interface
Authors: Missana T. , Turrero M. J. , Melon A. 
Published: April 2011

AbstractThe generation of bentonite colloids has been investigated by means of column experiments with bentonite compacted at the engineering barrier density and using water flows as low as 0.04 ml/h (≈350 ml/year). Results on generation of bentonite colloids in these “all-bentonite” columns suggested that most of the colloidal material observed in the eluted water is due to erosion of the plug. Therefore new column experiments in which granite and compacted bentonite are in contact have been designed and the effects of the water flow analysed. The physico - chemical characteristics of the colloidal material obtained in the “all-bentonite” and “bentonite plus granite” column experiments have been compared as well as its stability. Results show that bentonite colloids that have been in contact with granite always present a higher negative charge, this indicating that they could be stable in the natural environment.
Fundamental Properties of Monolithic Bentonite Buffer Material Formed by Cold Isostatic Pressing for High-Level Radioactive Waste Repository
Authors: Kawakami S. , Yamanaka Y , Kato K. , Asano H. , Ueda H. 
Published: April 2011

AbstractThe methods of fabrication, handling, and emplacement of engineered barriers used in a deep geological repository for high level radioactive waste should be planned as simply as possible from the engineering and economic viewpoints. Therefore, a new concept of a monolithic buffer material around a waste package have been proposed instead of the conventional concept with the use of small blocks, which would decrease the cost for buffer material. The monolithic buffer material is composed of two parts of highly compacted bentonite, a cup type body and a cover. As the forming method of the monolithic buffer material, compaction by the cold isostatic pressing process (CIP) has been employed.In this study, monolithic bentonite bodies with the diameter of about 333 mm and the height of about 455 mm (corresponding to the approx. 1/5 scale for the Japanese reference concept) were made by the CIP of bentonite powder. The dry densities: pd of the bodies as a whole were measured and the small samples were cut from several locations to investigate the density distribution. The swelling pressure and hydraulic conductivity as function of the monolithic body density for CIP-formed specimens were also measured.High density ( ρd: 1.4–2.0 Mg/m3) and homogeneous monolithic bodies were formed by the CIP. The measured results of the swelling pressure (3–15 MPa) and hydraulic conductivity (0.5–1.4×10−3 m/s) of the specimens were almost the same as those for the uniaxial compacted bentonite in the literature. It is shown that the vacuum hoist system is an applicable the handling method for emplacement of the monolithic bentonite.
Erosion Properties and Dispersion-Flocculation Behavior of Bentonite Particles
Authors: Kurosawa S. , Kato H. , Ueta S. , Yokoyama K. , Fujihara H. 
Published: April 2011

AbstractExperimental and theoretical studies have been performed to clarify the ability of flowing groundwater in contact with bentonite to generate bentonite colloidal particles and disperse such colloids. This information is required to determine (a) the long-term stability of bentonite as a buffer material for borehole disposal of radioactive wastes in deep geologic media and (b) the potential influence of bentonite colloidal particles on radionuclide transport, specifically for use in scenario analyses in the performance assessment of waste disposal.In this study, the minimum groundwater velocity required to erode particles of Nabentonite or Ca-bentonite from a bentonite surface in contact with groundwater was derived from shear strengths of aqueous bentonite gel suspensions, as determined by viscometer tests. The shear strengths were used to estimate the corresponding shear force on bentonite particle-particle bonds, using an estimated value for the number of initial bentonite particle-particle bonds in the experimental systems studied. The derived shear force was converted to corresponding groundwater velocity by using Stokes' equation and simplifying assumptions. The results indicate that groundwater velocities in a range of about 10−5 to 10−4 m/s would be required to initiate bentonite erosion. This range is higher than the groundwater flow velocity generally found in deep geologic media in Japan. In addition, known groundwater electrolyte concentrations were compared with theoretical estimates of aqueous electrolyte concentrations required to flocculate colloidal bentonite particles (for example 1 × 10−3 mol/l Na+). The comparison indicates that, even if erosion of bentonite occurred, the colloidal bentonite particles formed would flocculate. As a result, this study has shown that the effect of bentonite colloids on radionuclide transport is likely to be negligible in the performance assessment of radioactive waste disposal in deep geologic media.
Understanding of Relationship Between the Average Mass Transport Rate And The Moments Of Permeability
Authors: Niibori Y. , Tochiyama O. , Chida T. 
Published: April 2011

AbstractTo estimate the transport rate of radionuclides in the geosphere, we must consider the spatial variability of permeability. However, the borehole data of permeability are limited and we can not determine the type of probability density function, though the measurement data reflect the most significant hydraulic properties about geologic media including innumerable cracks or fast flow paths. While the recent models describing radioactive nuclide transport in near/far-field have assumed a certain probability density function (typically a lognormal distribution) as a permeability distribution, we cannot always obtain sufficient measurement data to define the function. However, the available data of permeability at least give us the moments such as the arithmetic mean, the standard deviation and the skewness for the distribution.The purpose of this paper is to get an understanding of the general relationship between the average mass transport rates and the moments. Using various types of probability density functions and pseudo random-numbers, hypothetical permeability distributions are generated. With these distributions, this paper obtains the average transport rates described as the numerical impulseresponse based on the advection-dispersion model for a two-dimensional region. The calculated results show that, for the dimensionless standard deviation up to around 1, the three moments are enough to characterize the permeability distribution for the purposes of the nuclide transport prediction.In this work, for five specified probability density functions, the upper and lower bounds of skewness are derived as a function of the dimensionless arithmetic mean and standard deviation. The obtained upper and lower bounds explicitly show that the Bernoulli trials (a discrete probability density function) yield the widest range in the skewness against the standard deviation. Since the response has lower peak and longer tail as the skewness goes to the lower bound value, we can predict the shape of the breakthrough curve from the three moments of the borehole data.
Characterization of U-Series Disequilibria at the Pena Blanca Natural Analogue Site, Chihuahua, Mexico
Authors: Wong Virgina , Goodell Philip C. , Anthony Elizabeth Y. 
Published: April 2011

AbstractThe purpose of this study was to evaluate radionuclide migration from a uranium-mineralized breccia pipe. The site provides an excellent opportunity to evaluate radionuclide mobility in a geochemical environment similar to that around the proposed high-level waste repository at Yucca Mountain, Nevada. Samples represent fracture-infillings from both within and outside the breccia pipe. Mineral assemblages within the fractures include 1) pure kaolinite, 2) a mixture of ironoxyhydroxides (goethite and hematite) with associated alunite and jarosite, which we refer to as the Fe-mineral assemblage, and 3) carbonates. Uranophane, weeksite, soddyite, and boltwoodite are associated with samples from within the breccia zone.We obtain radionuclide activities from gamma-ray rather than alpha spectroscopy, and the methodology for these measurements is presented in detail. Plots of 230Th/238U vs. 226Ra/230 Th show three distinct mobility trends. 1) The majority of the Fe-mineral samples from within the breccia pipe yield values between 1.0 and 1.1 for both ratios, 2) Fe-mineral samples from outside the ore zone and a kaolinite from within the ore zone have 230Th/ 238U of 0.58 to 0.83 and 226Ra/230Th of 1.09 to 1.42, and 3) some Fe-mineral samples from within the breccia pipe have values of 1.2 and 0.9 respectively. These data, combined with those from other studies at Peña Blanca suggest that U and Ra are sometimes mobile in the near-surface environment and that multiple episodes of enrichment and leaching are required to explain the trends.
Unsaturated Zone Waters From the Nopal I Natural Analog, Chihuahua, Mexico - Implications for Radionuclide Mobility at Yucca Mountain
Authors: Pickett David A. , Murphy William M. 
Published: April 2011

AbstractChemical and U-Th isotopic data on unsaturated zone waters from the Nopal I natural analog reveal effects of water-rock interaction and help constrain models of radionuclide release and transport at the site and, by analogy, at the proposed nuclear waste repository at Yucca Mountain. Geochemical reaction-path modeling indicates that, under oxidizing conditions, dissolution of uraninite (spent fuel analog) by these waters will lead to eventual schoepite precipitation regardless of initial silica concentration provided that groundwater is not continuously replenished. Thus, less soluble uranyl silicates may not dominate the initial alteration assemblage and keep dissolved U concentrations low. Uranium-series activity ratios are consistent with models of U transport at the site and display varying degrees of leaching versus recoil mobilization. Thorium concentrations may reflect the importance of colloidal transport of low-solubility radionuclides in the unsaturated zone.
On the Oxidation of Uraninite From Natural Reactor Cores
Authors: Cui Daqing , Eriksen Trygve , Eklund Ulla-Britt 
Published: April 2011

AbstractNatural nuclear reactors provide unique evidence in helping to understand the processes that might occur over long timescales in radioactive waste disposal sites. In the presented work, the extent and kinetics of oxidation of core material from the Oklo-Bangombé natural reactors are investigated. The X-ray powder diffraction analysis shows that the uraninites core samples from the Bangombé Reactor and Oklo Reactor 2. and Oklo Reactor 13 have the same unit-cell parameters as synthetic UO2.25. A significant amount of fourmarierite, Pb(UO2)4O3(OH)44H2O, was identified in the core samples from two shallow reactors Bangombé and Oklo 2, but not in the deeper reactor Oklo 13. The results of U(IV)/U(IV) measurements indicate that the extent of oxidative weathering of shallow reactors (Bangombé and Oklo 2) is greater than for the deeper reactor Oklo 13. Evaporable organic compounds found in the uraninite inclusion containing “bitumen” at the edge of Okelobondo Reactor (400 °C) and in the black shale immediately above the Bangombé Reactor (260 °C) may work as a reducing buffer or/and a hydrophobic water shield to depress the oxidative dissolution of the uraninite cores.