Potential Repository Research Articles

Testing claims about volcanic disruption of a potential geologic repository at Yucca Mountain, Nevada

Yucca Mountain is a potential repository site for high‐level radioactive waste. We examine claims that the frequency of future basaltic dike intersection could be 1 × 10−6/yr or higher (i.e., on average, one penetration every million yrs). These claims fail simple tests at four time scales. Spatial‐temporal models that predict intrusion frequencies >2 × 10−7/yr are overly pessimistic because no dikes have been found in the 13 Myr‐old potential repository footprint. More realistic models are developed by considering this non‐detection of dikes and by applying probabilistic hazard assessments based on known patterns of Pleistocene volcanism. Using a hazard assessment code, we estimate the frequency of dike intersection at 5.4 × 10−8/yr with an upper 95% confidence bound of 9.8 × 10−8/yr.

Yucca Mountain Nuclear Waste Containment Standard a Hot Topic

July is becoming a tough month for nuclear waste. In July 2003, US District Court Judge Lynn Winmill in Idaho ruled that all of the approximately 90 million gallons of tank wastes at the Savannah River Site in South Carolina and Hanford Nuclear Reservation in Washington State are high level and must be buried in repositories. In July 2004, the US Circuit Court of Appeals for the District of Columbia ruled that a 10 000-year radiation standard for Yucca Mountain is inadequate. Congress will no doubt have to intervene to short-circuit the latter decision, or repositories in the US will never open.Any geologic burial site will be fractured before—and especially after—tunnel boring. The very best that can be done is to supplement the rock’s containment ability with engineered barriers such as thick-wall steel casks and titanium drip shields.The least the tank wastes weigh is 360 000 tons, or nearly five times the weight that’s slated to go in Yucca Mountain. The Department of Energy is committed to removing tank wastes from Savannah River and Hanford. In 1984, the department had potential crystalline repositories in North Carolina, New Hampshire, Maine, and Wisconsin on its radar screen. Rocks in those states are granitic, which means each of the sites is already fractured and will be further fractured with drilling. Anyone wanting proof of that need only consult the Oskarshamn repository, a granite site about 150 miles south of Stockholm.At some point, we’re going to have to be practical. We’ve enjoyed nuclear energy, and we’ve enjoyed our status as the world’s greatest nuclear power, but everything comes with a price. We now have 410 000 tons of spent-fuel rods and tank wastes spread all over the country, and all of us would breathe easier if they were out of sight, out of mind.© 2004 American Institute of Physics.

Geohydromechanical processes in the Excavation Damaged Zone in crystalline rock, rock salt, and indurated and plastic clays—in the context of radioactive waste disposal

Abstract The creation of an excavation disturbed zone (EdZ) or excavation damaged zone (EDZ) is expected around all man-made openings in geologic formations. Macro- and micro-fracturing, and in general a redistribution of in situ stresses and rearrangement of rock structures, will occur in this zone, resulting in drastic changes of permeability to flow, mainly through the fractures and cracks induced by excavation. The EDZ may have significant implications for the operation and long-term performance of an underground nuclear waste repository. Various issues of concern need to be evaluated, such as processes creating fractures in the EDZ, the degree of permeability increase, and the potential for sealing or healing (with permeability reduction) in the zone. In recent years, efforts along these lines have been made for a potential repository in four rock types—crystalline rock, salt, indurated clay, and plastic clay—and these efforts have involved field, laboratory, and theoretical studies. The present work involves a synthesis of the ideas and issues that emerged from presentations and discussions on EDZ in these four rock types at a CLUSTER Conference and Workshop held in Luxembourg in November 2003. First, definitions of excavation disturbed and EDZs, that may be applicable to all four rock types, are proposed. Then, an approach is suggested for the synthesis and intercomparison of geohydromechanical processes in the EDZ for crystalline rock, salt, indurated clay, and plastic clay. Comparison tables of relevant processes, associated factors, and modelling and testing techniques are developed. A discussion of the general state-of-the-art and outstanding issues is also presented. A substantial bibliography of relevant papers on the subject is supplied at the end of the paper.

Characterizing Unsaturated Diffusion in Porous Tuff Gravel

Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high-level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using X-ray computed tomography (CT) in partially saturated gravel and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2- to 4-mm granular tuff at multiple water contents were calculated from electrical conductivity (EC) measurements using the Nernst–Einstein equation. In the second, we used laser ablation with inductively coupled plasma–mass spectrometry (LA/ICP-MS) to perform microscale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube–cube and cube–tetrahedron). The X-ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 × 10−14 m2 s−1 for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.

Rhizosheath of sinai desert plants is a potential repository for associative diazotrophs

Among 42 plant species representing the flora of north Sinai, two possessed sand grain sheath encasing the roots. They are Panicum turgidum Forssk. and Stipagrostis scoparia (Trin.and Rupr.) deWinter. Rhizosheaths, compared to surrounding free sand, accommodated higher population density of microorganisms including associative diazotrophs. Isolates secured belonged to the species of Bacillus circulans, Paenib. macerans ( Bacillus macerans), Enterobacter agglomerans, Agrobacterium radiobacter and Chryseomonas luteola ( Pseudomonos luteola). The rhizosheath potentiality in re-vegetating sand dunes and arid lands, through nitrogen fixation, plant–water relationship and root continuity for nutrient uptake, are discussed.

Use of X-ray absorption imaging to examine heterogeneous diffusion in fractured crystalline rocks

Heterogeneous diffusion in different regions of a fractured granodiorite from Japan has been observed and measured through the use of X-ray absorption imaging. These regions include gouge-filled fractures, recrystallized fracture-filling material and hydrothermally altered matrix. With the X-ray absorption imaging technique, porosity, relative concentration, and relative mass of an iodine tracer were imaged in two dimensions with a sub-millimeter pixel size. Because portions of the samples analyzed have relatively low porosity values, imaging errors can potentially impact the results. For this reason, efforts were made to better understand and quantify this error. Based on the X-ray data, pore diffusion coefficients ( D p) for the different regions were estimated assuming a single diffusion rate and a lognormal multirate distribution of D p. Results show D p for the gouge-filled fractures are over an order of magnitude greater than those of the recrystallized fracture-filling material, which in turn is approximately two times greater than those for the altered matrix. The recrystallized fracture-filling material was found to exhibit the greatest degree of variability. The results of these experiments also provide evidence that diffusion from advective zones in fractures through the gouge-filled fractures and recrystallized fracture-filling material could increase the pore space available for matrix diffusion. This evidence is important for understanding the performance of potential nuclear waste repositories in crystalline rocks as diffusion is thought to be an important retardation mechanism for radionuclides.

Dissolution Kinetics of Calcite in NaCl–CaCl2–MgCl2Brines at 25 °C and 1 bar pCO2

Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl2–MgCl2 solutions at 1 bar (0.1 MPa) pCO2 and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca2+:Mg2+= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca2+:Mg2+= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 − Omega;) n , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10−2 mol m−2 hr−1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10−2 mol m−2 hr−1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca2+:Mg2+ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO2 sequestration.

Surface‐to‐tunnel seismic tomography studies at Yucca Mountain, Nevada

A surface‐to‐tunnel seismic survey was conducted to estimate fracture intensity and distribution in the proposed nuclear waste repository area at Yucca Mountain, Nevada. A 5‐km‐long source line and a 3‐km‐long receiver line were located on top of Yucca Mountain ridge and inside the Exploratory Study Facility (ESF) tunnel, respectively. Numerical modeling showed that the first arrival P waves are mainly propagated in the repository horizon at depth. Therefore two‐dimensional travel time inversions were performed after the actual source locations on the ridge were projected onto the layer of interest at depth. Static corrections were applied to absorb the scatter in the new source locations and to correct for variations in source and receiver coupling and travel time shifts. Zero‐offset amplitude variations of the first arrivals across the receiver line inside the tunnel showed strong correlation with the fracture density determined by measurements along the tunnel wall. Wave guidance by subhorizontal fractures is suggested as a possible source for this phenomenon. Tomographic inversion of the travel time data revealed a low‐velocity zone in the south central area of the repository horizon. Conversion of the velocity tomography results to fracture‐density tomograms showed good correlation with fracture intensity mapped along tunnel walls. The fractured zone extends over a large area in the southern region of the potential repository, in particular, southwest of the intensely fractured zone encountered in the ESF tunnel. The rock in the northern section of the repository appears more competent compared to the highly fractured areas in the south.

Estimated Effects of Temperature-Relative Humidity Variations on the Composition of In-Drift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada

AbstractWe define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances.

Conductivity Behavior of Salt Deposits on the Surface of Engineered Barrier Materials for the Potential High-Level Nuclear Waste Repository at Yucca Mountain, Nevada

AbstractThe electrical conductance or conductivity of three salt mixtures, Na-K-Cl-NO3, Ca-K-Cl and Ca-Na-Cl, were measured at 25, 50 and 70°C [77, 122, and 158 °F] as a function of relative humidity (RH). Mutual deliquescence and efflorescence RH (MDRH and MERH) values were determined based on the conductivity measurements. It was found that the conductivity of the three salt mixtures started to increase at RH values that are approximately 40 % of their MDRH and increased by 1to 2 orders of magnitude just before reaching the MDRH. At the MDRH, a significant increase in conductivity was observed. The MDRH and MERH for the Ca-K-Cl and Ca-Na-Cl mixtures were found to be approximately 15 % in the temperature range of 50 to 70 °C [122 to 158 °F]. The MDRH and MERH for the Na-K-Cl-NO3system were found to be approximately 54 % at 50 °C [122 °F] and decreased significantly with an increase in temperature.

Analysis of CO 2 leakage through ‘low-permeability’ faults from natural reservoirs in the Colorado Plateau, east-central Utah

Abstract The numerous CO 2 reservoirs in the Colorado Plateau region of the United States are natural analogues for potential geological CO 2 sequestration repositories. To understand better the risk of leakage from reservoirs used for long-term underground CO 2 storage, we examine evidence for CO 2 migration along two normal faults that cut a reservoir in east-central Utah. CO 2 -charged springs, geysers, and a hydrocarbon seep are localized along these faults. These include natural springs that have been active for long periods of time, and springs that were induced by recent drilling. The CO 2 -charged spring waters have deposited travertine mounds and carbonate veins. The faults cut siltstones, shales, and sandstones and the fault rocks are fine-grained, clay-rich gouge, generally thought to be barriers to fluid flow. The geological and geochemical data are consistent with these faults being conduits for CO 2 moving to the surface. Consequently, the injection of CO 2 into faulted geological reservoirs, including faults with clay gouge, must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere.

Electrochemical Evaluation and Surface Characterization of Josephinite as a Natural Analog for Container Materials

AbstractA sample of josephinite, a rock containing predominantly a Ni-Fe metallic phase, was evaluated as a natural metal analog to increase confidence in the assessment of waste package performance for the potential high-level radioactive waste repository at Yucca Mountain, Nevada. The josephinite sample was characterized electrochemically in simulated groundwater environments using cyclic potentiodynamic polarization and potentiostatic tests. The passive surface layers formed potentiostatically were examined by X-ray photoelectron spectroscopy. These results were compared to those obtained with a synthetic Ni3Fe alloy with a chemical composition similar to that of josephinite. Electrochemical studies showed that josephinite exhibited passivity at a slightly higher pH than did the cast Ni3Fe alloy and was found to be slightly more susceptible to pitting corrosion. The passive films formed on the josephinite and the cast Ni3Fe alloy have a duplex structure consisting of an Fe-rich hydroxide outer layer and a Ni-rich oxide inner layer. Results obtained from this study provide an appropriate characterization of the environmental conditions leading to the passivity and localized corrosion of josephinite. Extended persistence of a stable passive film, however, is essential for the long-term stability of the josephinite sample. The relationship between the passive behavior and the formation of alteration layers needs to be established for assessing the survivability of josephinite.

Integration of perched water and chloride data in modeling flow processes within the unsaturated zone of Yucca Mountain, Nevada

This paper presents a modeling study incorporating field-measured perched water and chloride data into a three-dimensional (3-D) large-scale unsaturated zone (UZ) model of Yucca Mountain, Nevada, a potential underground nuclear waste repository site. This study integrates field measured perched water and pore-water chloride data into a single 3-D flow and transport model through model calibration. Using these field data in model calibration is intended to improve the model's capability to evaluate current conditions and predict future conditions of the unsaturated zone, so as to aid in the performance assessment of the proposed repository system. The calibration results show that the large-scale UZ flow and transport model may be able to predict both moisture and chloride geochemical conditions at the site.

Preferential Radionuclide Release Due to Alpha Decay: Effects on Repository Performance

AbstractWe model preferential release of 237Np, 234U, 230Th, 226Ra, and 210Pb from disposed commercial spent nuclear fuel as a result of alpha recoil damage, using the U.S. Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) model for the potential repository at Yucca Mountain. Time-dependent augmentation of the ingrown component is simulated by increasing the initial parent inventory; we have used a factor of five increase, based on natural system observations. For 237Np, the magnitude of preferential release is subject to solubility limits. Stochastic TPA runs show a significant effect on modeled dose of preferential 237Np release, but low impact from the other four radionuclides. The mechanism could be ineffective if 237Np is incorporated into secondary phases. While our results are exploratory in nature, this approach to modeling decay-related enhancement of release can be applied in other nuclear waste disposal settings.

Progress of R&D on total system performance assessment of a potential high level radioactive waste repository in Korea : Hwang, Y. S. et al. Progress in Nuclear Energy, 2003, 42, (2), 199–220

Progress of R&D on total system performance assessment of a potential high level radioactive waste repository in Korea : Hwang, Y. S. et al. Progress in Nuclear Energy, 2003, 42, (2), 199–220

Factor analytical approaches for evaluating groundwater trace element chemistry data

The multivariate statistical techniques principal component analysis (PCA), Q-mode factor analysis (QFA), and correspondence analysis (CA) were applied to a dataset containing trace element concentrations in groundwater samples collected from a number of wells located downgradient from the potential nuclear waste repository at Yucca Mountain, Nevada. PCA results reflect the similarities in the concentrations of trace elements in the water samples resulting from different geochemical processes. QFA results reflect similarities in the trace element compositions, whereas CA reflects similarities in the trace elements that are dominant in the waters relative to all other groundwater samples included in the dataset. These differences are mainly due to the ways in which data are preprocessed by each of the three methods. The highly concentrated, and thus possibly more mature (i.e. older), groundwaters are separated from the more dilute waters using principal component 1 (PC 1). PC 2, as well as dimension 1 of the CA results, describe differences in the trace element chemistry of the groundwaters resulting from the different aquifer materials through which they have flowed. Groundwaters thought to be representative of those flowing through an aquifer composed dominantly of volcanic rocks are characterized by elevated concentrations of Li, Be, Ge, Rb, Cs, and Ba, whereas those associated with an aquifer dominated by carbonate rocks exhibit greater concentrations of Ti, Ni, Sr, Rh, and Bi. PC 3, and to a lesser extent dimension 2 of the CA results, show a strong monotonic relationship with the percentage of As(III) in the groundwater suggesting that these multivariate statistical results reflect, in a qualitative sense, the oxidizing/reducing conditions within the groundwater. Groundwaters that are relatively more reducing exhibit greater concentrations of Mn, Cs, Co, Ba, Rb, and Be, and those that are more oxidizing are characterized by greater concentrations of V, Cr, Ga, As, W, and U.

A reaction-transport model for calcite precipitation and evaluation of infiltration fluxes in unsaturated fractured rock

The percolation flux in the unsaturated zone (UZ) is an important parameter addressed in site characterization and flow and transport modeling of the potential nuclear-waste repository at Yucca Mountain, NV, USA. The US Geological Survey (USGS) has documented hydrogenic calcite abundances in fractures and lithophysal cavities at Yucca Mountain to provide constraints on percolation fluxes in the UZ. The purpose of this study was to investigate the relationship between percolation flux and measured calcite abundances using reactive transport modeling. Our model considers the following essential factors affecting calcite precipitation: (1) infiltration, (2) the ambient geothermal gradient, (3) gaseous CO 2 diffusive transport and partitioning in liquid and gas phases, (4) fracture–matrix interaction for water flow and chemical constituents, and (5) water–rock interaction. Over a bounding range of 2–20 mm/year infiltration rate, the simulated calcite distributions capture the trend in calcite abundances measured in a deep borehole (WT-24) by the USGS. The calcite is found predominantly in fractures in the welded tuffs, which is also captured by the model simulations. Simulations showed that from about 2 to 6 mm/year, the amount of calcite precipitated in the welded Topopah Spring tuff is sensitive to the infiltration rate. This dependence decreases at higher infiltration rates owing to a modification of the geothermal gradient from the increased percolation flux. The model also confirms the conceptual model for higher percolation fluxes in the fractures compared to the matrix in the welded units, and the significant contribution of Ca from water–rock interaction. This study indicates that reactive transport modeling of calcite deposition can yield important constraints on the unsaturated zone infiltration-percolation flux and provide useful insight into processes such as fracture–matrix interaction as well as conditions and parameters controlling calcite deposition.

The site-scale saturated zone flow model for Yucca Mountain: calibration of different conceptual models and their impact on flow paths

This paper presents several different conceptual models of the Large Hydraulic Gradient (LHG) region north of Yucca Mountain and describes the impact of those models on groundwater flow near the potential high-level repository site. The results are based on a numerical model of site-scale saturated zone beneath Yucca Mountain. This model is used for performance assessment predictions of radionuclide transport and to guide future data collection and modeling activities. The numerical model is calibrated by matching available water level measurements using parameter estimation techniques, along with more informal comparisons of the model to hydrologic and geochemical information. The model software (hydrologic simulation code FEHM and parameter estimation software PEST) and model setup allows for efficient calibration of multiple conceptual models. Until now, the Large Hydraulic Gradient has been simulated using a low-permeability, east–west oriented feature, even though direct evidence for this feature is lacking. In addition to this model, we investigate and calibrate three additional conceptual models of the Large Hydraulic Gradient, all of which are based on a presumed zone of hydrothermal chemical alteration north of Yucca Mountain. After examining the heads and permeabilities obtained from the calibrated models, we present particle pathways from the potential repository that record differences in the predicted groundwater flow regime. The results show that Large Hydraulic Gradient can be represented with the alternate conceptual models that include the hydrothermally altered zone. The predicted pathways are mildly sensitive to the choice of the conceptual model and more sensitive to the quality of calibration in the vicinity on the repository. These differences are most likely due to different degrees of fit of model to data, and do not represent important differences in hydrologic conditions for the different conceptual models.

Modeling thermal–hydrological response of the unsaturated zone at Yucca Mountain, Nevada, to thermal load at a potential repository

This paper presents a numerical study on the response of the unsaturated zone (UZ) system of Yucca Mountain to heat generated from decaying radioactive wastes emplaced at the proposed repository. The modeling study is based on the current thermal–hydrological (TH) mountain-scale model, which uses a locally refined 2D north–south cross-section and dual-permeability numerical approach. The model provides a prediction of the mountain-scale TH response under the thermal-load scenario of 1.45 kW/m, while accounting for future climatic changes and the effects of drift ventilation. The TH simulation results show that ventilation of the repository drifts has a large impact on thermal–hydrologic regimes and moisture-flow conditions at the repository. In both cases, with and without ventilation, the TH model predicts dry or reduced liquid saturation near the drifts for over 1000 years, during which liquid flux through the drifts is reduced to either zero or less than the ambient flux. Without ventilation, the model predicts higher temperatures at the repository, but no major moisture redistribution in the UZ except in the areas very near the heated drifts.

Performance assessment model development and analysis of radionuclide transport in the unsaturated zone, Yucca Mountain, Nevada

This paper describes the development and use of a particle-tracking model to perform radionuclide-transport simulations in the unsaturated zone at Yucca Mountain, Nevada. The goal of the effort was to produce a computational model that can be coupled to the project's calibrated 3D site-scale flow model so that the results of that effort could be incorporated directly into the Total System Performance Assessment (TSPA) analyses. The transport model simulates multiple species (typically 20 or more) with complex time-varying and spatially varying releases from the potential repository. Water-table rise, climate-change scenarios, and decay chains are additional features of the model. A cell-based particle-tracking method was employed that includes a dual-permeability formulation, advection, longitudinal dispersion, matrix diffusion, and colloid-facilitated transport. This paper examines the transport behavior of several key radionuclides through the unsaturated zone using the calibrated 3D unsaturated flow fields. Computational results illustrate the relative importance of fracture flow, matrix diffusion, and lateral diversion on the distribution of travel times from the simulated repository to the water table for various climatic conditions. Results also indicate rapid transport through fractures for a portion of the released mass. Further refinement of the model will address several issues, including conservatism in the transport model, the assignment of parameters in the flow and transport models, and the underlying assumptions used to support the conceptual models of flow and transport in the unsaturated zone at Yucca Mountain.