Yucca Mountain Project Research Articles

Synthesis of boron carbide reinforced aluminum castings through mechanical stir casting

Nuclear energy is the leading clean energy source in the United States with 99 nuclear power plants generating approximately 20% of the country’s electricity. While the growth of the industry helps to reduce our reliance on fossil fuels, there is also an increase in nuclear waste. The demand for on-site dry cask spent nuclear fuel storage has increased due to complications with the Yucca Mountain Project and diminishing pool capacity. A novel dry cask consisting of an aluminum metal matrix for effective thermal conductivity with boron carbide as a neutron absorbing additive was investigated using gravity sand casting techniques. X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and optical microscopy were used to characterize the resultant microstructure, including boron carbide incorporation and heterogeneity. Results indicate vortex and nonvortex mixing in air with boron carbide (1–10 µm diameter) produces large amounts of porosity and insufficient wetting. Use of a larger particle size distribution of boron carbide (20–60 µm diameter) during high speed vortex mixing prior to casting has shown significant dispersion of up to 12 wt% sufficient for neutron shielding with appropriate wall thickness. These results validate the use of gravity sand casting as a means to produce borated aluminum for an effective alternative to fuel storage.

Verification and comparison of two thermodynamic databases through conversion to PHREEQC and multicomponent geothermometrical calculations

The databases SOLTHERM and Yucca Mountain Project (YMP) were converted from GeoT/TOUGHREACT to PHREEQC format, to not only allow a higher accessibility of freely available data for thermodynamic modelling in the temperature range of 0−350 °C at the pressure of water vapour saturation, but also show possible causes of differences between simulations using two programs. Converted databases were verified by comparing the results obtained with the same simulations performed with PHREEQC and GeoT. Four natural geothermal solutions from previously published works were used in the simulations, covering a wide range of temperatures, salinities, pH, and equilibrium minerals. Consecutively, a comparison was performed through geothermometrical calculations for the test solutions, by using the converted databases with PHREEQC and applying the same methods as in the original publications.Minimal differences were found between simulations using the original and converted databases, especially when solutions with low ionic strengths were involved. Differences found using solutions with ionic strengths above 1.0 mol/kg are likely caused by differences in the calculation methods for activity coefficients of dissolved species between converted databases in PHREEQC and original databases in GeoT.The geothermometrical calculations showed a good agreement between published results, using different databases and programs, and results obtained using the transformed databases in PHREEQC, especially considering calculated saturation indices of rock-forming minerals such as quartz, feldspars, calcite and dolomite, and muscovite. A higher deviation was found for clay minerals, as the thermodynamic data for this type of minerals are more variable and differ per type (and version) of database.

Siting Deep Boreholes for Disposal of Radioactive Waste: Consequences for Tight Coupling between Natural and Engineered Systems.

Since the Yucca Mountain project in the U.S. was defunded in 2010, the notion of disposing of spent nuclear fuel (SNF) in deep boreholes has been reinvigorated, most recently by private companies proposing to utilize lateral drilling technology to excavate boreholes for SNF disposal in sedimentary rock. It is claimed that this approach will alleviate site characterization efforts and expand the availability of potential disposal sites. However, long-term safety will hinge upon the prevalence of geochemically reducing, highly saline, and slow-flowing fluids around the waste emplacement zone, and to quantify these parameters in fluids sampled from depths >1 km will present a challenge. Regional data indicate only a narrow geographical extent of such conditions in the conterminous United States. Furthermore, models of radionuclide transport from disposal boreholes must take into account processes that may accelerate degradation of the canisters, plug, and SNF itself, such as radiolysis and attack by constituents of hydrothermal brines, coupled with hydrogeologic features that promote advective groundwater flow. This review summarizes some geologic considerations, most notably those related to geochemistry, that challenge the long-term safety case for deep borehole disposal of SNF.

Modelling groundwater contamination above a potential nuclear waste repository in the Columbia River Basalt, USA

Potential groundwater contamination above the one-time candidate repository for high-level nuclear waste in the Columbia River Basalt (CRB), USA, is evaluated in a flow-transport model with 260,895 elements for a period of 1 million years. The hydrogeologic database originates from the Basalt Waste Isolation Project, which ended in 1987, giving way to the Yucca Mountain project. The CRB data have been scanned recently with the object of finding a storage site for natural gas and a sequestration site for CO2. However, the impact on the nuclear issue in the USA or other countries with similar large flood-basalt provinces has never been evaluated since the suspension of the project. The peak radioactive dose predicted for a potential repository in the CRB is at least eight orders of magnitude lower than the US generic regulatory limit (0.15 mS/a) even for the most pessimistic scenario.

Probability and Consequences of Nuclear Criticality at a Geologic Repository—II: Potential Repository in Volcanic Tuff as an Example

This paper, Part II of two companion papers, demonstrates the concepts for evaluating the criticality scenario class after closure of a geologic repository for spent nuclear fuel (SNF) and high-level radioactive waste. As an example, the low-probability rationale used to exclude consideration of criticality in the performance assessment of the potential Yucca Mountain (YM) repository in southern Nevada is summarized. The Yucca Mountain Project (YMP) presented a quantitative rationale that the probability of criticality inside breached waste containers was <10−4 over 104 yr to show that criticality was not necessary to consider. The dominant probability occurred when neutron absorber material was inadvertently left out for a package disposing of SNF from experimental reactors owned by the U.S. Department of Energy. In addition, this paper develops a quantitative estimate of the low probability of criticality outside the package in either the engineered or geologic barrier to complement the qualitative rationale developed by YMP. Because consequence may also be used as the basis of screening, consequences of criticality at the potential YM repository are roughly estimated, based on results from the literature. The consequences are then combined with the low-probability estimates as a complementary cumulative distribution function to place the corresponding estimated consequences in context and, thereby, provide further perspective on excluding the criticality scenario class.

Hydrology and Geochemistry of Yucca Mountain and Vicinity, Southern Nevada and California

John S. Stuckless Editor GSA Memoir 209 (Stuckless, 2012) summarizes over 20 years of site characterization conducted as part of the U.S. Department of Energy's work to determine the suitability of Yucca Mountain for storing high-level nuclear waste. Memoir 209 is a companion volume to an earlier publication Memoir 199, The Geology and Climatology of Yucca Mountain and Vicinity, Southern Nevada and California , edited by Stuckless and Levich (2007), which summarized the background information and set the stage for Memoir 209. Memoir 209 reports the field observations and model simulations carried out in support of the construction license for the Yucca Mountain Program. This well-written compendium summarizes a vast number of studies on hydrology and geochemistry and develops a working conceptual model of the site, although the conceptual models do not always correspond with some of the models used by the U.S. Department of Energy in support of a License Application for Construction Authorization (U.S. Department of Energy, 2008). If there is one shortfall for this Memoir, it is that a few of the summaries do not include a discussion of uncertainties, shortcomings, or rationales for the numerical models selected or why experiments were performed. This is understandable in some cases because including significant detail would produce an unwieldy tome that would likely be too long to be read by many. But there are cases outlined below where additional detail, if only a paragraph or two, would have greatly benefited the reader in providing context for interpreting the results and understanding the courses of action taken. This is particularly true for a GSA Memoir. Chapter 1 provides the background information for the Yucca Mountain project including a brief …

Multiphysics processes in partially saturated fractured rock: Experiments and models from Yucca Mountain

The site investigations at Yucca Mountain, Nevada, have provided us with an outstanding data set, one that has significantly advanced our knowledge of multiphysics processes in partially saturated fractured geological media. Such advancement was made possible, foremost, by substantial investments in multiyear field experiments that enabled the study of thermally driven multiphysics and testing of numerical models at a large spatial scale. The development of coupled‐process models within the project have resulted in a number of new, advanced multiphysics numerical models that are today applied over a wide range of geoscientific research and geoengineering applications. Using such models, the potential impact of thermal‐hydrological‐mechanical (THM) multiphysics processes over the long‐term (e.g., 10,000 years) could be predicted and bounded with some degree of confidence. The fact that the rock mass at Yucca Mountain is intensively fractured enabled continuum models to be used, although discontinuum models were also applied and are better suited for analyzing some issues, especially those related to predictions of rockfall within open excavations. The work showed that in situ tests (rather than small‐scale laboratory experiments alone) are essential for determining appropriate input parameters for multiphysics models of fractured rocks, especially related to parameters defining how permeability might evolve under changing stress and temperature. A significant laboratory test program at Yucca Mountain also made important contributions to the field of rock mechanics, showing a unique relation between porosity and mechanical properties, a time dependency of strength that is significant for long‐term excavation stability, a decreasing rock strength with sample size using very large core experiments, and a strong temperature dependency of the thermal expansion coefficient for temperatures up to 200°C. The analysis of in situ heater experiments showed that fracture closure/opening caused by changes in normal stress across fractures was the dominant mechanism for thermally induced changes in intrinsic fracture permeability during rock mass heating/cooling and that fracture shear dilation appears to be less significant. Significant effort was devoted to predicting the long‐term stability of underground excavations under (mechanical) strength degradation and seismic loading, perhaps one of the most challenging tasks within the project. We note that such long‐term strength degradation is actually an example of a chemically mediated process governed by underlying (microscopic) stress corrosion and chemical diffusion processes. In the Yucca Mountain Project, such chemically mediated mechanical changes were considered implicitly through model calibrations against laboratory and in situ heater experiments at temperatures anticipated to be experienced by the rock. A possible future research direction would be to simulate such processes mechanistically in a complete coupled THMC framework where C denotes chemical processes.

Sacred Land or National Sacrifice Zone: The Role of Values in the Yucca Mountain Participation Process

Local participation in environmental decision making is a fundamental tenet of environmental justice. This essay examines the participation process for nuclear waste siting decisions and suggests that the lack of a viable means for discussion of competing values is a flaw in the currently used model of participation. Through analysis of the Yucca Mountain high-level nuclear waste site in the USA, I show how the lack of discussion of values occludes participation by marginalized American Indians. In particular, I examine the incommensurability between American Indian nations that value Yucca Mountain as sacred land and the federal government that values Yucca Mountain as a national sacrifice zone. I argue that Yucca Mountain acts as a polysemous value term in the controversy. My findings suggest that an environmentally just model of participation in environmental decision making must include a way to account for incommensurable values and cultural differences. Further, I highlight the lessons we can learn from the Yucca Mountain project as we deliberate about what to do with nuclear waste.

Failure of the Yucca Mountain Project and the Blue Ribbon Commission Report

Failure of the Yucca Mountain Project and the Blue Ribbon Commission Report

Lessons Learned from the Yucca Mountain Nuclear Waste Repository Project The Engineered Barrier System

ABSTRACTNuclear waste isolation programs both inside and outside the United States have provided evidence that there are many geologic options for a repository, but virtually all of them rely to some degree on an engineered barrier system (EBS) to isolate and/or retard the migration of radionuclides to the biosphere. At Yucca Mountain, the design of the EBS was unexpectedly challenging because of uncertainties in quantitatively determining the local environment of the EBS particularly during the thermal pulse. The EBS design for the Yucca Mountain site evolved from a thin-walled, limited-lifetime, corrosion-resistant canister through a corrosion-allowance canister, to the present design, which may have a lifetime of more than 106years. The EBS proposed for the Yucca Mountain repository has many individual sub-barriers, beginning with the spent fuel and waste, the cladding of the spent fuel, the geometry of the package, etc. The anticipated modes of degradation of engineering materials, including corrosion of the fuel, of the canister, and of the drip shield proposed specifically for the Yucca Mountain project, and the consequences of the materials degradation on the performance of the repository are presented. The roles of conservative modeling and simplifying assumptions for radionuclide mobilization and transport in the EBS on characterization of the source term are addressed.

Materials and Corrosion Research in the Yucca Mountain Project

There is a current nuclear energy renaissance around the world, mainly in Asia. However, after 60 years of using nuclear power, the issue of nuclear waste still needs to be solved. All the countries currently addressing the issue of nuclear wastes are considering disposing them in stable geologic formations. Most repositories designs in Europe are considering materials such as carbon steel and copper for the container. The US repository in Yucca Mountain is currently on hold waiting for new recommendations from the Blue Ribbon Commission. A robust corrosion and materials testing program was carried out over the last 20 years at universities and National Laboratories to support the Yucca Mountain Project. For example, the intensive corrosion and electrochemical research helped understanding of how the different variables affect the resistance of C-22 to localized corrosion. Even if the repository of Yucca Mountain is not materialized, the knowledge gained on the corrosion behavior of materials is permanent.

Research in the Yucca Mountain Project Contributed to the Understanding of Corrosion Processes in Engineering Alloys

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Temperature, Humidity, and Airflow in the Emplacement Drifts Using Convection and Dispersion Transport Models

A coupled thermal-hydrologic-airflow model is developed, solving for the transport processes within a waste emplacement drift and the surrounding rock mass together at the proposed nuclear waste repository at Yucca Mountain. Natural, convective airflow as well as heat and mass transport in a representative emplacement drift, embedded in a three-dimensional, mountain-scale rock mass with edge cooling, are explicitly simulated for the first time in the literature, using the MULTIFLUX model. The conjugate, thermal-hydrologic transport processes in the rock mass are solved with the TOUGH2 porous-media simulator in a coupled way to the in-drift processes. The new simulation results show that large-eddy turbulent flow, as opposed to small-eddy flow, dominates the drift airspace for at least 5000 years following waste emplacement. The size of the largest, longitudinal eddy is equal to half of the drift length, providing a strong axial heat and moisture transport mechanism from the hot drift sections to the cold drift sections. The in-drift results are compared to those from simplified models using a surrogate, dispersive model with an equivalent dispersion coefficient for heat and moisture transport. Results from the explicit, convective velocity simulation model provide higher axial heat and moisture fluxes than those estimated from the previously published, simpler, equivalent dispersion models, in addition to showing differences in temperature, humidity, and condensation rate distributions along the drift length. A new dispersive model is also formulated for comparison, giving a time- and location-variable function that runs generally about ten times higher in value than the highest dispersion coefficient currently used in the Yucca Mountain Project.

Practical Postcalibration Uncertainty Analysis: Yucca Mountain, Nevada

The values of parameters in a groundwater flow model govern the precision of predictions of future system behavior. Predictive precision, thus, typically depends on an ability to infer values of system properties from historical measurements through calibration. When such data are scarce, or when their information content with respect to parameters that are most relevant to predictions of interest is weak, predictive uncertainty may be high, even if the model is "calibrated." Recent advances help recognize this condition, quantitatively evaluate predictive uncertainty, and suggest a path toward improved predictive accuracy by identifying sources of predictive uncertainty and by determining what observations will most effectively reduce this uncertainty. We demonstrate linear and nonlinear predictive error/uncertainty analyses as applied to a groundwater flow model of Yucca Mountain, Nevada, the United States' proposed site for disposal of high-level radioactive waste. Linear and nonlinear uncertainty analyses are readily implemented as an adjunct to model calibration with medium to high parameterization density. Linear analysis yields contributions made by each parameter to a prediction's uncertainty and the worth of different observations, both existing and yet-to-be-gathered, toward reducing this uncertainty. Nonlinear analysis provides more accurate characterization of the uncertainty of model predictions while yielding their (approximate) probability distribution functions. This article applies the above methods to a prediction of specific discharge and confirms the uncertainty bounds on specific discharge supplied in the Yucca Mountain Project License Application.

High‐level radioactive waste management in the USA

High‐level radioactive waste (HLW) disposal policy in the USA since 1987 has focused on a site in volcanic tuffs 305 meters beneath Yucca Mountain, Nevada, with current plans calling for the repository to be opened in 2017 subject to approval by the Obama Administration. Yet the offsite radiation release standards of the US Environmental Protection Agency are still being finalized, and there is significant doubt over whether a repository at Yucca Mountain can be successfully licensed by the Nuclear Regulatory Commission. Moreover, the proposed repository has a capacity cap insufficient to accept the total HLW stream from the civilian and military sectors, and focus on a single site is unnecessarily risky. Thus, the HLW problem in the USA remains far from being solved. After providing an overview of the nuclear waste regime of the USA, this paper reviews the technical, legal, and political status of the Yucca Mountain project. A large range of current issues and social values are discussed in this context, including the recently proposed Global Nuclear Energy Partnership that would revive spent fuel reprocessing and has important implications for national security and nuclear non‐proliferation.

Multiple-point statistical prediction on fracture networks at Yucca Mountain

In many underground nuclear waste repository systems, such as Yucca Mountain project, water flow rate and amount of water seepage into the waste emplacement drifts are mainly determined by hydrological properties of fracture network in the surrounding rock mass. Natural fracture network system is not easy to describe, especially with respect to its connectivity which is critically important for simulating the water flow field. In this paper, we introduced a new method for fracture network description and prediction, termed multi-point-statistics (MPS). The process of Multi-point Statistical method is to record multiple-point statistics concerning the connectivity patterns of fracture network from a known fracture map, and to reproduce multiple-scale training fracture patterns in a stochastic manner, implicitly and directly. It is applied to fracture data to study flow field behavior at Yucca Mountain waste repository system. First, MPS method is used to create fracture network with original fracture training image from Yucca Mountain dataset. After we adopt a harmonic and arithmetic average method to upscale the permeability to a coarse grid, THM simulation is carried out to study near-field water flow in surrounding rock of waste emplacement drifts. Our study shows that connectivity or pattern of fracture network can be grasped and reconstructed by Multi-Point-Statistical method. In theory, it will lead to better prediction of fracture system characteristics and flow behavior. Meanwhile, we can obtain variance from flow field, which gives us a way to quantify uncertainty of models even in complicated coupled THM simulation. It indicates that Multi-Point Statistics is a potential method to characterize and reconstruct natural fracture network in a fractured rock mass with advantages of quantifying connectivity of fracture system and its simulation uncertainty simultaneously.

Modeling plutonium solubility for Yucca Mountain performance assessment

AbstractThe Yucca Mountain performance assessment requires a Pu solubility model over a broad range of environmental conditions. Several Pu solubility models have been developed and evaluated for the Yucca Mountain Project. These models are 1) the theoreticalfO2model; 2) the empiricalEhupper limit model; and 3) the adjustedEhupper limit model. It is found that over the expected environmental conditions, Pu solubility is very sensitive to redox conditions and properly selecting the redox conditions to be modeled is the key to developing a realistic and defensible Pu solubility model. Analyzing the modeling results carefully and using experimental results as benchmarks suggest that the adjustedEhupper limit model is a realistic and yet defensible model to be used by the PA.

Coupled THM Model and Simulation of the Yucca Mountain and FEBEX Case Study within DECOVALEX-THMC Framework

Task_D of the DECOVALEX_THMC project focus on predictive analysis of the long-term coupled processes(up to 10,000 years) in two generic repositories, FEBEX type and Yucca Mountain Project type for comparison. To better understand the coupled THM processes and their influence on the system behavior, we have introduced a set of generic coupled THM governing equations. Basing on these equations, we develop simplified models according to given Task_D model inception phase request. Boiling model and empirical bentonite swelling model are introduced into general simulation which makes model more practical. Our numerical code FRT-THM upon this practical models is developed and used in two BMT case study of Task_D. Simulation results are shown and verified in the 3rd and 4th workshop of DECOVALEX_THMC in 2005. There is a good agreement with results of different participant teams which enhances confidence in prediction of coupled THM processes.

Modeling Alloy 22 general corrosion in the Yucca Mountain nuclear waste repository

This paper presents the general corrosion model used by the Yucca Mountain Project to account for general corrosion degradation of the Alloy 22 waste package outer barrier. The general corrosion rates were determined from mass-loss measurements of specimens exposed to repository-relevant solutions at 60°C and 90°C for about five years. An Alloy 22 general corrosion temperature dependence was obtained from short-term polarization resistance measurements and found to be consistent with values quoted in the literature. Uncertainty in mass-loss data and model fitting was evaluated and incorporated into the resultant Alloy 22 general corrosion model.

Experimental observation of the drift shadow effect using X-ray absorption imaging

X-ray absorption imaging is used to test the concept of the drift shadow in geological samples. The drift shadow model predicts that water travels around underground tunnels, or drifts, leaving areas of high saturation along the sides of the drift (roof-drip lobe) and an area of low saturation beneath the drift (drift shadow). The drift shadow model could impact nuclear waste repositories designed with open tunnels, such as Yucca Mountain, by impacting the flux available to transport waste beneath the repository. However, without strong evidence for the drift shadow effect, it is difficult to justify its inclusion in performance assessment calculations. Twelve experiments were run looking at the impact of fracture aperture, inflow rate and geological heterogeneity on flow in the vicinity of a drift. Test cells of dimensions 10.1 cm × 15.0 cm × 2.5 cm were constructed using geological samples relevant to the Yucca Mountain project. A semi-circle of sample was removed from one side of the cell creating an artificial drift (assuming symmetry). An artificial fracture was created through the middle of each test cell parallel to the face of the sample. Potassium iodide tracer solution was dripped into the artificial fracture at the top of the sample. X-ray imaging allowed for visualization of the tracer flow paths over the duration of the experiment. In addition, samples were collected at the bottom of the flow cell to determine the lateral distribution of the outflow. Results showed distinct flow paths of tracer solution around the drift followed by shedding at the edge of the drift (roof-drip lobes), as predicted by the drift shadow model. In addition, the distribution of discharge under the drift supported the drift shadow model with less discharge directly under the drift and greater discharge to the side of the drift in most experiments. In the experiments with smaller fracture apertures and lower flow rates, discharge beneath the drift was greater than expected than if flow was uniformly vertical, indicating that under certain conditions some water could be carried back under the drift due to capillary action. Very low mass discharge into the drift (generally less than 1% of the inflow mass) along with the X-ray images of the tracer flow paths provide evidence that the drift is acting as a capillary barrier. X-ray imaging also showed that heterogeneities have some control on the flow paths. Experimental results such as these coupled with numerical modeling that better quantifies the effects of heterogeneities and uncertainties in input parameters may allow performance assessment calculations to include the impact of the drift shadow on radionuclide transport. If implemented, such studies could lead to improved natural barrier performance in future calculations.