Yucca Mountain Project Research Articles

Glass fabrication and product consistency testing of lanthanide borosilicate glass for plutonium disposition

The Department of Energy Office of Environmental Management (DOE/EM) plans to conduct the Plutonium Disposition Project at the Savannah River Site (SRS) in Aiken, SC, to disposition excess weapons-usable plutonium. A plutonium glass waste form is a leading candidate for immobilization of the plutonium for subsequent disposition in a geologic repository. The objectives of this present task were to fabricate plutonium-loaded lanthanide borosilicate (LaBS) Frit B glass and perform testing to provide near-term data that will increase confidence that LaBS glass product is suitable for disposal in the proposed Federal Repository. Specifically, testing was conducted in an effort to provide data to Yucca Mountain Project (YMP) personnel for use in performance assessment calculations. Plutonium-containing LaBS glass with the Frit B composition with a 9.5 wt% PuO 2 loading was prepared for testing. Glass was prepared to support glass durability testing via the ASTM product consistency testing (PCT) at Savannah River National Laboratory (SRNL). The glass was characterized with X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) prior to performance testing. This characterization revealed some crystalline PuO 2 inclusions with disk-like morphology present in the as-fabricated, quench-cooled glass. A series of PCTs was conducted at SRNL with varying exposed surface area and test durations. Filtered leachates from these tests were analyzed to determine the dissolved concentrations of key elements. The leachate solutions were also ultrafiltered to quantify colloid formation. Leached solids from select PCTs were examined in an attempt to evaluate the Pu and neutron absorber release behavior from the glass and to investigate formation of alteration phases on the glass surface. A series of PCTs was conducted at 90 °C in ASTM Type 1 water to compare the Pu LaBS Frit B glass durability to current requirements for high level waste (HLW) glass in a geologic repository. The PCT (7-day static test with powdered glass) results on the Pu-containing LaBS Frit B glass at SA/V of ≈2000 m −1 showed that the glass was very durable with an average normalized elemental release value for boron of 0.013 g/m 2. This boron release value is ≈640× lower than normalized boron release from current environmental assessment (EA) glass used for repository acceptance. The PCT-B (7-, 14-, 28- and 56-day, static test with powdered glass) normalized elemental releases were similar to the normalized elemental release values from PCT-A testing, indicating that the LaBS Frit B glass is very durable as measured by the PCT. Normalized plutonium releases were essentially the same within the analytical uncertainty of the ICP-MS methods used to quantify plutonium in the 0.45 μm-filtered leachates and ultra-filtered leachates, indicating that colloidal plutonium species do not form under the PCT conditions used in this study.

Modeling of Ni–Cr–Mo based alloys: Part II — Kinetics

The CALPHAD approach is applied to kinetic studies of phase transformations and to aging of prototypes of Ni–Cr–Mo-based alloys selected for waste disposal canisters in the Yucca Mountain Project (YMP). Based on a previous study on alloy stability for several candidate alloys, the thermodynamic driving forces, together with a newly developed mobility database, have been used to analyze diffusion-controlled transformations in these Ni-based alloys. Results on precipitation of the Ni 2Cr-ordered phase in Ni–Cr and Ni–Cr–Mo alloys, and of the complex P and σ phases in a surrogate of Alloy 22 are presented, and the output from the modeling are compared with experimental data on aging.

An Evaluation of the Active Fracture Concept in Modeling Unsaturated Flow and Transport in a Fractured Meter‐Sized Block of Rock

Numerical simulation is an effective and economical method for optimally designing laboratory experiments and deriving practical experimental conditions. We executed a detailed numerical simulation study to examine the active fracture concept (AFC) using a 1‐m3‐sized block model. The numerical simulations for this study were performed by applying various experimental conditions, including different bottom flow boundaries, varying injection rates, and different fracture–matrix interaction (i.e., by increasing absolute matrix permeability at the fracture–matrix boundary) for a larger fracture interaction under transient or balanced‐state flow regimes. Two conceptual block models were developed based on different numerical approaches: a two‐dimensional discrete‐fracture‐network model (DFNM) and a one‐dimensional dual‐continuum model (DCM). The DFNM was used as a surrogate for a natural block to produce synthetic breakthrough curves of water and tracer concentration under transient or balanced‐state conditions. The DCM is the approach typically used for the Yucca Mountain Project because of its computational efficiency. The AFC was incorporated into the DCM to capture heterogeneous flow patterns that occur in unsaturated fractured rocks. The simulation results from the DCM were compared with the results from the DFNM to determine whether the DCM could predict the water flow and tracer transport observed in the DFNM at the scale of the experiment. It was found that implementing the AFC in the DCM improved the prediction of unsaturated flow and that the flow and transport experiments with low injection rates in the DFNM compared better with the AFC implemented DCM at the 1‐m3 scale. However, the estimated AFC parameter varied from 0.38 to 1.0 with different flow conditions, suggesting that the AFC parameter was not sufficient to fully capture the complexity of the flow processes in a 1‐m3 discrete fracture network.

Neutron analysis of spent fuel storage installation using parallel computing and advance discrete ordinates and Monte Carlo techniques

In the United States, the Nuclear Waste Policy Act of 1982 mandated centralised storage of spent nuclear fuel by 1988. However, the Yucca Mountain project is currently scheduled to start accepting spent nuclear fuel in 2010. Since many nuclear power plants were only designed for -10 y of spent fuel pool storage, > 35 plants have been forced into alternate means of spent fuel storage. In order to continue operation and make room in spent fuel pools, nuclear generators are turning towards independent spent fuel storage installations (ISFSIs). Typical vertical concrete ISFSIs are -6.1 m high and 3.3 m in diameter. The inherently large system, and the presence of thick concrete shields result in difficulties for both Monte Carlo (MC) and discrete ordinates (SN) calculations. MC calculations require significant variance reduction and multiple runs to obtain a detailed dose distribution. SN models need a large number of spatial meshes to accurately model the geometry and high quadrature orders to reduce ray effects, therefore, requiring significant amounts of computer memory and time. The use of various differencing schemes is needed to account for radial heterogeneity in material cross sections and densities. Two P3, S12, discrete ordinate, PENTRAN (parallel environment neutral-particle TRANsport) models were analysed and different MC models compared. A multigroup MCNP model was developed for direct comparison to the SN models. The biased A3MCNP (automated adjoint accelerated MCNP) and unbiased (MCNP) continuous energy MC models were developed to assess the adequacy of the CASK multigroup (22 neutron, 18 gamma) cross sections. The PENTRAN SN results are in close agreement (5%) with the multigroup MC results; however, they differ by -20-30% from the continuous-energy MC predictions. This large difference can be attributed to the expected difference between multigroup and continuous energy cross sections, and the fact that the CASK library is based on the old ENDF/B-II library. Both MC and SN calculations were run in parallel on a BEOWULF PC-cluster (eight processors). Timing results indicate that the SN calculation yielded a detailed dose distribution at over 318,426 points in -164 h. Unbiased continuous energy MC required 214 h to calculate dose rates with a 1% relative error in only 18 regions on the surface of the cask. The biased A3MCNP calculations yields dose rates with -0.8% relative error in only 2.5 h on one processor. This study demonstrates that a parallel code, such as the 3-D parallel SN transport code, PENTRAN can solve a complex large problem, such as the storage cask, accurately and efficiently. Moreover, this calculation was performed on a relatively inexpensive PC-cluster. Possible inadequacies of the CASK cross section library still need to be evaluated.

Modeling of Ni–Cr–Mo based alloys: Part I—phase stability

The CALPHAD approach is applied to the study of the stability of several candidate Ni–Cr–Mo based alloys for use in waste disposal canisters for the Yucca Mountain Project (YMP). The stability of occurring phases in the as-fabricated materials is investigated since temporal evolution of precipitates at service temperatures can significantly alter their corrosion resistance and mechanical properties. To account for the stability of the Ni 2Cr phase and the hypothetical Ni 2Mo and Ni 2W phases, available thermodynamic data and ab initio electronic structure-based results were employed. A comparative study of property diagrams for the nominal composition of several YMP candidate alloys is presented. These results form the basis for experimental and theoretical studies of kinetics of phase transformation and aging to be presented in Part II.

Stochastic Study of Solute Transport in a Nonstationary Medium

A Lagrangian stochastic approach is applied to develop a method of moment for solute transport in a physically and chemically nonstationary medium. Stochastic governing equations for mean solute flux and solute covariance are analytically obtained in the first-order accuracy of log conductivity and/or chemical sorption variances and solved numerically using the finite-difference method. The developed method, the numerical method of moments (NMM), is used to predict radionuclide solute transport processes in the saturated zone below the Yucca Mountain project area. The mean, variance, and upper bound of the radionuclide mass flux through a control plane 5 km downstream of the footprint of the repository are calculated. According to their chemical sorption capacities, the various radionuclear chemicals are grouped as nonreactive, weakly sorbing, and strongly sorbing chemicals. The NMM method is used to study their transport processes and influence factors. To verify the method of moments, a Monte Carlo simulation is conducted for nonreactive chemical transport. Results indicate the results from the two methods are consistent, but the NMM method is computationally more efficient than the Monte Carlo method. This study adds to the ongoing debate in the literature on the effect of heterogeneity on solute transport prediction, especially on prediction uncertainty, by showing that the standard derivation of solute flux is larger than the mean solute flux even when the hydraulic conductivity within each geological layer is mild. This study provides a method that may become an efficient calculation tool for many environmental projects.

Studies of the Corrosion Properties of Ni-Cr-Mo-Gd Neutron-Absorbing Alloys

Abstract The corrosion properties of Ni-Cr-Mo-Gd alloys that are being developed for use as a neutron-absorbing structural material were examined. The corrosion work was part of a larger alloy development program. The corrosion properties were examined by both electrochemical and longer-term immersion testing in standard test solutions and in simulated solutions used in corrosion testing by the Yucca Mountain Project. The addition of Gd to a Ni-Cr-Mo alloy results in the formation of a gadolinide (Ni5Gd) secondary phase. This phase was observed to preferentially dissolve under electrochemical testing at anodic potentials. The reaction appears to be mostly limited to the secondary phase exposed to the surface. A brief comparison with another neutron-absorbing alloy, borated stainless steel, is made.

Deliquescence of NaCl-NaNO3, KNO3-NaNO3, and NaCl-KNO3 salt mixtures from 90 to 120°C.

We conducted reversed deliquescence experiments in saturated NaCl–NaNO3–H2O, KNO3–NaNO3–H2O, and NaCl–KNO3–H2O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO3, and KNO3 represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO3 and K–NO3 ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions.

Deliquescence of NaCl–NaNO[sub 3], KNO[sub 3]–NaNO[sub 3], and NaCl–KNO[sub 3] salt mixtures from 90 to 120 °C

We conducted reversed deliquescence experiments in saturated NaCl–NaNO3–H2O, KNO3–NaNO3–H2O, and NaCl–KNO3–H2O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO3, and KNO3 represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO3 and K–NO3 ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions.

Measurement of laser peening residual stresses

Laser peening is an emerging surface treatment for metals that imparts compressive residual stress near the surface by introducing cold work. The present paper describes two residual stress measurement techniques that are well suited for the measurement of residual stresses induced by laser peening. Since residual stress is a key outcome of the laser peening process, its measurement is an important process diagnostic. The two residual stress measurement techniques described are the slitting method and the contour method. In the present research, the slitting method is used to establish relationships between various laser peening parameters (irradiance, pulse duration, number of layers, etc.) and the residual stress profile (residual stress versus depth) in flat plate coupons that have been uniformly peened on one surface. This helps to identify an effective set of laser peening parameters for the given material. Next, laser peening is applied to a specimen(s) with representative geometry and the contour method is used to measure the spatial distribution of residual stress around critical geometric features. The information gathered from the contour method measurement(s) is used to identify an effective laser peening pattern (coverage area) for the given geometry. This two step procedure is illustrated using recent measurements on sample Alloy 22 (UNS N06022) welds for the Yucca Mountain Project. Details of both measurement procedures are given including representative results for each.

Three-Dimensional Numerical Method of Moments for Linear Equilibrium-Adsorbing Solute Transport in Physically and Chemically Nonstationary Formations

A Lagrangian perturbation method is applied to develop a method of moments for reactive solute flux through a three-dimensional, nonstationary flow field. The flow nonstationarity may stem from medium nonstationarity, finite domain boundaries, and/or fluid pumping and injecting. The reactive solute flux is described as a space–time process where time refers to the solute flux breakthrough in a control plane at some distance downstream of the solute source and space refers to the transverse displacement distribution at the control plane. The analytically derived moments equations for solute transport in a nonstationary flow field are too complicated to solve analytically; therefore, a numerical finite difference method is implemented to obtain the solutions. This approach combines the stochastic model with the flexibility of the numerical method to boundary and initial conditions. The approach provides a tool to apply stochastic theory to reactive solute transport in complex subsurface environments. Several case studies have been conducted to investigate the influence of the physical and chemical heterogeneity of a medium on the reactive solute flux prediction in nonstationary flow field. It is found that both physical and chemical heterogeneity significantly affect solute transport behavior in a nonstationary flow field. The developed method is also applied to an environmental project for predicting solute flux in the saturated zone below the Yucca Mountain Project area, demonstrating the applicability of the method in practical environmental projects.

Comparison of streamtube and three-dimensional models of reactive transport in heterogeneous media

Field-scale transport of reactive solutes is influenced strongly by physical and geochemical heterogeneity over a wide range of spatial scales. Accurate numerical simulation requires fine spatial and temporal discretization to resolve the smallest scales of heterogeneity, in addition to coupling large numbers of chemical components participating in geochemical and microbiological reactions. Standard approaches based upon finite difference or finite element discretization of the governing set of advection-dispersion-reaction equations can lead to computational difficulties for large problems. To overcome these difficulties, several streamtube-based models have been presented in the literature. These models neglect transverse dispersion, and hence reactive transport occurs along independent one-dimensional streamtubes. Therefore, streamtube models are well-suited for applications where the objective is to calculate solute flux breakthrough across a control plane. We develop a reactive streamtube model by modification of the Finite Element Heat and Mass transfer code (FEHM), an existing flow and transport code. FEHM can simulate unsaturated zone conditions, heat flow, and multicomponent reactive transport, and has been used extensively by the Yucca Mountain Project. Because the same underlying numerical algorithms are used in both the FEHM and streamtube models, direct comparison of their relative performance is possible. We perform this comparison for a three-dimensional reactive transport example, to demonstrate the large potential computational advantages of the streamtube approach.

A three-dimensional numerical method of moments for groundwater flow and solute transport in a nonstationary conductivity field

A three-dimensional numerical method of moments has been developed for solute flux through nonstationary flows in porous media. The solute flux is described as a space–time process where time refers to the solute flux breakthrough and space refers to the transverse displacement distribution at a control plane. Flow nonstationarity may stem from various sources, such as the medium’s conductivity nonstationarity and complex hydraulic boundary conditions. The first two statistics of solute flux are derived using a Lagrangian framework and are expressed in terms of the probability density functions (PDFs). These PDFs are given in terms of one- and two-parcel moments of travel time and transverse locations, and these moments are related to the Eulerian velocity moments. The moment equations obtained analytically for flow and transport are so complex that numerical techniques are used to obtain solutions. In this study, we investigate the influence of various factors, such as the grid resolution relative to correlation length and the number of solute parcels comprising a source, on the accuracy of the calculation results. It has been found that for the computation of means and variances using the developed moment equations, hydraulic head requires at least one numerical grid element per correlation length scale. At least two grid elements are required for velocity, and 1–2 grid elements for the solute flux variance. Five parcels are required per correlation length scale to approximate the initial solute source distribution. The effects of boundary and hydraulic conductivity nonstationarity on flow and transport are also considered. Flow nonstationarity caused by either hydraulic boundary condition or conductivity nonstationarity significantly influences the transport process. The calculation results of numerical method of moments are compared with Monte Carlo simulations. The comparison indicates that the two methods are consistent with each other for head variance, velocity covariance in longitudinal direction, and mean and variance of total solute flux, but numerical method of moment underestimates the velocity variance in transverse direction. The method is applied to an environmental project for predicting the solute flux in the saturated zone below the Yucca Mountain project area, demonstrating the applicability of the method to complex subsurface environments.

Applications of nonstationary stochastic theory to solute transport in multi-scale geological media

In this study, we make use of a nonstationary stochastic theory in studying solute flux through spatially nonstationary flows in porous media. The nonstationarity of flow stems from various sources, such as multi-scale, nonstationary medium features and complex hydraulic boundary conditions. These flow nonstationarities are beyond the applicable range of the ‘classical’ stochastic theory for stationary flow fields, but widely exist in natural media. In this study, the stochastic frames for flow and transport are developed through an analytical analysis while the solutions are obtained with a numerical method. This approach combines the stochastic concept with the flexibility of the numerical method in handling medium nonstationarity and boundary/initial conditions. It provides a practical way for applying stochastic theory to solute transport in complex groundwater environments. This approach is demonstrated through some synthetic cases of solute transport in multi-scale media as well as some hypothetical scenarios of solute transport in the groundwater below the Yucca Mountain project area. It is shown that the spatial variations of mean log-conductivity and correlation function significantly affect the mean and variance of solute flux. Even for a stationary medium, complex hydraulic boundary conditions may result in a nonstationary flow field. Flow nonstationarity and/or nonuniform distribution of initial plume (geometry and/or density) may lead to nonGaussian behaviors (with multiple peaks) for mean and variance of the solute flux. The calculated standard deviation of solute flux is generally larger than its mean value, which implies that real solute fluxes may significantly deviate from the mean predictions.

Thermally induced natural convection effects in Yucca Mountain drifts

Thermally induced natural convection from the heat produced by emplaced waste packages is an important heat and mass transfer mechanism within the Yucca Mountain Project (YMP) drifts. Various models for analyzing natural convection have been employed. The equivalent porous medium approach using Darcy's law has been used in many YMP applications. However, this approach has questionable fidelity, especially for turbulent flow conditions. Computational fluid dynamics (CFD), which is based on the fundamental Navier–Stokes equations, is currently being evaluated as a technique to calculate thermally induced natural convection in YMP. Data–model comparisons for turbulent flow conditions show good agreement of CFD predictions with existing experiments including YMP-specific data.

Analysis of thermohydrologic behavior for above-boiling and below-boiling thermal-operating modes for a repository at Yucca Mountain

We report results from a multi-scale thermohydrologic modeling study for two alternative thermal-operating modes for the potential repository system recently analyzed by the Yucca Mountain Project. These include a Higher-Temperature Operating Mode (HTOM), which results in a localized boiling zone around each emplacement drift, and a Lower-Temperature Operating Mode (LTOM), which always maintains sub-boiling temperatures throughout the repository. The HTOM places all waste packages nearly end to end, making the lineal power density greater than in the LTOM. The lower lineal power density in the LTOM was achieved by placing some waste packages farther apart (which results in a larger repository footprint), and through an increased reliance on pre-closure ventilation to remove the waste-package-generated heat. We focus on temperature T and relative humidity RH at the waste-package and drift-wall surfaces, and on in-drift evaporation. In general, HTOM temperatures are greater than corresponding LTOM temperatures, exhibit similar spatial variability and have a stronger dependence on infiltration flux. The duration of RH reduction on waste packages is similar for the LTOM and HTOM. A major difference between the LTOM and HTOM is the lower waste-package temperature at any given value of waste-package RH for the LTOM. Waste-package temperatures in the LTOM, by design, remain below ∼85 °C; the absence of RH reduction arising from host-rock dryout causes waste-package RH to remain above about 40%. The HTOM waste packages experience higher temperatures and correspondingly lower RH conditions as a result of RH reduction arising from host-rock dryout. For most of the repository area, the HTOM delays the potential onset of gravity-driven seepage compared to the LTOM (as indicated by the duration of boiling at the drift wall). Boiling conditions in the HTOM also delays the onset of capillary-driven seepage into the granular invert, causing the HTOM to have less evaporation in the invert during the first 800–1500 years than the LTOM; subsequent evaporation rates are higher in the HTOM, due to the higher power density.

Application of classification trees in the sensitivity analysis of probabilistic model results

The complexity of some integrated-system models necessitates using a probabilistic approach to quantify uncertainty in model projections. In this work, we demonstrate how classification trees can be used to perform sensitivity analyses on probabilistic results. The classification tree technique is applied to results from the probabilistic total system performance assessment model used in the Yucca Mountain project. The technique proves effective in delineating the variables that most influence low and high outcomes.

Stability and Aging of Candidate Alloys for the Yucca Mountain Project: CALPHAD Results

ABSTRACTThe CALPHAD approach is applied to the study of stability and aging of several Ni-Cr-Mo alloys similar to those considered for use in waste disposal containers for the Yucca Mountain Project (YMP). The stability of occurring phases in the as-fabricated materials must be investigated since their precipitation with time at temperature can reduce their corrosion resistance and mechanical properties. We first show that excellent agreement exists between assessed and calculated phase diagrams for the three major binary alloys, Ni-Cr, Ni-Mo, and Cr-Mo, and for the ternary Ni-Cr-Mo alloy. Then we present a comparative study of phase fractions versus temperature for the nominal composition of several multi-component Ni-Cr-Mo alloys. Finally we discuss the aging properties of the Ni2Cr-ordered phase in Ni-Cr and in Ni-Cr-Mo alloys, and of the complex P phase in Alloy 22.

Solubility Evaluation for Yucca Mountain TSPA-SR

ABSTRACTA systematic evaluation of radionuclide solubility has been conducted for the Yucca Mountain Project. The conventional thermodynamic approach was employed and geochemical model calculations were used to estimate radionuclide solubilities for the base case. The study uses the computer code EQ3/6 as the major geochemical modeling tool. The water composition and environmental conditions are based on the results of in-package chemistry and in-drift chemistry studies. Field observations, laboratory measurements, and thermodynamic and kinetic considerations are utilized to ensure the resulting solubilities are conservative. Fourteen radioelements have been studied (U, Np, Pu, Th, Am, Ac, Tc, I, C, Cs, Sr, Ra, Pa, and Pb) and their solubilities are presented as either functions of environmental conditions or statistical distributions. An alternative source term model for Np, which was based on measurements of spent fuel dissolution experiments rather than conventional thermodynamic considerations, was also developed. Comparison of it with the Np base case solubility model suggests that it is necessary to advance our understanding about the behaviors of Np during the process of spent fuel corrosion.

Waste Package Environment for the Yucca Mountain Site Characterization Project

ABSTRACTEnvironmental parameters that would affect the degradation of engineered materials, including waste packages and drip shields, in the potential high level nuclear waste repository at Yucca Mountain, Nevada are being characterized as part of the Yucca Mountain Project. These parameters include: temperature, relative humidity, range of water chemistry, deliquescence of salts, pH, and electrochemical potential (Eh). The likelihood of various brine compositions forming on the engineered components under repository conditions, and the implications, is discussed. Relative humidity controls the ionic strength and composition of the aqueous solutions, and hence strongly influences the corrosion processes that could occur. Studies are underway to more fully characterize the redox state of aqueous solutions in contact with engineered system components.