Waste Isolation Pilot Plant Site Research Articles

Interpretation of actinide-distribution data obtained from non-destructive and destructive post-test analyses of an intact-core column of Culebra dolomite

The US Department of Energy (DOE), with technical assistance from Sandia National Laboratories, has successfully received EPA certification and opened the Waste Isolation Pilot Plant (WIPP), a nuclear waste disposal facility located approximately 42 km east of Carlsbad, NM. Performance assessment (PA) analyses indicate that human intrusions by inadvertent, intermittent drilling for resources provide the only credible mechanisms for significant releases of radionuclides from the disposal system. For long-term brine releases, migration pathways through the permeable layers of rock above the Salado formation are important. Major emphasis is placed on the Culebra Member of the Rustler Formation because this is the most transmissive geologic layer overlying the WIPP site. In order to help quantify parameters for the calculated releases, radionuclide transport experiments have been carried out using intact-core columns obtained from the Culebra dolomite member of the Rustler Formation within the WIPP site. This paper deals primarily with results of analyses for 241Pu and 241Am distributions developed during transport experiments in one of these cores. Transport experiments were done using a synthetic brine that simulates Culebra brine at the core recovery location (the WIPP air-intake shaft (AIS)). Hydraulic characteristics (i.e., apparent porosity and apparent dispersion coefficient) for intact-core columns were obtained via experiments using the conservative tracer 22Na. Elution experiments carried out over periods of a few days with tracers 232U and 239Np indicated that these tracers were weakly retarded as indicated by delayed elution of the species. Elution experiments with tracers 241Pu and 241Am were attempted but no elution of either species has been observed to date, including experiments of many months' duration. In order to quantify retardation of the non-eluted species 241Pu and 241Am after a period of brine flow, non-destructive and destructive analyses of one intact-core column were carried out to determine distribution of these actinides in the rock. Analytical results indicate that the majority of the 241Am remained very near the injection surface of the core (possibly as a precipitate), and that the majority of the 241Pu was dispersed with a very high apparent retardation value. The 241Pu distribution is interpreted using a single-porosity advection–dispersion model, and an approximate retardation value is reported.

Thermodynamic modeling of actinide complexation with oxalate at high ionic strength

A geochemical model describing the solubility of actinides in underground water at the Waste Isolation Pilot Plant (WIPP) Project is under development. The database for this model consists of standard chemical potentials and Pitzer model parameters for hundreds of species that may be present in the WIPP disposal room. Organic ligands used in separation and decontamination processes may be present in the nuclear wastes placed in the WIPP site and could have a significant impact on mobile actinide concentrations. In this work the β1 and β2 stability constants of NpO2 +, UO2 2+, Am3+ and Th4+ with the oxalate anion have been measured in 0.3–5.0M NaCl media at 25 °C by a solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO2 +<Am3+<UO2 2+<Th4+, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction model was used to model the data. Because the data were collected mainly in the high ionic strength region, values of β(1) were estimated from these plus literature values. The Pitzer model gives a good representation of the data using three interaction parameters β(0), β(1), and cφ.

Conceptual structure of the 1996 performance assessment for the Waste Isolation Pilot Plant

The conceptual structure of the 1996 performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) is described. This structure involves three basic entities (EN1, EN2, EN3): (1) EN1, a probabilistic characterization of the likelihood of different futures occurring at the WIPP site over the next 10,000 yr, (2) EN2, a procedure for estimating the radionuclide releases to the accessible environment associated with each of the possible futures that could occur at the WIPP site over the next 10,000 yr, and (3) EN3, a probabilistic characterization of the uncertainty in the parameters used in the definition of EN1 and EN2. In the formal development of the 1996 WIPP PA, EN1 is characterized by a probability space (S{sub st}, P{sub st}, p{sub st}) for stochastic (i.e., aleatory) uncertainly; EN2 is characterized by a function {line_integral} that corresponds to the models and associated computer programs used to estimate radionuclide releases; and EN3 is characterized by a probability space (S{sub su}, P{sub su}, p{sub su}) for subjective (i.e., epistemic) uncertainty. A high-level overview of the 1996 WIPP PA and references to additional sources of information are given in the context of (S{sub st}, P{sub st}, p{sub st}), {line_integral} and (S{sub su}, P{sub su}, p{sub su}).

Development of the conceptual models for chemical conditions and hydrology used in the 1996 performance assessment for the Waste Isolation Pilot Plant

The Waste Isolation Pilot Plant (WIPP) is a US Department of Energy (DOE) facility for the permanent disposal of defense-related transuranic (TRU) waste. US Environmental Protection Agency (EPA) regulations specify that the DOE must demonstrate on a sound basis that the WIPP disposal system will effectively contain long-lived alpha-emitting radionuclides within its boundaries for 10,000 years following closure. In 1996, the DOE submitted the 40 CFR Part 191 Compliance Certification Application for the Waste Isolation Pilot Plant (CCA) to the EPA. The CCA proposed that the WIPP site complies with EPA's regulatory requirements. Contained within the CCA are descriptions of the scientific research conducted to characterize the properties of the WIPP site and the probabilistic performance assessment (PA) conducted to predict the containment properties of the WIPP disposal system. In May 1998, the EPA certified that the TRU waste disposal at the WIPP complies with its regulations. Waste disposal operations at WIPP commenced on 28 March 1999. The 1996 WIPP PA model of the disposal system included conceptual and mathematical representations of key hydrologic and geochemical processes. These key processes were identified over a 22-year period involving data collection, data interpretation, computer models, and sensitivity studies to evaluate the importance of uncertainty and of processes that were difficult to evaluate by other means. Key developments in the area of geochemistry were the evaluation of gas generation mechanisms in the repository; development of a model of chemical conditions in the repository and actinide concentrations in brine; selecting MgO backfill and demonstrating its effects experimentally; and, determining the chemical retardation capability of the Culebra. Key developments in the area of hydrology were evaluating the potential for groundwater to dissolve the Salado Formation (the repository host formation); development of a regional model for hydrologic conditions; development of a stochastic, probabilistic representation of hydraulic properties in the Culebra Member of the Rustler Formation; characterization of physical transport in the Culebra; and, the evaluation of brine and gas flow in the Salado. Additional confidence in the conceptual models used in the 1996 WIPP PA was gained through independent peer review in many stages of their development.

Effects of spatially heterogeneous porosity on matrix diffusion as investigated by X-ray absorption imaging

High-resolution X-ray absorption imaging was used to investigate the effects of spatially heterogeneous porosity on matrix diffusion. Experiments were performed on four, centimeter-scale slabs of Culebra dolomite taken from the Waste Isolation Pilot Plant (WIPP) site. These tests involved the diffusion of potassium iodide into a single edge of each brine-saturated rock slab, while X-ray absorption imaging was used to measure the two-dimensional relative concentration distribution at different times during the experiment. X-ray imaging was also used to measure the heterogeneous, two-dimensional porosity distribution of each rock slab. The resulting high-resolution data provide unique insight into the spatially varying diffusion characteristics of each heterogeneous rock sample, which traditional methods such as through-diffusion experiments cannot. In these tests, significant variations in the diffusion coefficient were calculated over the relatively small length (centimeter) and time scales (months) investigated. Results also indicated that these variations were related to the heterogeneous porosity characteristics of each rock sample. Not only were the diffusion coefficients found to depend on the magnitude of the porosity but also on its spatial distribution. Specifically, the geometry, position, and orientation of the heterogeneous porosity features populating each rock slab appeared to influence the diffusion characteristics.

Deformation Mechanism of the Underground Excavations at the WIPP Site

The deformational behavior of underground excavations in rock salt is difficult to predict because of the time-dependent properties of rock salt, and its response to many other parameters, which influence its behavior. To investigate the influence of important parameters such as time, temperature, geology, opening geometry, excavation sequence, and effects of the immediate roof layer on underground excavations in rock salt, deformation measurements from the Waste Isolation Pilot Plant (WIPP) were used.

Field investigation of the waste isolation pilot plant (WIPP) site (New Mexico) using a nonstationary stochastic model with a trending hydraulic conductivity field

Verification of a stochastic model which models the phenomena of flow and transport in a nonstationary conductivity field [Rubin and Seong, 1994] is attempted with field data from the Waste Isolation Pilot Plant (WIPP) site near Carlsbad, New Mexico. The nonstationarity is manifested as a spatial linear trend in the mean log conductivity field. Analysis of the log conductivity and the hydraulic head data shows that the site corresponds to Rubin and Seong's α2 model, where the mean flow direction and the gradient of mean log conductivity are normal to each other. The experimental semivariogram of the log conductivity obtained from field data strongly suggests that it is nonstationary and can be fit with a Gaussian covariance function. The α2 model is successful in predicting the experimental semivariogram of the head obtained from the field data, especially in capturing the finite sill characteristics. The stationary model predicts an unbounded semivariogram at large distances. Solute travel times are also investigated by comparing the travel time cumulative distribution function (CDF) resulting from the stationary and the nonstationary hydraulic conductivity models. The stationary model predicts a higher CDF at early times and hence overestimates the very early travel times to the regulatory plane of compliance than the α2 model.

Actinide Biocolloid Formation in Brine by Halophilic Bacteria

AbstractWe examined the ability of a halophilic bacterium (WIPP 1A) isolated from the Waste Isolation Pilot Plant (WIPP) site to accumulate uranium in order to determine the potential for biocolloid facilitated actinide transport. The bacterial cell surface functional groups involved in the complexation of the actinide were determined by titration. Uranium, added as uranyl nitrate, was removed from solution at pH 5 by cells but at pH 7 and 9 very little uranium was removed due to its limited solubility. Although present as soluble species, uranyl citrate at pH 5, 7, and 9, and uranyl carbonate at pH 9 were not removed by the bacterium because they were not bioavailable due to their neutral or negative charge. Addition of uranyl EDTA to brine at pH 5, 7, and 9 resulted in the immediate precipitation of U. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) analysis revealed that uranium was not only associated with the cell surface but also accumulated intracellularly as uranium-enriched granules. Extended X-ray absorption fine structure (EXAFS) analysis of the bacterial cells indicated the bulk sample contained more than one uranium phase. Nevertheless these results show the potential for the formation of actinide bearing bacterial biocolloids that are strictly regulated by the speciation and bioavailability of the actinide.

Recovery and Healing of Damage in WIPP Salt

Creep damage in rock salt, which generally manifests in the form of microcracks, can be recovered or healed when subjected to sufficiently high pressures and temperatures. In this paper, the phenomena of damage recovery and healing in rock salt are treated using the continuum damage mechanics approach. A healing term is formulated and incorporated into an existing constitutive model for describing coupled creep, fracture, and healing in rock salt. The constitutive model is then evaluated against experimental data of rock salt from the Waste Isolation Pilot Plant (WIPP) site. Satisfactory results are obtained between model calculations and experimental measurements of axial, lateral, and volumetric strains, as well as acoustic wave velocity and attenuation recovered during damage healing. Furthermore, analyses of experimental data revealed that healing anisotropies exist in WIPP salt. Most, though not all, of the healing anisotropies can be modeled with an appropriate power-conjugate equivalent stress, kinetic equation, and evolution equation for damage healing based on a scalar damage variable. This work represents the first attempt to simulate healing of damaged intact WIPP salt; however, further evaluations and improvement of the model can be anticipated.

Creep-Induced Cleavage Fracture in WIPP Salt Under Indirect Tension

The phenomenon of cleavage fracture initiation in rock salt undergoing concurrent creep was studied experimentally using the Brazilian indirect tension test technique. The tensile creep and cleavage fracture behaviors were characterized for rock salt from the Waste Isolation Pilot Plant (WIPP) site. The Brazilian test consists of a compressive line load applied diametrically on a disk specimen to produce a region of tensile stress in the center of the disk. The damage processes were documented using video photography. The experimental results were analyzed in terms of a wing-crack fracture model and an independently developed, coupled time-dependent, mechanism-based constitutive model whose parameters were obtained from triaxial compression creep tests. Analytical results indicate that coupling between creep and cleavage fracture in WIPP salt results in a fracture behavior that exhibits time-dependent characteristics and obeys a failure criterion involving a combination of stress difference and tensile stress. Implications of creep-induced cleavage fracture to the integrity of WIPP structures are discussed.

A Damage Mechanics Treatment of Creep Failure in Rock Salt

Recent progress in the formulation of a constitutive model for describing coupled creep and damage development in rock salt is summarized. The constitutive model is based on the assumption that both dislocation slip and creep damage in the form of microcracks with possible wing-tips contribute to the macroscopic inelastic strain rate. The relevant kinetic equations, flow law, and damage evolution equation are presented. Evaluations are made of the coupled creep and fracture model against the measured creep response of clean salt from the Waste Isolation Pilot Plant (WIPP) site. In addition, the development of creep damage and the rupture of WIPP salt subjected to either triaxial compression or indirect tension have been analyzed to evaluate several creep rupture criteria.

Pilot Point Methodology for Automated Calibration of an Ensemble of conditionally Simulated Transmissivity Fields: 1. Theory and Computational Experiments

A new methodology for solution of the inverse problem in groundwater hydrology is proposed and applied to a site in southeastern New Mexico with extensive hydrogeologic data. The methodology addresses the issue of nonuniqueness of the inverse solutions by generating an ensemble of transmissivity fields considered to be equally likely, each of which is in agreement with the measured transmissivity and pressure data. It consists of generating a selected number of conditionally simulated transmissivity fields and then calibrating each of the fields to match the measured steady state or transient pressures, in a least squares sense. The calibration phase involves an iterative implementation of an automated pilot point approach coupled with conditional simulations. Pilot points are the parameters of calibration. They are synthetic transmissivity data which are added to the transmissivity database to produce a revised conditional simulation during calibration. Coupled kriging and adjoint sensitivity analysis is employed for the optimal location of pilot points, and gradient search methods are used to derive their optimal transmissivities. The pilot point methodology is well suited for characterizing the spatial variability of the transmissivity field in contrast to methods using zonation. Pilot points are located where their potential for minimizing the objective function is the highest. This minimizes the perturbations in the transmissivities which are optimally assigned to the pilot point and results in minimal changes to the covariance structure of the transmissivity field. The calibrated fields honor the transmissivity measurements at their locations, preserve the variogram, and match the measured pressures in a least squares sense. Since there are numerous options in the execution of this methodology, computational experiments have been conducted to identify the most efficient among them. The method has been applied to the Waste Isolation Pilot Plant (WIPP) site, in southeastern New Mexico, where the U.S. Department of Energy is conducting probabilistic system assessment for the permanent disposal of transuranic nuclear waste. The resulting calibrated transmissivity fields are input to a Monte Carlo analysis of the total system performance. The present paper, paper 1 of a two‐paper presentation, describes the methodology. Paper 2, a companion paper, presents the methodology's application to the WIPP site.

Geochemical evolution of groundwater in the Culebra dolomite near the Waste Isolation Pilot Plant, southeastern New Mexico, USA

The Culebra Dolomite Member of the Rustler Formation, a thin (10 m) fractured dolomite aquifer, lies approximately 450 m above the repository horizon of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico, USA. Salinities of water in the Culebra range roughly from 10,000 to 200,000 mg/L within the WIPP site. A proposed model for the post-Pleistocene hydrochemical evolution of the Culebra tentatively identifies the major sources and sinks for many of the groundwater solutes. Reaction-path simulations with the PHRQPITZ code suggest that the Culebra dolomite is a partial chemical equilibrium system whose composition is controlled by an irreversible process (dissolution of evaporites) and equilibrium with gypsum and calcite. Net geochemical reactions along postulated modern flow paths, calculated with the NETPATH code, include dissolution of halite, carbonate and evaporite salts, and ion exchange. R-mode principal component analysis revealed correlations among the concentrations of Si, Mg, pH, Li, and B that are consistent with several clay-water reactions. The results of the geochemical calculations and mineralogical data are consistent with the following hydrochemical model: 1. (1) solutes are added to the Culebra by dissolution of evaporite minerals 2. (2) the solubilities of gypsum and calcite increase as the salinity increases; these minerals dissolve as chemical equilibrium is maintained between them and the groundwater 3. (3) equilibrium is not maintained between the waters and dolomite; sufficient Mg is added to the waters by dissolution of accessory carnallite or polyhalite such that the degree of dolomite supersaturation increases with ionic strength 4. (4) clays within the fractures and rock matrix exert some control on the distribution of Li, B, Mg, and Si via sorption, ion exchange, and dissolution.

Fall Meeting Subsurface Hydrology Highlights

As has become the norm in recent years, subsurface hydrology sessions took place every day of AGU's Fall 1992 Meeting in San Francisco, Calif. Sessions were organized around various approaches to groundwater modeling, hydrogeology field studies, hydrochemical investigations, vadose‐zone studies, and measurement methods. Session chairs contributed the following highlights.A well‐attended special session looked at the “History of Groundwater Hydrology in the Nuclear Era.” F. L. Parker, Vanderbilt University, Nashville, Tenn., reviewed early developments in U.S. radioactive‐waste disposal and offered the provocative viewpoint that a disposal program is characterized by a high degree of inflexibility. W. W. Dudley of the U.S. Geological Survey, Denver, Colo., summarized the hydrological studies that have been such an important component of the nuclear explosion program at the Nevada Test Site. Two papers presented results of investigations in connection with the Waste Isolation Pilot Plant site, where military waste is to be stored in salt.

Geochemistry of the waste isolation pilot plant (WIPP) site, southestern New Mexico, U.S.A.

An extensive geochemical data base, including analyses of major and minor solutes, mineralogical studies of core samples, and isotopic studies of waters, carbonates and sulfates, has been assembled for evaporites and related rocks in the northern Delaware Basin of southeastern New Mexico. These data were compiled for the geological and hydrological characterization of the Waste Isolation Pilot Plant (WIPP), which is excavated in the evaporites of the Salado Formation. These data were evaluated in order: (1) to determine the stability of the evaporite mineralogy over geological time; (2) to compare the aqueous geochemistry with host rock mineralogy; (3) to delineate the nature and timing of water-rock interactions, such as dissolution and recrystallization; (4) to determine the geological and climatic conditions that have governed groundwater recharge. The resulting synthesis of data and current hypotheses concerning the origin, composition and history of waters in the evaporite rocks and related units of the Delaware Basin provides a tentative conceptual model for the behavior of the water-rock system since the deposition of the evaporites in the Permian. Essential components of this model include: (1) widespread Late Triassic/Early Jurassic evaporite recrystallization; (2) accumulation of deep-basin brines isolated from meteoric recharge; (3) evaporite dissolution by meteoric waters flowing in carbonates and sulfates interbedded in the uppermost Permian section and at the basin margin; (4) lateral rather than vertical infiltration of pre-Holocene meteoric waters in the uppermost Permian section; and (5) climatic conditions presently less conducive to recharge than in the Late Pleistocene.

Site-characterization studies at the Waste Isolation Pilot Plant (WIPP) site, southeastern New Mexico : Lappin, A R Radioact Waste ManageV14, N1/2, 1990, P101–121

Site-characterization studies at the Waste Isolation Pilot Plant (WIPP) site, southeastern New Mexico : Lappin, A R Radioact Waste ManageV14, N1/2, 1990, P101–121

Interactions between Concrete and Brine at the Waste Isolation Pilot Plant (WIPP) Site, New Mexico

ABSTRACTA concrete liner emplaced in 1984 in a shaft at the Waste Isolation Pilot Plant has served as a natural laboratory for observing interactions among concrete, evaporite rocks, and brine. During a routine inspection of the liner in the spring of 1990, discoloration, deposition of secondary salts, wet areas with exposed aggregate grains, softening of paste, surficial spalling, and cracking were observed locally on the concrete surface of the liner. Some construction joints showed apparent leakage of brine from behind the liner, which was nominally 50 cm thick. Seepage brines were nearly saturated relative to CaCl2 and contained lesser amounts of MgCl2 and KCl, and minor NaCl. The liner surface was locally altered to a 1–2 cm friable hygroscopic layer containing little cement paste; concrete cores (7 or 10 cm diameter) through the liner at depths of 248, 254, 255, and 271 m showed similar degrees of alteration at the liner/rock interface. The most profound alteration of concrete was developed in a ˜7 cm zone sub-parallel to and straddling the construction joint cored at a depth of ˜254.5 m. This zone was extensively microfractured, transected aggregate grains, and contained brucite, gypsum, magnesium hydroxychloride hydrate, and locally calcium chloroaluminate instead of the usual phases of hydrated portland cement. Several mechanisms of chemical degradation have been proposed, the most likely being attack by magnesium ions.

In situ testing program at the Waste isolation pilot plant : Matalucci, R V Radioact Waste ManageV14, N1/2, 1990, P123–154

The In Situ Testing Program is an integral part of the Waste Isolation Pilot Plant (WIPP) Technology Development Program, providing full-sized experiments for evaluation of constitutive models and laboratory data. The in situ tests are being performed about 659 m underground, in the rock-salt environment of the WIPP site, and consist of three technical areas: Thermal/Structural Interactions (TSI); Plugging and Sealing (PS and Waste Package Performance (WPP). The in situ tests are designed to (a) provide data for evaluating theoretical models and predictive techniques; and (b) demonstrate the actual behavior of the salt, engineered barrier backfills and seals, and waste packages in an underground salt environment

A prequalifying program for evaluating the analytical performance of commercial laboratories

Soil and water samples were spiked with known activities of radionuclides and sent to seven commercial laboratories that had expressed an interest in analyzing environmental samples for the Waste Isolation Pilot Plant (WIPP). This Prequalifying Program was part of the selection process for an analytical subcontractor for a three-year program of baseline radiological surveillance around the WIPP site. Both media were spiked at three different activity levels with several transuranic radionuclides, as well as tritium, fission products, and activation products. Laboratory performance was evaluated by calculating relative error for each radionuclide in each sample, assigning grade values, and compiling grades into “report cards” for each candidate. Results for the five laboratories that completed the Prequalifying Program were pooled to reveal differing degrees of difficulty among the treatments and radionuclides. Interlaboratory comparisons revealed systematic errors in the performance of one candidate. The final report cards contained clear differences among overall grades for the five laboratories, enabling analytical performance to be used as a quantitative criterion in the selection of an analytical subcontractor.

International Collaboration in Nuclear Waste Solidification

A number of collaborative research projects between Belgium, Canada, France, Japan, Sweden, Switzerland, the United States, and the Federal Republic of Germany are now in progress to test the relative surface reactions or nuclear waste glasses under a wide variety of simulated repository conditions. The studies include 1-, 3-, 6-, 12-, 24-, and 32-month deep burial in granite boreholes in the Stripa mine in Sweden at 90 and 10/sup 0/C. Nearly 2000 interactive interfaces are being studied in salt in the Waste Isolation Pilot Plant site in the United States. Several glasses from these tests are also being evaluated in clay in Belgium. Comparisons of the simulated burial conditions with glasses containing radioactivity close to that expected for commercial operations as LaHague, France, are being made by a Japan-Sweden-Switzerland consortium with collaboration from the Commissariat a l'Energie Atomique, Marcoule and the Hahn Meitner Institute, Berlin. These studies are leading toward an international consensus on the relative performance of high-level waste forms including borosilicate glasses, waste packages, and repository variables.