Publications

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Monitoring programs for nuclear-waste repositories are now conceived as an exercise in performance confirmation: the purpose of the monitoring program is to provide objective evidence that the repository system is functioning as expected. However, monitoring is still largely seen as being entirely focused on the near-repository environment, with the aim of verifying predictions made about such things as temperature or rock mass behavior that are relevant to the designed functioning of the repository. A more comprehensive approach comes from recognizing that the repository is embedded in a larger natural system that may be affected by factors in addition to the repository. At some current or prospective repository sites, various human activities that are amenable to monitoring might potentially (or actually) affect the natural system. Observing these anthropogenically induced changes in the natural system and being able to simulate them accurately using the models already constructed for repository performance assessment provide an additional opportunity for performance confirmation. Water-level changes induced by oil and gas exploration and/or potash mining activities near the Waste Isolation Pilot Plant site provided an opportunity to confirm important features of the groundwater model used in performance assessment calculations. Repository programs should evaluate the potential for ongoing or future human activities to affect the systems in which their repositories are situated and provide an opportunity for performance confirmation.

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The WIPP Case Study describes the compliance monitoring program, record keeping requirements, and passive institutional controls that are used to help ensure the Waste Isolation Pilot Plant (WIPP) will safety contain radioactive waste and indicate dangers and location of the wastes. The radioactive components in the waste are regulated by the U.S. Environmental Protection Agency (EPA) while the hazardous components in the waste are regulated by the New Mexico Environment Department (NMED). This paper addresses monitoring relating to radionuclide containment performance, passive institutional controls, and record keeping over a 10,000-year time frame. Monitoring relating to the hazardous components and the associated regulator are not addressed in this paper. The WIPP containment performance is mandated by release limits set by regulation. Regulations also require the radioactive waste containment performance of the WIPP to be predicted by a ''Performance Assessment.'' The EPA did not base the acceptance of the WIPP solely on predicted containment but included additional assurance measures. One such assurance measure is monitoring, which may be defined as the on-going measurement of conditions in and around the repository. This case study describes the evolution of the WIPP monitoring program as the WIPP project progressed through the planning, site characterization, regulatory promulgation, and eventual operational stages that spanned a period of over 25 years. Included are discussions of the regulatory requirements for monitoring, selection of monitoring parameters, trigger values used to identify unexpected conditions, assessment of monitoring data against the trigger values, and plans for post-closure monitoring. The United EPA established the requirements for Passive Institutional Controls (PICs) for disposal sites. The requirements state the a disposal site must be designated by the most permanent markers, records, and other passive institutional controls practicable to indicate the dangers of the wastes and their location. The PIC Task Force assessed the effectiveness of PICs in deterring inadvertent human intrusion and developed a conceptual design for permanently marking the Waste Isolation Pilot Plant (WIPP), establishing records, and identifying other practicable controls to indicate the dangers of the wastes and their location. The marking system should provide information regarding the location, design, contents, and hazards associated with WIPP. This paper discuss these controls including markers, records, archives, and government ownership and land-use restrictions.

The Permian Salado Formation in the Delaware Basin of New Mexico is an extensively studied evaporite deposit because it is the host formation for the Waste Isolation Pilot Plant (WIPP), a repository for transuranic wastes. Geologic and hydrologic studies of the Salado conducted since the mid-1970s have led to the development of a conceptual model of the hydrogeology of the formation that involves far-field (i.e. beyond the disturbance created by the repository) permeability in anhydrite layers and at least some impure halite layers. Pure halite layers and some impure halite layers may not possess an interconnected pore network adequate to provide permeability. Pore pressures are probably very close to lithostatic pressure. In the near field around an excavation, dilation, creep, and shear have created and/or enhanced permeability and decreased pore pressure. Whether flow occurs in the far field under natural gradients or only after some threshold gradient is reached is unknown. If far-field flow does occur, mean pore velocities are probably on the order of a meter per hundreds of thousands to tens of millions of years. Most hydraulic-test responses in the Salado do not appear to reflect radial flow, but instead imply subradial (e.g. intermediate between linear and radial) flow dimensions. We believe these subradial dimensions reflect channeling of flow through fracture networks, or portions of fractures, that occupy a diminishing proportion of the radially available space, or through percolation networks that are not 'saturated' (percolation terminology meaning fully interconnected). This is probably related to the directional nature of the permeability created or enhanced by excavation effects. Inferred values of permeability cannot be separated from their associated flow dimensions. Therefore, numerical models of flow and transport should include heterogeneity that is structured to provide the same flow dimensions as are observed in hydraulic tests. Modeling of the Salado Formation around the WIPP repository should also include coupling between hydraulic properties and the evolving stress field because hydraulic properties change as the stress field changes. © 2002 Elsevier Science B.V. All rights reserved.

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This report provides (1) an overview of all tracer testing conducted in the Culebra Dolomite Member of the Rustler Formation at the Waste Isolation Pilot Plant (WPP) site, (2) a detailed description of the important information about the 1995-96 tracer tests and the current interpretations of the data, and (3) a summary of the knowledge gained to date through tracer testing in the Culebra. Tracer tests have been used to identify transport processes occurring within the Culebra and quantify relevant parameters for use in performance assessment of the WIPP. The data, especially those from the tests performed in 1995-96, provide valuable insight into transport processes within the Culebra. Interpretations of the tracer tests in combination with geologic information, hydraulic-test information, and laboratory studies have resulted in a greatly improved conceptual model of transport processes within the Culebra. At locations where the transmissivity of the Culebra is low (< 4 x 10{sup -6} m{sup 2}/s), we conceptualize the Culebra as a single-porosity medium in which advection occurs largely through the primary porosity of the dolomite matrix. At locations where the transmissivity of the Culebra is high (> 4 x 10{sup -6} m{sup 2}/s), we conceptualize the Culebra as a heterogeneous, layered, fractured medium in which advection occurs largely through fractures and solutes diffuse between fractures and matrix at multiple rates. The variations in diffusion rate can be attributed to both variations in fracture spacing (or the spacing of advective pathways) and matrix heterogeneity. Flow and transport appear to be concentrated in the lower Culebra. At all locations, diffusion is the dominant transport process in the portions of the matrix that tracer does not access by flow.

This report presents interpretations of hydraulic tests conducted in bedded evaporates of the Salado Formation from May 1992 through May 1995 at the Waste Isolation Pilot Plant (WIPP) site in southeastern New Mexico. The WIPP is a US Department of Energy research and development facility designed to demonstrate safe disposal of transuranic wastes from the nation's defense programs. The WIPP disposal horizon is located in the lower portion of the Permian Salado Formation. The hydraulic tests discussed in this report were performed in the WIPP underground facility by INTERA inc. (now Duke Engineering and Services, Inc.), Austin, Texas, following the Field Operations Plan and Addendum prepared by Saulnier (1988, 1991 ) under the technical direction of Sandia National Laboratories, Albuquerque, New Mexico.

Many lessons have been learned over the past 24 years as the Waste Isolation Pilot Plant (WIPP) project has progressed from initial site characterization to final licensing that may be of relevance to other nuclear-waste-disposal projects. These lessons pertain to the manner in which field and laboratory investigations are planned, how experiments are interpreted, how conceptual and numerical models are developed and simplified~ and how defensibility and credibility are achieved and maintained. These lessons include 1) Site characterization and performance assessment (PA) should evolve together through an iterative process, with neither activity completely dominating the other. 2) Defensibility and credibility require a much greater depth of understanding than can be represented in PA models. 3) Experimentalists should be directly involved in model and parameter abstraction and simplification for PA. 4) External expert review should be incorporated at all stages of a project~ not just after an experiment or modeling activity is completed. 5) Key individuals should be retained for the life of a project or a process must be established to transfer their working knowledge to new individuals. 6) An effective QA program needs to be stable and consistent for the duration of a project and rests on best scientific practices. All of these lessons relate to the key point that consideration must be given from the earliest planning stages to maximizing the defensibility and credibility of all work.

Most analytical solutions and computer codes for well-test analysis assume a radial flow geometry around a well even though actual flow geometries can be quite different particularly in fractured media. Accurate estimation of hydraulic parameters requires knowledge of the flow geometry. Flow dimensions, representing the combined effects of flow geometry and variations in hydraulic properties, em be interpreted from the late-time slope of the pressure derivative on a log-log plot. However, the interpreted flow dimensions could be caused by an infinite number of flow geometry and hydraulic property combinations. Identifying the correct flow geometry so that appropriate hydraulic properties can be calculated is a difficult process, requiring additional information from a variety of sources. Defining a "conservative" model for a system with nonradial flow dimensions is problematic at best. Errors are compounded when hydraulic properties interpreted by force-fitting radial model to tests in nonradial systems are used in flow and transport models that also fail to take proper account of flow geometry. Whatever the flow dimension of a system might be, proper test interpretation and careful model construction, calibration, and testing are required to provide accurate modeling of flow and transport in that system.