Mark Matthews
U. S. Department of Energy, Carlsbad Area Office
P.O. Box 3090
Carlsbad, New Mexico, 88221, USA
505-234-7467
matthem@wipp.carlsbad.nm.us

Anita S. Reiser
Sandia National Laboratories
P.O. Box 5800
Albuquerque, New Mexico, 87185,
505-848-0131,
asreise@sandia.gov

 The WIPP is a transuranic waste disposal project of the U.S. Department of Energy and administered by the Carlsbad Area Office (CAO). In support of the Compliance Certification Application (CCA), CAO undertook a major experimental program. There were four major experimental program areas: shaft seals and rock mechanics; disposal room interactions; fluid flow and transport; and direct releases due to human intrusion. The results were used to develop conceptual models of the repository and to predict its behavior over the 10,000-year regulatory period. Ongoing experimental activities are addressing issues identified by EPA during its review of the CCA. EPA issued a draft rulemaking approving the CCA on October 30, 1997.

CAO's rock mechanics and seal system effort has resulted in the current seal system design, which includes several materials (compacted salt, salt-saturated concrete, clay, and asphalt). These materials are incorporated into multiple independent seal components, and the redundant components are designed to provide additional assurance of seal performance. Planned experimental activities are focused on refining the design and enhancing the model.

 The area of disposal room interactions is complex since it includes creep closure of the room, consolidation of the waste, and chemical interactions with brine that may flow into the room from the surrounding formation. These processes are all closely coupled and can significantly influence one another. In order to provide additional performance assurance, the DOE will use magnesium oxide backfill in the disposal rooms to modify the chemical conditions in the repository and control the actinide source term. Planned experimental activities are focused on refining the models of coupled processes and magnesium oxide behavior.

In the area of fluid flow and transport, CAO determined that in order to characterize the processes that determine repository performance, it was necessary to understand the presence and flow of brines in Salado, in the overlying Culebra, and in the Castile. A series of measurements around the repository has provided data on the hydrologic properties of the underlying strata. In addition, extensive tests have been conducted to establish retardation factors for the important actinides. Reasonable yet conservative chemical retardation values were established and used in the compliance calculations to account for the uncertainty in these processes.

 Finally, in the area of system response to human-initiated activities, direct releases can contribute significantly to the total releases of radionuclides. The DOE will develop a new mechanistic model of the spallings process, in which materials are carried up the intrusion borehole to the surface. CAO is committed to preventing adverse activities at the WIPP site for as long as it has jurisdiction at the site. To mitigate the potential of human intrusion into the repository, the DOE will incorporate passive institutional controls to reduce the probability of inadvertent human intrusion.

 The Pre-Disposal Phase Experimental Program has provided the necessary information to support the WIPP performance assessment process. The results of the program have been instrumental in demonstrating that the WIPP meets the regulatory requirements. Planned activities will provide additional relevant information that will be used to enhance design, reduce costs, and show continued compliance with regulatory requirements.

 The U. S. Department of Energy (DOE) is responsible for the disposition of transuranic (TRU) waste generated by the production of nuclear weapons and other defense-related activities that began in the mid-1940s. TRU waste is defined as radioactive waste containing more than 3,700 becquerels of alpha-emitting transuranic isotopes per gram of waste, with half-lives greater than 20 years. Some of these radioactive wastes are mixed with hazardous wastes and are also subject to the regulations that apply to the management and disposal of hazardous wastes.

In 1970, the Atomic Energy Commission, the predecessor of the DOE, determined that TRU wastes require more stringent management and more secure disposal facilities than low-level radioactive waste. Since 1970, DOE has temporarily stored this waste in a manner that will facilitate retrieval and emplacement in an appropriate disposal facility. Approximately 74,500 cubic meters of these wastes have been generated and are stored at government installations across the country. It is currently projected that an additional 54,400 cubic meters of these wastes will be generated, although this projection may change as DOE's decontamination and decommissioning programs, environmental restoration programs, and waste treatment programs proceed. Before the DOE can open, operate, and decommission a TRU waste geologic repository, the facility must be certified by the U. S. Environmental Protection Agency (EPA).

 The Waste Isolation Pilot Plant (WIPP), located 42 kilometers (km) east of Carlsbad, New Mexico (Figure 1), was constructed for disposal of TRU waste and TRU mixed waste. The WIPP site, comprising 41.2 square kilometers, is situated in the northern part of the Delaware Basin. With the exception of a few thin Quaternary surface deposits, most geologic units of interest at the WIPP site are of Permian age. The underground disposal facility lies approximately 650 meters below the surface in the lower portion of the 600-meter-thick Salado Formation comprised of bedded salt. The underground facility consists of an experimental area at the north end, a waste-disposal area at the south end and four vertical shafts located in an intermediate operations area (Figure 2). The waste-disposal area will consist of 10 waste-disposal-panel equivalents, including panels one through eight, designed with seven disposal rooms to each panel, and two equivalent panels consisting of central access drifts and cross-cuts south of the four shafts where waste will also be emplaced. At present, only a single waste panel has been constructed.

 Experimental activities during the pre-disposal phase of WIPP (1990 to 1997) focused on those processes that were considered most important to repository performance: rock mechanics; seal system materials, characteristics, and component performance; source term chemistry; and flow and transport characteristics of the geological units overlying the Salado Formation. Additional activities were implemented based on interactions with scientific oversight groups and other interested parties. These activities focused on Salado Formation hydrology, the disturbed rock zone (DRZ), and gas generation. The specific areas of study were also modified as our understanding of the natural and engineered system improved. With the development of a comprehensive total system performance assessment that included the key site processes, experimental objectives have changed to place a greater emphasis on developing parameter value distributions for use in the performance assessment calculations while continuing to refine the conceptual models. In particular, evaluation of the importance of coupled processes, and the need to include specific coupled processes in the performance assessment calculations, have been (and still are) a continuing objective of the experimental program.

In October 1996, the DOE submitted a Compliance Certification Application (CCA) (1996) to EPA. After requesting additional information from DOE, EPA determined in May 1997 that the application was complete, and continued with an extensive review of the CCA and supporting information. The review included an EPA-mandated performance assessment of the WIPP using EPA's values for key parameters (those parameters identified through sensitivity analyses to have strong effects on system performance). In October 1997, the EPA issued a draft decision to certify the WIPP for operation with four conditions:

• EPA requires use of salt-saturated concrete (the same type of concrete specified for the shaft seals) in panel closures  

Fig. 1. Location of the WIPP facility (42 kilometers east of Carlsbad, NM) 

• DOE generator sites may not ship waste to WIPP until EPA evaluates and approves the generators' QA programs for waste characterization activities and assumptions. 
• DOE generator sites may not use process knowledge to characterize waste until EPA determines the generators' system of controls is adequate to characterize the waste 
• A schedule for implementing passive institutional controls after closure is required, together with documentation that the control system can be constructed. 

DOE anticipates receiving waste in 1998 following completion of the rulemaking process (a public comment period followed by EPA review and final decision).

During the disposal phase, scheduled to begin by June 1998 and expected to last 35 years, the DOE Carlsbad Area Office (CAO) will receive, handle, and emplace TRU waste in the repository. Experimental program activities are planned during the disposal phase to enhance WIPP operations, to reduce uncertainties associated with key parameters, and to build confidence in the long-term performance of the repository system. Information gathered during this phase will also be incorporated into the re-certification of WIPP that is mandated by the regulations every five years after disposal operations begin.

 The disposal phase experimental program will focus on maintaining compliance with all applicable regulations and improving operation of the WIPP and the national TRU system. The improvements will be achieved by focusing efforts to develop new methods and procedures for reducing worker exposure to radiation and other hazards, increasing operational efficiency, reducing system complexity, and reducing costs. The planned experimental efforts fall into a number of technical areas, which are discussed below. Results from these experiments will be used to update the technical baseline incorporated into the performance assessment calculations as part of the re-certification process.

Work during the disposal phase will focus on refining and optimizing the seal system design and requisite construction-related technologies, and on verifying those modeling parameters shown to be important to seal system performance. The seal system design included in the WIPP CCA can be constructed with existing technology. Uncertainties in material properties, or in construction methodology, have been accounted for in this design by including multiple components (crushed salt, salt-saturated concrete, clay, grout, asphalt, and earthen fill). Each component is designed to contribute to the effectiveness of the shaft sealing system over the 10,000-year regulatory period, and the overall design includes appropriate redundancy to accommodate material and construction uncertainties and modeling limitations. Activities during the disposal phase are designed to reduce these uncertainties.

 Since the requirements for the shaft seal system are to function at full-scale, and the seals are to be installed using practical field construction methods and controls, much of the disposal-phase work will be aimed at obtaining and evaluating data under field conditions. Additional work will be aimed at improving the modeled representation of the system to improve accuracy. Specific activities to be performed include:

• Obtaining additional data on the properties of crushed salt and its behavior during reconsolidation in the seals. The data will be used to improve modeling capabilities, to optimize the specifications for crushed salt in the seals to enhance long-term performance, and ultimately to reduce redundancy in seal components where appropriate.
• Monitoring the development and healing of the DRZ around the WIPP shafts through an on-going program at the WIPP to measure the extent and nature of the DRZ surrounding the air intake shaft (Figure 2). The DRZ is an important element in the performance of the seal systems, as it provides the only potential continuous pathway for fluid flow adjacent to seal components.
• Obtaining additional data and further evaluating the properties of dynamically compacted clay.
• Evaluating the emplacement methodology for asphaltic seals. The asphalt component of the current seal system is designed to function as a waterstop, isolating the crushed salt component from water which may be present in the DRZ. Construction requires special tools and construction techniques that will be used in the confined space of a shaft. Further information on the characteristics and the extent of the DRZ may show that it is not continuous or that flow through it is minimal, possible eliminating the need for the asphalt.
• Continuing to develop the numerical models used to represent the seal system in performance assessment calculations in conjunction with the experimental activities. In particular, the DOE will address modeling salt reconsolidation coupled with fluid flow to improve the modeling, identify opportunities to enhance computational efficiency, and identify opportunities to simplify the seal system design. The data obtained during the experimental activities will be used in the models, which will be used in turn to identify additional opportunities for simplification and improvement prior to recertification.

The current understanding of the chemical and physical processes which govern conditions in the disposal room, the remaining repository, and the adjacent areas of the Salado Formation was adequate to allow the CAO to develop conceptual and numerical models and calculate long-term disposal system performance in the CCA. The models and calculations incorporate the uncertainties in the processes expected to occur in the disposal room (e.g., brine inflow, creep closure, waste heterogeneity, actinide solubility, colloids, microbial degradation, and gas generation). Magnesium oxide backfill has been incorporated into repository design to provide additional assurance. The current design for the backfill includes conservatism to account for the uncertainties listed above, particularly uncertainties in actinide solubility and in the quantity of gas generated through microbial degradation.

 Additional experimental activities are being planned in those areas most likely to improve operational efficiency or to reduce costs. These areas include the actinide source term (actinide solubilities); gas generation (quantity and composition of gas generated); and backfill (chemical and physical properties and behavior). These are discussed in the following sub-sections.

 Actinide Source Term: The actinide source term defines the actinides that are available for transport to the accessible environment. The model is being evaluated and enhanced through several experimental programs:

• An on-going source term test program (STTP) in which samples of TRU waste are inundated in WIPP brine under controlled conditions. Sampling of the brine mixed with TRU waste in the STTP test containers will be continued until the concentrations of key components in the brine approach equilibrium.
• Measurements of solubility and stability of hexavalent uranium compounds and metal cations to obtain data for enhancing models of thermodynamic behavior.
• Measurements of actinide solubility and chemical behavior in the presence of humic materials.

The results will be used to further enhance modeling capabilities, leading to more accurate results and a more realistic prediction of repository behavior. Refining the modeling capabilities is expected to ultimately lead to significant computational efficiencies at recertification. Refining models is also expected to contribute to optimizing the magnesium oxide backfill design.

 Gas Generation: The model used to predict gas generation under repository conditions is currently being evaluated through on-going gas generation experiments (GGE). The GGE are designed to assess the contributions of the major waste constituents to gas generation. These include: organic materials (cellulosics, plastics, rubbers, and leaded rubber); iron-base metal; and sludges (particularly sludges that contain potential nutrients for microbial degradation). In the GGE, the waste has been inundated with simulated WIPP brine in test containers that are initially pressurized with nitrogen to simulate lithostatic pressure at the WIPP repository level. The container pressure, temperature, and gas composition are being periodically measured. The GGE data, when combined with other disposal phase experimental results, may be used to reduce the volume of magnesium oxide backfill, resulting in significant cost savings.

Magnesium Oxide Backfill: The CAO will continue laboratory tests to obtain information to further define the chemical and physical properties of magnesium oxide under repository conditions. The experiments will:

• Provide data on the extent of water uptake and resulting volume expansion and reductions in porosity and permeability
• Provide data on the physical effect of hydration and carbonation of the magnesium oxide and the resulting effects on cementation of waste particles
• Provide additional data on carbon dioxide uptake at conditions closely resembling those expected at WIPP.

 The data will be used to enhance the performance assessment models of repository behavior, particularly with respect to coupled processes (gas generation, actinide solubility, brine inflow, and room closure).

 The magnesium oxide backfill will be contained in bags that are placed over and between the drums and waste containers in each room. These bags are intended to minimize contact between the backfill and mine air until decommissioning is complete and the disposal rooms have closed. The CAO is also considering room-scale studies designed to optimize procedures for placing the bags of magnesium oxide over and around the waste containers in the disposal rooms. The potential benefits of the magnesium oxide tests and analyses include:

• Reducing the amount of magnesium oxide required to achieve long-term chemical control in the repository, thereby reducing backfill material costs and operating costs associated with backfill placement;
• Enhancing the ease and efficiency of the backfill placement process, which will reduce operating costs associated with placement, and may also reduce material handling requirements and thereby reduce the potential for worker exposure to powdered backfill, and;
• Refining the backfill packaging specifications to enhance package performance and reduce cost. This will also reduce potential worker exposure through reduced handling, enhance chemical delivery and performance (by providing assurance that the magnesium oxide will be available soon after repository closure), and reduce packaging materials and their associated costs.

In conjunction with these efforts, the CAO is also considering development of robotic systems to improve processes for waste emplacement and backfilling and to further reduce the potential for worker exposure.

 Waste Characterization: Waste characterization will be a major contributor to cost throughout the national TRU program, and significant cost savings may be realized if the characterization requirements can be reduced. In conjunction with the WIPP CCA, the CAO assessed waste components and characteristics to identify those that are significant to disposal system performance. These waste characteristics and components are currently incorporated into the WIPP CCA performance assessment models with some conservatism to account for uncertainty and heterogeneity in the wastes. The CAO may perform additional evaluations and laboratory tests to assess the degree of uncertainty and thereby provide greater confidence in the manner in which waste characteristics should be applied. Also, characterization techniques, equipment, and instrumentation may be evaluated to develop faster and less expensive processes and equipment.

Because of the uncertainty inherent in consideration of future events, EPA requires consideration of the consequences of direct intrusion into the repository, and also requires controls as an assurance measure to reduce the chance of inadvertent human intrusion.

 During inadvertent intrusion by drilling directly into the repository, there are three pathways for direct releases of repository materials: cuttings (solid material cut by the drill bit and removed to the surface by the drilling fluid), cavings (solid material eroded from the sides of the borehole and carried to the surface by drilling fluid), and spallings (particulate material carried up the borehole to the surface by pressurized gas). The type of release and the amount of material released are strongly dependent on repository conditions and waste properties at the time of intrusion. These direct releases contribute significantly to the overall releases during the 10,000-year regulatory period.

 Passive institutional controls (PICs) are required under the current regulations as an assurance measure of reducing human intrusion into the repository during the 10,000-year regulatory time frame. The current design for the PICs system involves multiple types and levels of passive controls to make human intrusion into the WIPP repository site unlikely.

 Both programs were evaluated as part of EPA's evaluation of the CCA. DOE's additional activities in both areas will focus on addressing issues identified during the review.

 Direct release (spallings): The conceptual model for spallings included in the CCA was found inadequate by the Conceptual Model Peer Review Panel on April 24, 1997, although the release volumes predicted by the CCA model were considered reasonable when compared to those releases predicted using a new, mechanistically-based conceptual model. DOE will focus experimental efforts on:

• Obtaining laboratory test data relevant to physical and chemical characteristics of waste in the closed repository, with emphasis on compaction, cementation of waste particles, and encapsulation in the Salado salt. In particular, waste surrogates are undergoing tests to obtain data on critical shear stress, a parameter that captures the effects of cavings.
• Continuing development of the new spallings model, enhancing its capabilities to address the coupled processes (brine inflow, room closure, evolution of the DRZ, gas generation) which are important in predicting the repository conditions at the time of intrusion
• Integrating the new spallings model into the suite of models used in performance assessment for re-certification.

DOE believes the new model will provide a more realistic estimate of releases due to intrusion. Since direct releases through spallings, cuttings, and cavings are the most significant pathway for releases to the environment, enhancements in these models will have significant effects on compliance.

 Passive institutional controls: Such controls, once established, are expected to remain effective with no human surveillance and maintenance; however, in the performance assessment calculations, the CAO only takes credit for 700 years of performance. Components of the passive institutional controls system include: (1) physical markers that warn of the presence of buried nuclear waste and identify the boundary of the disposal area footprint and the controlled area; (2) external records about the WIPP repository; and (3) continued federal land ownership. DOE will develop additional documentation for the system as required by EPA in the proposed rulemaking. 

The DOE has conducted site characterization and laboratory tests related to the safe disposal of long-lived radioactive waste at WIPP since the mid-1970s. As a direct result, it has developed an underground facility that is anticipated to begin receiving waste in 1998. A disposal phase experimental program is being planned to enhance operations and reduce costs associated with waste disposal while continuing to increase confidence in the long-term performance of the TRU repository. These programs are intended to enhance confidence in predicting long-term repository performance and in extending the state-of-the art capabilities for characterizing natural and engineered systems. These activities will have an immediate and direct impact on the WIPP by showing continued compliance with regulatory requirements, contributing to cost effective operational improvements, and reducing uncertainty associated with key parameters that influence repository performance.

United States Department of Energy, 1996, Title 40 CFR 191 "Compliance Certification Application for the Waste Isolation Pilot Plant", DOE/CAO 1996-2184, Carlsbad Area Office, Carlsbad, New Mexico.

 United States Department of Energy, 1997, "Disposal Phase Experimental Program Plan", DOE/CAO-97-1223, Carlsbad Area Office, Carlsbad, New Mexico.