The Waste Isolation Pilot Plant (WIPP) is a deep underground mined facility for the disposal of transuranic waste resulting from the nation's defense program. Transuranic waste is defined as waste contaminated with transuranic radionuclides with half-life greater than 20 years and activity greater than 100 nanocuries per gram. The waste mainly consists of contaminated protective clothing, rags, old tools and equipment, pieces of dismantled buildings, chemical residues, and scrap materials. The total activity of the waste expected to be disposed at the WIPP is estimated to be approximately 7 million curies, including 12,900 kilograms of plutonium distributed throughout the waste in very dilute form. The WIPP is located near the community of Carlsbad, in southeastern New Mexico. The geological setting is a 600-meter thick, 250 million-year-old saltbed, the Salado Formation, lying 660 meters below the surface.
The National Research Council (NRC) has been providing the U.S. Department of Energy (DOE) scientific and technical evaluations of the WIPP since 1978. This is the first full NRC report issued following the certification of the facility by the U.S. Environmental Protection Agency (EPA) on May 18, 1998. An interim report was issued by the committee in April 2000 and is reproduced in this report as Appendix A1. The main findings and recommendations from the interim report have been incorporated into the body of this report.
The committee's task is twofold: (1) to identify technical issues that can be addressed to enhance confidence in the safe and long-term performance of the repository and (2) to identify opportunities for improving the National Transuranic (TRU) Program for waste management, especially with regard to the safety of workers and the public. The complete statement of task is reported in Sidebar P.1 of the Preface.
The overarching finding and recommendation of this report is that the activity that would best enhance confidence in the safe and long-term performance of the repository is to monitor critical performance parameters during the long pre-closure phase of repository operations (35 to possibly 100 years). Indeed, in the first 50 to 100 years the rates of important processes such as salt creep, brine inflow (if any), and microbial activity are predicted to be the highest and will be less significant later. The committee recommends that the results of the on-site monitoring program be used to improve the performance assessment for recertification purposes. These results will determine whether the need for a new performance assessment is warranted. For the National TRU Program, the committee finds that the DOE is
implementing many of the recommendations of its interim report. It is important that the DOE continue its efforts to improve the packaging, characterization, and transportation of the transuranic waste.
The committee's specific findings and recommendations have been grouped into three categories: (1) site performance, (2) site characterization, and (3) the National TRU Program.
Every five years, the WIPP must obtain recertification from the EPA by showing that the repository is performing as predicted. Site performance refers to activities, phenomena, or events that occur as a result of repository construction and waste emplacement in the time frame between placement of the waste and final sealing1 of the repository shaft. Site performance has been evaluated by the DOE in its Compliance Certification Application (CCA) (DOE, 1996). The CCA relies on a model, called a “performance assessment,” that calculates the probability and consequence of several scenarios by which radionuclides could be released into the environment. The performance assessment also identifies the major uncertainties and their impact on the overall performance of the system. To reduce some of the uncertainties in the performance assessment and to add confidence in the containment performance of the repository, the committee recommends taking advantage of the long (35 to possibly 100 years) preclosure operating period to monitor selected performance indicators, including those listed below:
1. Brine migration is a key issue because it provides the most realistic mechanism for mobilizing and transporting radionuclides from the waste. The mixing of brine and waste could also result in the generation of gas in the repository. The committee recommends pre-closure monitoring to gain information on brine migration and moisture access to the repository. Observation should continue at least until the repository shafts are sealed and longer if possible. The committee recommends that the results of the on-site monitoring program be used to improve the performance assessment for recertification purposes.
2. Gas pressure generation is an important issue in the assessment of human intrusion scenarios. In the committee's opinion, there are uncertainties in some of the assumptions about gas generation used in the performance assessment of the CCA. The committee recommends pre-closure monitoring of gas generation rates, as well as of the volume of hydrogen, carbon dioxide, and methane produced. Such monitoring could enhance confidence in the performance of the repository, especially if no gas generation is observed. Observation should continue at least until the repository shafts are sealed and longer if possible. The results of the gas generation monitoring program should be used to improve the performance assessment for recertification purposes.
3. Magnesium oxide (MgO) is used as backfill in WIPP to provide some control of the chemical environment of the waste and, to a lesser extent, to fill voids in the disposal locations, thus enhancing the healing process. The chemical performance of MgO depends on gas generation and brine inflow as well as other chemical processes taking place in the repository. The committee finds that there is uncertainty about the effectiveness of MgO in controlling the chemical environment of the waste. Therefore, the
1The terms “sealing” and “healing” are both used in this report in relation to the repository. Repository sealing refers to the emplacement of engineered barriers preventing access or leakage to and from the repository. Repository healing indicates a natural process by which the mined salt creeps in around the waste to fill all the void spaces in the repository. See also “salt creep” and “engineered barriers” in the Glossary.
committee recommends that the net benefit of MgO used as backfill be reevaluated. The option to discontinue emplacement of MgO should be considered.
4. Deformation of rock salt and interaction of salt with TRU waste containers are of interest as a part of the pre-closure performance confirmation. The creep of salt is expected to entomb the waste drums in 100 to 150 years; thus, the radionuclide mobility values used in the performance assessment might have been overestimated. This implies less migration of radionuclides from the repository into the environment. The committee recommends pre-closure monitoring of the status of room deformation and of the disturbed rock zone2 (DRZ) healing. Seal performance should also be assessed. Observation should continue at least until the repository shafts are sealed and longer if possible. The results of the monitoring of room deformation and DRZ healing should be included in the PA and used for recertification purposes.
The WIPP program has engaged in a comprehensive program of site characterization that, in general, has been adequate to support certification of the facility. The committee identified four areas in which additional site characterization or monitoring is recommended. The four site characterization programs are described below:
1. A program for the hydrologic characterization of the Culebra, the most transmissive unit in the Rustler Formation. The Culebra could provide a pathway for the release of radionuclides into the environment in the event of human intrusion. The committee recommends a monitoring program to characterize the geohydrology of the Culebra Dolomite. Tests and measurements that should be considered include angled boreholes, natural gradient tracer tests, and additional pump or injection tests. These new data should be used to confirm, or modify, the conceptual and numerical models now proposed as reasonable simulation of the actual system.
2. A program for the detection of deep brine reservoirs below the waste disposal horizon. To improve site characterization and increase confidence in repository performance in view of the recertification application, the committee recommends the use of seismic survey techniques for detecting large brine reservoirs below the repository.3 In case a brine reservoir were found beneath the WIPP and its size were larger than what is already taken into account in the PA, then the DOE should conduct an extensive review of the impact of such reservoir on the repository performance. A basis would then exist to take appropriate action to ensure the safety of the repository.
3. A program for monitoring oil, gas, and mineral production in the area. Oil, gas, and mineral extraction activities in the vicinity of the repository could threaten its integrity. The committee recommends the development of a database to collect information on drilling, production enhancement,
2The disturbed rock zone is the zone around an excavation, in the host rock salt, where the stress field has been modified sufficiently to cause the formation of microfractures in the rock salt.
3The committee recognizes that small brine reservoirs, including brine occurring as a saturated continuum, could not be detected by seismic surveys, or other noninvasive remote sensing techniques.
mining operations, well abandonments, and unusual events (accidents and natural events) in the vicinity of the WIPP site.
4. A program for establishing the baseline for naturally occurring radioactive material (NORM) in subsurface brines and hydrocarbons in the vicinity of the site. The NRC interim report recommended that the DOE develop and implement a plan to sample oil-field brines, petroleum, and solids associated with current hydrocarbon production to identify the background concentrations of naturally occurring radioactive material in the vicinity of the WIPP site, for baselining purposes. In response to this recommendation, the DOE has started to collect data and is developing a database on NORM. The committee recommends that the DOE continue the implementation of its plan to sample oil-field brines, petroleum, and solids associated with current and future hydrocarbon production, as necessary to assess the magnitude and variability of NORM in the vicinity of the WIPP site for baselining purposes.
THE NATIONAL TRU PROGRAM
The National TRU Program, administered by the DOE Carlsbad Field Office, is a program to accommodate all applicable external regulations and internal requirements that are associated with the characterization, certification, packaging, and transportation of TRU waste to the WIPP facility. The committee addressed two main issues pertaining to the National TRU Program: (1) waste characterization and packaging and (2) waste transportation.
Waste Characterization and Packaging
The committee reviewed some of the waste characterization and packaging requirements established by the National TRU Program from a safety and cost point of view. This issue was detailed in the committee's interim report. A new issue concerning the total inventory of organic material allowed in the repository surfaced after the committee visited the WIPP site.
1. Waste characterization and packaging requirements. A principal finding of the interim report (Appendix A1) was that many requirements and specifications having to do with waste characterization and packaging lacked a safety or legal basis. In fact, the committee concluded that some of the requirements penalized the program by adding unnecessary costs and safety risks. Examples of self-imposed waste characterization requirements are waste repackaging to dilute the hydrogen-producing components and visual examination to verify the content of waste drums and avoid miscertifications. Therefore, the committee recommended in the interim report that the DOE should eliminate self-imposed waste characterization requirements that lack a safety or legal basis. The DOE has responded to this recommendation by initiating a review of all waste characterization and packaging requirements (Appendix A2). The committee recommends that the DOE's efforts to review waste characterization and packaging requirements continue and that changes be implemented over the entire National TRU Program. The committee recommends that the resources required to complete these improvements be made available by the DOE.
2. Total inventory of organic materials allowed in the repository. Buried with the waste is a considerable inventory of organic materials, such as plastic film used to stabilize the drums, plastic bags and corrugated cardboard, wooden waste boxes, plastic liners of waste drums, and pressed wood “slip sheets.”
The principal concern of the committee is that the DOE does not appear to keep an accurate inventory of such organic material. The committee recommends a risk-based analysis of the total organic material regulatory limits in WIPP. If accounting for the organic material is important to the safety of the repository, an inventory record system should be implemented as soon as possible to provide a basis for meaningful safety analysis.
The committee has examined various aspects of the WIPP TRU waste transportation system, focusing on system safety and the cost-effectiveness of planned and ongoing activities. In its interim report (Appendix A1), the committee reviewed DOE's TRANSportation Tracking and COMmunication (TRANSCOM) system and its emergency response program. In addition to the DOE's communication and notification program and its emergency response training, two other issues have been revisited in this report: the potential use of rail as a transportation option for certain TRU waste, and gas generation safety analysis for Transuranic Package Transporter, Model II (TRUPACT-II) containers.
1. DOE's communication and notification program. The committee's interim report ( Appendix A1) reviewed the transportation system for WIPP waste and particularly addressed the issue of the DOE's communication and notification system TRANSCOM and its emergency response program. The committee raised questions about the reliability and ease of use of the TRANSCOM system. Meanwhile, the DOE appears to be moving systematically toward the implementation of an efficient, comprehensive, and state-of-the-art communication and notification system, called TRANSCOM 2000. The committee recommends that the DOE implement as soon as possible the new TRANCOM 2000 communication and notification system. Moreover, because the human factor is an important element of transportation system quality, TRANSCOM 2000 should include methods to minimize the occurrence and impact of human errors.
2. DOE's emergency response training. Although the committee is aware of the fact that the DOE is not directly responsible for the emergency response program, DOE should nevertheless identify the resources (e.g., responders, medical facilities, recovery equipment, response teams) that might be necessary to respond to a transportation incident. The committee recommends that the DOE facilitate the involvement of states in developing and maintaining an up-to-date, practical, and cost-effective spatial information database system to coordinate emergency responses. The DOE should also develop an ongoing assessment program for states' emergency response capabilities and allocate training resources to address deficiencies in coverage along WIPP routes.
3. Rail as a transportation option for certain TRU waste. Among the generator sites, some have rail-loadings and tracking capabilities that could be used for railway shipping of TRU waste to WIPP. The objective of the following recommendation is to minimize the number of road shipments, and therefore the related risk, and to optimize the waste load for shipments of inner waste packages that are unsuitable for placement in TRUPACT-II overpacks. The committee recommends that all reasonable transportation options including reduction in the number of shipments, such as rail and road transportation with better-adapted containers, should be part of the decision-making process of transporting TRU waste from generator and storage sites to the WIPP. Future transportation studies should consider railway shipments and their impact on both the safety and the cost of the program. The
DOE should also continue to pursue the development of packaging alternatives for materials not suitable for TRUPACT-II containers.
4. Gas generation safety analysis for TRUPACT-II containers. Hydrogen gas is generated in the shipping containers by radiolytic decomposition of the organic materials in waste during transportation of TRU waste to the WIPP. The root issue is the interpretation of the U.S. Nuclear Regulatory Commission's (USNRC's) regulations on shipments involving possible flammable gases. The questions of interpretation center around the allowed volume fractions of flammable gases and the definition of the confinement barrier. Depending on interpretation, the regulations can become a severe constraint on TRU waste shipments, with no apparent benefit. In particular, the committee was unable to verify the technical basis for some of the interpretations of the regulations as they relate to the safety of the workers and the public. The committee recommends a risk-informed analysis of WIPP specific shipment issues to identify core problems related to hydrogen generation and, perhaps, provide a basis for alternative cost-effective criteria while reducing the risk. The committee recommends the use of such risk-informed analysis in the application for revision of the USNRC certificate of compliance concerning hydrogen generation limits for transportation purposes.