The Waste Isolation Pilot Plant (WIPP) is an underground facility in bedded salt approximately 658 m (2,160 ft.) below the surface in a semi-arid region near Carlsbad, New Mexico, in the southeastern corner of the state (see Figure 1.1). A U.S. Department of Energy (DOE) facility, WIPP has been studied intensively to determine its suitability as a permanent repository for disposal of the category of intermediate-level radioactive waste known as defense-related transuranic (TRU) waste.
WIPP is a pioneering effort in the assessment of geological site suitability and design procedures for a waste repository. It is the first geological repository in the nation for which an application to begin permanent geological isolation is being submitted for a regulatory decision. If approved, WIPP will be the first "deep" designed repository in the world. (The low and intermediate-level radioactive waste repository at Olkiluoto, Finland, is located at a shallower depth of 125 m; the German repository near Morsleben is an abandoned salt mine, not designed initially as a repository.)
This report discusses the key technical issues that influence the suitability of WIPP for isolation of TRU waste. This chapter describes the radioactive wastes under consideration for storage at WIPP, the design concept of long-term storage in geological formations, and a brief description of the history of work at the WIPP site. Following chapters address specific technical issues that arise in considering the various pathways or scenarios by which radionuclides could move from the repository and be released to the accessible environment.
Transuranic waste results chiefly from the production of nuclear weapons from plutonium and enriched uranium. The term transuranic indicates that the waste contains radionuclides with atomic numbers greater than 92, that is, greater than that of uranium (see Box 1.1). Existing amounts of the principal isotopes of these and other elements and estimates of their projected total quantity in DOE facilities are listed in Table 1.1.
TRU waste consists of a wide variety of contaminated materials from laboratory and production operations, including discarded protective clothing, laboratory test equipment and reagents, machine components, and solidified sludge. This waste has accumulated over the past 50 years as a result of weapons development and production at U.S. defense facilities. Future TRU waste generation is expected to come from cleanup of the contaminated sites and decommissioned facilities of the U.S. weapons complex. Packed in 55-gallon steel drums and wooden boxes, TRU waste currently is being stored at various sites across the nation.
Although not as hazardous as high-level waste (see Table 1.1), TRU waste contains long-lived radionuclides that, if released to the biosphere, pose a risk to humans and the environment for many thousands of years into the future. Of all the actinide isotopes, plutonium-239 (Pu-239) is the one of greatest concerns substantially beyond about 500 years because of its high concentration in the materials to be stored at WIPP (Table 1.1) and its long half-life. Because of its long-lived toxicity, TRU waste, like high-level waste, must be isolated from the biosphere for times greater than 10,000 years. Disposal in deep, stable geological formations, so that very little of its radioactive content will reach the accessible environment by any natural means, has been proposed. The WIPP facility has been designed for the disposal of defense-related TRU waste to meet these conditions.
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--> Chapter 1 Introduction The Waste Isolation Pilot Plant (WIPP) is an underground facility in bedded salt approximately 658 m (2,160 ft.) below the surface in a semi-arid region near Carlsbad, New Mexico, in the southeastern corner of the state (see Figure 1.1). A U.S. Department of Energy (DOE) facility, WIPP has been studied intensively to determine its suitability as a permanent repository for disposal of the category of intermediate-level radioactive waste known as defense-related transuranic (TRU) waste. WIPP is a pioneering effort in the assessment of geological site suitability and design procedures for a waste repository. It is the first geological repository in the nation for which an application to begin permanent geological isolation is being submitted for a regulatory decision. If approved, WIPP will be the first "deep" designed repository in the world. (The low and intermediate-level radioactive waste repository at Olkiluoto, Finland, is located at a shallower depth of 125 m; the German repository near Morsleben is an abandoned salt mine, not designed initially as a repository.) This report discusses the key technical issues that influence the suitability of WIPP for isolation of TRU waste. This chapter describes the radioactive wastes under consideration for storage at WIPP, the design concept of long-term storage in geological formations, and a brief description of the history of work at the WIPP site. Following chapters address specific technical issues that arise in considering the various pathways or scenarios by which radionuclides could move from the repository and be released to the accessible environment. Transuranic Waste: What It Is, Where It Comes From, Where It Must Go Transuranic waste results chiefly from the production of nuclear weapons from plutonium and enriched uranium. The term transuranic indicates that the waste contains radionuclides with atomic numbers greater than 92, that is, greater than that of uranium (see Box 1.1). Existing amounts of the principal isotopes of these and other elements and estimates of their projected total quantity in DOE facilities are listed in Table 1.1. TRU waste consists of a wide variety of contaminated materials from laboratory and production operations, including discarded protective clothing, laboratory test equipment and reagents, machine components, and solidified sludge. This waste has accumulated over the past 50 years as a result of weapons development and production at U.S. defense facilities. Future TRU waste generation is expected to come from cleanup of the contaminated sites and decommissioned facilities of the U.S. weapons complex. Packed in 55-gallon steel drums and wooden boxes, TRU waste currently is being stored at various sites across the nation. Although not as hazardous as high-level waste (see Table 1.1), TRU waste contains long-lived radionuclides that, if released to the biosphere, pose a risk to humans and the environment for many thousands of years into the future. Of all the actinide isotopes, plutonium-239 (Pu-239) is the one of greatest concerns substantially beyond about 500 years because of its high concentration in the materials to be stored at WIPP (Table 1.1) and its long half-life. Because of its long-lived toxicity, TRU waste, like high-level waste, must be isolated from the biosphere for times greater than 10,000 years. Disposal in deep, stable geological formations, so that very little of its radioactive content will reach the accessible environment by any natural means, has been proposed. The WIPP facility has been designed for the disposal of defense-related TRU waste to meet these conditions.
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--> BOX 1.1 What Is Transuranic Waste? The Department of Energy, which is responsible for the management and disposal of defense-related TRU waste, uses the following definition: TRU waste is waste that is not high-level waste and that is "contaminated with alpha1-emitting radionuclides of atomic number greater than 92 and half-lives greater than 20 years in concentrations greater than 100 nanocuries per gram" (DOE Order 5820.2A). The Land Withdrawal Act of 1992 and the EPA 40 CFR part 191 contain the legal, regulatory definitions that agree with this language. Further details on exclusions and inclusions to WIPP-bound waste are contained in these references. The most important transuranic elements for judging the suitability of WIPP as a TRU waste repository are plutonium (Pu), the major component; americium (Am), a moderate component; and neptunium (Np), a minor component because of its relatively lower inventory. Common usage of the term TRU waste often includes all elements in the actinide group with atomic number 90 (thorium) or higher. Thorium (Th) and uranium (U) isotopes are minor components of the WIPP inventory (Table 1.1), smaller components than americium. Other radioactive elements are present in the waste but in amounts so small that they have a less significant influence on the determination of the performance of the proposed repository. Some of the isotopes of the actinide elements are long-lived alpha emitters with half-lives ranging up to billions of years. For many isotopes, however, the concentrations are low. Plutonium-239 (Pu-239), with a half-life of about 24,000 years, is the isotope of greatest abundance beyond about 500 years. The radioactive constituents of TRU waste pose a radiological health hazard and are regulated by EPA in 40 CFR 191 and 40 CFR 194. Some TRU waste is mixed with chemically hazardous materials (e.g., certain toxic metals and organic compounds); the health consequences of exposure are derived from the chemical effect of these materials on the human body. EPA standards in 40 CFR 268 (also known as the Resource Conservation and Recovery Act of 1976, or RCRA) regulate the chemically hazardous constituents. An amendment (P.L. 104-201) to the WIPP Land Withdrawal Act (P.L. 102-579) exempts WIPP from federal RCRA requirements. 1 Radionuclides that emit positively charged (alpha) particles in the process of decaying to more stable nuclides. Alpha radiation is the least penetrating of the three common forms (beta and gamma radiation are the other two) and cannot penetrate human skin. However, alpha emitters can be harmful if ingested or inhaled, or if they enter the body through other means, for example, through contact with a cut in the skin. Geologic Disposal Of Radioactive Waste In Salt The objective of the U.S. nuclear waste disposal program is to place the waste in a location where harmful quantities cannot return to the biosphere by any foreseeable processes. The decision to develop a waste disposal facility in salt arose from an assessment by a committee of the National Research Council (NRC) appointed to study the problem of how to dispose of accumulating inventories of high-level radioactive waste. In its report, The Disposal of Radioactive Waste in Land, published in 1957, the committee recommended further work to assess the suitability of burying high-level radioactive wastes in stable geological formations at depths on the order of 500-1,000 m below the surface (NRC, 1957). Every other country that must deal with radioactive waste isolation, including Belgium, Canada, China, Finland, France, Germany, Japan, Russia, Sweden, Switzerland, Taiwan, and the United Kingdom, plans to use geological disposal to isolate radioactive waste. The 1957 committee was particularly attracted to the notion of burying the waste in rock salt, either in salt domes or in thick salt beds It cited the following advantages: Salt can be mined easily. It is known to flow slowly under the pressure of overlying beds, and so will consolidate around the waste and isolate it in place. It is essentially impermeable.
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--> TABLE 1.1 December 1995 Estimates of Inventory of Actinide Wastea CH-TRU RH-TRU Total anticipated activity (Ci), all isotopes 7,880,000 1,020,000 Design capacity of WIPP (m3) 168,500 7,080 Total anticipated volume of WIPP (m3) 110,000 27,000 Average activity WIPP TRU waste (Ci/m3)b 46.8 143 Average activity of anticipated volume (Ci/m3)c 71.6 37.8 Isotope Half-lifee (years) Total CH-TRU (Ci) Total RH-TRU (Ci) Th-232d 1.41 × 1010 9.11 × 10-1 9.24 × 10-2 U-233d 1.59 × 105 2 × 103 3.18 × 101 U-234d 2.45 × 105 5.53 × 102 3.93 × 101 U-235d 7.0 × 108 1.28 × 101 5.2 × 100 U-238d 4.47 × 109 3.96 × 101 1.44 × 100 Np-237 2.14 × 106 5.49 × 101 4.85 × 10-2 Pu-238 8.77 × 101 3.8 × 106 1.45 × 103 Pu-239 2.4 × 104 7.82 × 105 1.03 × 104 Pu-240 6.5 × 103 2.08 × 105 5.07 × 103 Pu-241d 1.44 × 101 2.61 × 106 1.42 × 105 Pu-242 3.7 × 105 1.17 × 103 1.5 × 10-1 Am-241 4.33 × 102 4.39 × 105 5.96 × 103 NOTE: CH-TRU = contact-handled transuranic waste; RH-TRU = remote-handled transuranic waste. The design capacity of RH-TRU is legally specified as 250,000 cubic feet (7,080 cubic meters). As the table shows, the anticipated volume specified in (DOE, 1995c) exceeds this quantity. This discrepancy could be resolved by a future change in the numerical estimates of anticipated volumes, or by a future renegotiation to augment the legally specified limit, or by waste processing to reduce the volume of the RH-TRU transported to WIPP to be less than the volume generated. Both DOE documents and EPA standards measure radioactivity in curies, where one curie, 3.7×1010 disintegrations per second, is the historical unit representing approximately the radioactivity of one gram of radium 226. The System International (SI) unit for radioactivity is the Becquerel, 1 disintegration per second. U. S. regulations express dose equivalent in roentgen-equivalent-man (rem) units, another historical unit differing from the SI unit, the Sievert (Sv), where one Sv is 100 rem. Because the U. S. work on WIPP uses these older units, they are used in this report. a Not shown are minor concentrations of many other isotopes. b The Ci/m3 values shown here derive from the total activity (Ci) and the design capacity of the WIPP repository (m3). By comparison, the average activity of U.S. spent fuel high level waste is 140 × 103 Ci/m3 (10-year-old waste) and 140 × 102 Ci/m3 (100-year-old waste). See Roddy et al. (1986). c The Ci/m3 values shown here indicate the ratio of total activity in curies (Ci) to the total anticipated volume (m3) of waste to be shipped to WIPP. d Because the half-life of Pu-241 is less than 20 years, it falls outside the EPA 40 CFR 191 definition of TRU waste, although it is a transuranic isotope. The thorium and uranium isotopes are likewise actinides shown here that are excluded from classification as TRU waste, since they are not transuranic isotopes. e The level of radioactivity decreases exponentially with time, with the half-life denoting the time at which the initial amount of any radionuclide is halved. After ten half lives, approximately 0.1 percent of the initial radioactivity remains. For long times, the WIPP inventory is dominated by Pu-239, ten half lives of which is a quarter of a million years. This time, at which approximately 0.1 percent of the initial amount of Pu-239 remains, is only roughly 0.1 percent of the present age of the geologic formation in which WIPP is excavated. Source: (DOE, 1995c, Tables ES-3, ES-4, and 3-4).
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--> Salt that has existed underground for millions of years will almost certainly remain stable for thousands of years into the future. These advantages, and others also, were noted in a subsequent study (NRC, 1970). History Of WIPP Selection of the present WIPP site came after rejection of a previous candidate site, an abandoned salt mine near Lyons, Kansas. Investigated from the mid-1960s to the early 1970s, the Kansas site was abandoned because of the large number of boreholes penetrating the formation and salt dissolution issues (Powers et al., 1978). Renewed efforts at a nationwide site selection and characterization process resulted in a focus of attention on the New Mexico region of the Delaware Basin (see Appendix A). Exploratory drilling began in 1974 for a possible deep geologic TRU-waste disposal site in salt beds in New Mexico. The first location selected for study was rejected as the permanent site principally because of irregular subsurface geology (Powers et al., 1978). Recommendation of the present site was made in 1975. In 1976, the name "Waste Isolation Pilot Plant" was given to the project, and ERDA-9, the first exploratory hole located at the present site, was drilled (Powers et al. 1978; NRC, 1984). In 1979, the WIPP site, about 40 km, (25 miles), east of Carlsbad, NM in the southeastern corner of the state (Figure 1.1), was authorized, after preliminary testing, as a "research and development facility for demonstrating safe disposal of radioactive waste from defense activities and programs" (U.S. Congress, 1980). Construction of WIPP, together with studies of its suitability as a TRU repository, proceeded through the 1980s. The WIPP Underground Facility Today The WIPP facility is located approximately 658 m below the surface in the layered salt of the Permian Salado Formation (see Figure ES.1). Four vertical shafts from the surface provide access to the repository horizon and underground excavations. Drifts (underground roadways) link the shafts to the waste disposal area, which is planned to consist of eight panels (the rectangular sections off the main haulage ways in Figure ES.1), each subdivided into seven rectangular rooms excavated between access drifts (corridors that provide passage to the individual disposal rooms, bounded by 30.5-m-wide pillars on each side). To date, only one of these panels has been excavated. Disposal room dimensions are 4 m high by 10 m wide by 91 m long. The drifts are essentially the same height but narrower than the rooms and are of varying lengths. Rooms have been excavated to follow the nonhalite marker beds that occur throughout the Salado, which dip at about 1° (from horizontal) to the south. Thus, the system of excavations is essentially horizontal. The Disposal Plan The disposal plan involves stacking TRU waste-filled drums in each room until the room is filled. Some material may be packed into the spaces around the drums to fill the void spaces, a process termed "backfilling." Waste-filled rooms could be backfilled with crushed salt (excavated from the repository); crushed salt mixed with bentonite (a water-absorbing clay mineral); other materials combined with the salt (e.g., additives to reduce the solubility of plutonium in brine); or grout (Butcher, 1990; DOE, 1991). The specific plans for backfill have not been revealed in repository designs published prior to 1996. A separate series of drifts and rooms for underground experiments has been excavated at the north end of the facility, several hundred meters from the disposal area (Figure ES.1). Various underground tests have been performed since the 1980s (see, for example, Butcher and Mendenhall, 1993), but these are now considered complete. The experimental area will not be used for waste disposal. Regulation And Licensing Of WIPP In 1992, the WIPP Land Withdrawal Act (LWA; P.L. 102-579) transferred control of land at the WIPP site from the Secretary of the Interior to the Secretary of Energy and granted authority to the Secretary of Energy to close the area and its immediate surroundings to public use. A major provision of the LWA requires
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--> FIGURE 1.1 Location of the Waste Isolation Pilot Plant site. Inset map shows approximate location of map area in the state of New Mexico. Source: Modified slightly from Sewards et al. (1991, Figure III-1).
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--> BOX 1.2 Compliance and Safety The U.S. Environmental Protection Agency is responsible for ensuring that any U.S. geological repository for radioactive waste does not result in significantly adverse effects on human health and the natural environment. Federal standards have been promulgated to this end, and any proposed repository must be shown to be in compliance with these standards before it can be opened and operated to accept waste. When applied to a particular repository such as WIPP, features not anticipated in formulating the compliance standards could affect the overall safety of the repository. It is conceivable that a repository could comply with the standard, yet be significantly more or less safe than the measure of safety implicitly understood when the standard was formulated. This distinction could be important, because the federal standard (40 CFR 191) has never been applied to a real repository licensing situation. The difference between compliance and safety can be illustrated by a simple example from the WIPP repository. EPA has defined the "accessible environment" as the region outside a 5-km-radius vertical cylinder around the center of the WIPP site, extending downward without limit from the surface (see Figure 1.2). DOE has chosen to define the boundary used in the compliance calculations as a smaller region, a 4 mile (N-S) by 34 mile (E-W) square region (6.4 km by 6.4 km square region) placed centrally within the 5-km-radius circle and also extending downwards without limit from the surface. Any migration of radionuclides across this boundary in excess of limits specified in the standard (40 CFR 191) constitutes a violation—that is, a failure to comply. This is independent of the depth at which the migration occurs and without consideration of the pathway by which the radionuclides would reach the surface. Most of any "escaping" radionuclides would reach the accessible environment by transport in water. At WIPP, the most probable "escape route" to the surface is in the brine of the Culebra Dolomite, but this brine is for the most part undrinkable, even to cattle. This greatly reduces the safety risk compared to transport of the same quantity of radionuclides in fresh water. However, the containment requirement of the EPA standard does not address the potability of water containing the released radionuclides. Releases to salty, unpotable water near the surface thus may pose a compliance problem while presenting little safety hazard. The committee found no probable situation for which the WIPP repository could be found to be in compliance with 40 CFR 191 and yet not be safe. DOE to show that the repository will comply with federal regulations. The LWA designated the Environmental Protection Agency (EPA) as the regulator to determine whether or not compliance with appropriate regulations has been achieved (see Box 1.2). Chief among EPA regulations is the 40 CFR 191 standard, which describes general radiation protection requirements. Two other important regulations are EPA's 40 CFR 194, which describes the specific requirements at the WIPP site for compliance with 40 CFR 191; and EPA's 40 CFR 268 Resource Conservation and Recovery Act (RCRA) requirements on the chemically hazardous constituents of WIPP-bound waste. Over the past two decades, DOE has conducted an extensive program of investigations to assess the suitability of WIPP as a TRU waste repository—that is, as a site that will adequately isolate the waste from the environment. Sandia National Laboratories has served as chief technical adviser, with help from many sources, including the nearby Los Alamos National Laboratory, several other national laboratories, the U.S. Geological Survey, universities, and private engineering and consulting firms. Framework For The Report Presently, DOE is preparing an application to the EPA for a certification of compliance of the WIPP facility to demonstrate that the proposed repository meets regulatory requirements. Because the compliance certification application consists largely of conclusions drawn from DOE's scientific and technical investigations, it is timely to comment on results of the committee's investigations to date and their implications with regard to the overall performance of WIPP as a repository for TRU waste. Like the previous studies of the WIPP Committee (NRC, 1984), this
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--> report is a review of ongoing activities and should be viewed as a progress report rather than a final evaluation (see also Box 1.3). Analyses of the performance and regulatory compliance of WIPP are addressed via performance assessment (PA), a process for identifying, for hypothetical future conditions, the potential pathways by which radionuclides could most probably be released to the environment. Chapter 2 reviews the allowable releases of radionuclides in EPA standards, describes the PA methodology used to demonstrate compliance with these release limits, and discusses PA results related to WIPP. A detailed examination by DOE has been carried out over the past decade to study possible ways in which releases could occur, ranging from inadequate sealing of the repository to meteorite impact, climatic change, glacial erosion of the overlying formations to expose the waste, and many others. These possibilities were reduced by DOE to the 75 most credible features, events, and processes (FEPs), which were then examined in greater detail. This examination indicated that eight combinations of events, or scenarios, warranted careful scrutiny (see Figure 2.2). From this group, the so-called ''E1E2" scenario emerged as potentially the most serious way in which the isolation integrity of the repository could be violated. This scenario, which is discussed in more detail in Chapter 2 (see Figure 2.5), involves the drilling, some time within the next 10,000 years, of two oil or gas wells (either for exploration or for production), both of which intersect the repository. One of the wells also connects into a large reservoir of pressurized brine in the Castile Formation below the repository. It is further assumed that both wells would be "capped" (that is, sealed) near the surface and that the well casings would have corroded so that brine from the Castile could flow under high pressure through the repository, up the second hole, and into the Rustler Formation (principally the Culebra Dolomite), which lies above the Salado Formation. From there, the radionuclide-contaminated brine could flow more or less horizontally in the regional ground-water system, eventually reaching the accessible environment via a well drilled into the Rustler Formation to provide drinking water for cattle. Detailed investigation of the factors that could influence the radionuclides released by the E1E2 and other scenarios has led to the identification of several issues that could be important in demonstrating the suitability of WIPP as a TRU waste repository. Chapters have been arranged to cover each of these issues. The performance assessment methodology used to calculate compliance integrates information from geological, hydrological, chemical, and repository design influences on radionuclide releases to the environment. The geological and hydrological influences of the salt beds that serve as host rock for the repository are discussed in Chapter 3. The significance of repository design options and waste treatment are discussed in Chapter 4. The chemistry of the radionuclides in brine and mechanisms of radionuclide BOX 1.3 NRC Committee on WIPP In March 1978, DOE asked the NRC to review the scientific and technical criteria and guidelines for designing, constructing, and operating the Waste Isolation Pilot Plant to isolate radioactive wastes from the biosphere. The panel (now committee) on the WIPP was established within what is now the NRC's Board on Radioactive Waste Management. Since 1978, the committee has provided guidance to DOE on the scientific and technical adequacy of DOE programs designed to assess both the capability of the facility to isolate TRU waste and the overall performance of the facility. Two current members (Fred Ernsberger and former Chair Konrad Krauskopf) served on the original 1978 panel; three others (John Blomeke, Rodney Ewing, and Charles Fairhurst) began service in 1984. The last comprehensive report issued by the committee was in 1984 (NRC, 1984). Eight letter reports on specific issues at WIPP (NRC, 1979a,b, 1987, 1988a, b, 1989, 1991, 1992) also have been produced by the committee. The committee's current official statement of task is as follows: The Waste Isolation Pilot Plant Committee would prepare a report on the current status and progress of the scientific and technical issues that form the core of a submission by DOE to EPA for certification of the WIPP facility as a safe long-term repository for radioactive transuranic waste that complies with relevant regulations.
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--> FIGURE 1.2 The EPA's "accessible environment" is shown here as the region outside a vertical cylinder of 5-km radius, centered on the WIPP repository. DOE calculations are aimed at demonstrating compliance within a smaller region, a volume bounded by vertical sides that extend downwards without limit from a 4 mile by 4 mile (6.4 km by 6.4 km) square cross section. Source: Adapted from Marietta et al. (1989, p. II-3). transport in the subsurface environment are discussed in Chapter 5. Chapter 6 discusses the state of knowledge of the hydrological characteristics of the geological strata above the salt at WIPP, that is, the strata that would serve as a final barrier to releases of radionuclides to the accessible environment. Finally, Chapter 7 provides a summary of the conclusions reached in the committee's investigations. The first seven appendixes to the report provide supplementary information on specific aspects of the chapter discussions. An overview of the WIPP program, intended to assist readers in following the main themes and findings of the report, is presented in Appendix G. The focus of the committee's review has been on the long-term, post-closure performance of WIPP. The issues associated with the operational phase, both surface and sub-surface, as well as the activities that must occur at other DOE sites, are not included in this evaluation. It is the committee's belief that the scientific basis for these other activities, both by DOE and by the various regulatory agencies involved, has been considered adequately in previous reports, except for a few issues that cut across both the operational phase and the post-closure phase. An example of this type of activity is waste characterization, which is discussed in Chapter 2.