Contaminant isolation is the second of the three measures embedded in the long-term institutional management approach. This chapter addresses current technologies and methodologies used for isolation, desired characteristics of such measures, constraints and limitations for their application, and future directions for improvement, including the role of scientific and technical research.
Contaminant isolation refers to measures intended to prevent or limit contaminant migration into the environment adjoining a site. It becomes a necessary component of long-term institutional management in part because of the limitations on contaminant reduction discussed in the previous chapter. Contaminant isolation measures consist of engineered barriers, but also include groundwater pumping (hydraulic barriers) and waste stabilization approaches. As a group, these measures must be planned and coordinated closely with contaminant reduction measures, since the need for them is driven by the extent to which contaminant reduction measures are feasible or effective in reducing risk. Once in place, the ongoing effectiveness of contaminant isolation requires monitoring and maintenance and application of other aspects of the institutional management system. Over the longer term, monitoring of the groundwater and the unsaturated (or vadose) zone, as well as surface water, becomes important whenever contaminant isolation measures are in use.
DESCRIPTION OF THE TECHNOLOGIES
A recent report from the National Research Council (2000b) found increasing use and acceptance of waste containment and stabilization at U.S. Department of Energy (DOE) sites in recent years. Containment can be the low-risk, low-cost option of choice for some problems. Nevertheless, understanding of the long-term performance of containment and stabilization systems is limited, and there is a general absence of robust and cost-effective methods to validate that such systems are installed properly or that they can provide effective long-term protection.
Engineered barriers, either on the surface or subsurface, are generally used to limit the contact of surface water or groundwater with wastes and migration into the surrounding environment. In special cases they may be used to limit the release of contaminated fluids and gases from leaking waste storage tanks, liquid waste transfer systems, or buried wastes. By far the most common engineered barrier is the surface barrier, often called a “cap,” which is
placed over waste deposits (see Sidebar 4-1). Surface barriers typically have multiple layers, with natural and synthetic materials of differing sizes and composition chosen to stabilize the barrier, prevent intrusion by animals and plants, limit movement of wastes, prevent infiltration of water into the waste deposit, and provide a mechanism to slow the release of radioactive or toxic gases. Vegetation is often planted to stabilize the top layer of the barrier, enhance evapotranspiration, and minimize water infiltration through the barrier; in some cases, however, vegetation may increase infiltration by slowing runoff. Two major wildfires in 2000 at the Los Alamos National Laboratory and the Hanford Site demonstrate the fallibility of barriers that depend on vegetation for stability; loss of vegetation from such fires may cause possible releases due to unpredicted environmental exposure and subsequent erosion. An example of a different approach is found with caps over uranium mill tailings burial sites, such as at Rifle, Colorado, that are sculptured to promote runoff of rainwater.
Subsurface barriers are not in widespread use at DOE sites, but they are receiving increasing attention as the problems of waste infiltration and transport by groundwater require more attention. Subsurface barriers may be either vertical or horizontal, and may function in several ways, depending on the specific nature of the groundwater transport situation. One approach is to divert water physically around or away from buried waste. Less common applications include chemical alteration or retention, attenuation, or destruction of wastes as they pass through a permeable barrier. Subsurface barriers may be either physical or chemical in their basic mechanism of waste retention. Mechanical barriers may include “walls” of concrete or metal, fused soil, or even horizontal barriers of concrete under such objects as leaking tanks. Chemical barriers formed from materials that react chemically with radionuclides or toxic materials may retain them or retard their movement by groundwater. Examples of relatively simple chemical barriers are clays such as bentonite and clinoptilolite that bind cesium ions and other ionic species of concern, and thus slow their movement. More complex examples might be phosphate-bearing materials to bind phosphate-insoluble ions chemically, or barrier materials capable of chemically reducing ions whose reduced ionic forms are much less mobile than the oxidized forms (e.g., technetium and neptunium).
Fusing the soil-containing contaminants into an impermeable or near-impermeable mass can protect the contaminants from intrusion and water transport. Usually the soil is composed predominantly of sand, and melting and fusion is accomplished through electrical resistance heating. A similar but temporary engineered barrier may be formed by the freezing of soil containing water. This type of barrier finds application where contaminated water is likely to leak from a container or other source during transfer of liquid. Subsurface barriers are sometimes made by injecting grout into the soil. The grout may incorporate materials such as zeolitic clays to bind certain mobile species. Alternatively, the subsurface barrier may be made of clay, without the use of grout as a host material, or it may include flexible synthetic membranes.
The use of engineered barriers for contaminant isolation was the subject of a recent workshop and report. During discussions at a joint National Research Council and DOE Workshop on Barrier Technologies for Environmental Management (National Research Council, 1997a), several recurring themes arose:
The importance of proper installation techniques and quality control measures during construction, including the use of contractors with demonstrated experience and skill.
The insufficient knowledge of effective lifetimes for barrier materials and systems.
The importance of periodic inspection, maintenance, and monitoring, both short- and long-term, of containment barriers.
The dearth of barrier performance monitoring data, and consequently the importance of compiling data on both successful and unsuccessful barrier installations.
The advantages of using barriers in combination with pump-and-treat approaches to increase effectiveness.
Other good sources of information with respect to engineered barriers include Gee and Wing (1994), Rumer and Ryan (1995), Rumer and Mitchell (1996), and a recently published report on groundwater and soil cleanup issued by the National Research Council (1999e).
HANFORD SITE GROUNDWATER/VADOSE ZONE INTEGRATION PROJECT
(by Shlomo P. Neuman)
Since 1959, 67 of the Hanford Site's 149 single-shell high-level waste tanks have leaked or are suspected to have leaked about one million gallons (about 4 million liters) of waste into the ground. For years, the sorptive ability of sediments was expected to hold most leaked waste high above the water table. Upon closer examination of groundwater chemistry, contaminant distributions beneath tank farms, and geophysical data collected in wells, the U.S. Department of Energy (DOE) acknowledged that cesium-137, technetium-99, and cobalt-60 had migrated deeper than previously expected. Other tank-originated metals such as chromium, sodium, and nitrate are also likely in the groundwater. This admission resulted in negative national media attention and the reorganization of Hanford groundwater and vadose zone studies. It could also impact if and how waste is removed from high-level tanks for vitrifying and the technologies required to permanently close those tanks.
The Hanford Site Groundwater/Vadose Zone Integration Project (U.S. Department of Energy, 1998, 1999) was established in 1997 to coordinate and integrate the collection and interpretation of scientific information needed to deal with soil and groundwater contamination at the Hanford Site, Washington, on a site-wide basis. The project emphasizes characterization of the vadose zone, groundwater, and the Columbia River, and assessment of the risk that site contamination may pose to human health and the environment. Its intent is to help inform and influence key decisions by regulators and DOE concerning cleanup and environmental management of Hanford. To this end, the project aims to identify and address uncertainties and gaps in scientific understanding that influence such decisions, to initiate research that may help reduce these uncertainties and gaps, and to enhance the role of science and technology as a basis for site-related decisions. Through coordination and streamlining of site characterization efforts, the project hopes to eliminate redundancies and overlaps among these efforts. Additional project goals include development of risk assessment methods that are applicable across the site and the Columbia River system, rendering site information readily accessible to those who need it, facilitating public involvement in decisions concerning the cleanup and disposition of Hanford, and insuring independent technical reviews and management oversight of the Integration Project itself.
The Integration Project is envisioned as influencing Hanford Site decisions and operations such as high-level waste tank retrieval and closure, remediation of 200 Areas waste sites, and final closure of the Hanford Site, all toward protection of water resources, including the Columbia River. It is focused on five endeavors (U.S. Department of Energy, 1999):
The contaminants at the reservation include radionuclides (e.g., carbon-14, chlorine-36, iodine-129, cesium-137, strontium-90, selenium-79, technectium-99, uranium-238, plutonium-239 and -240, tritium) and hazardous chemicals such as carbon tetrachloride, trichloroethylene, nitrate, nitrite, cyanide, and chromium. The sources of radioactive and hazardous waste contaminants to the vadose zone, groundwater, and, ultimately, the Columbia River, include planned disposal as well as leakage and spillage of high-level wastes from storage tanks and transfer lines in the central plateau (200 Areas) of the site. In addition, some tank liquids containing fission products from processing of the spent fuel for recovery of plutonium were directed into subsurface drainage “cribs,” drains, ditches, and ponds that flowed directly into the soil
In some cases the existing geology may be able to act as a barrier to migration. Wastes can be placed in such relatively impermeable strata as clay or some rock formations. For example, uranium ore residues have been interred in clay layers at the Niagara Fails Storage Site in New York (National Research Council, 1995a). In general, however, it is difficult to predict the long-term performance of such natural materials because of the general inhomogeneity in the formations (e.g., fractures, changes in the physical and chemical properties, inclusions in and intrusions into the formations) and changes over time that may result in the presence of difficult-to-detect preferred pathways for migration. The geological repositories for transuranic waste at the Waste Isolation Pilot Plant in Carlsbad, New Mexico, and the proposed geological repository for commercial spent nuclear fuel and high-level waste at Yucca Mountain, Nevada, both depend on engineered barriers to contain the waste in addition to the attributes of their natural geological barriers.
Groundwater Management and Hydraulic Barriers
Enhanced recharge and/or groundwater collection and extraction are often used to control the direction of local groundwater flow and to prevent the further migration of groundwater contaminants. Although not usually thought of as a barrier technology, groundwater collection and extraction, with subsequent treatment of the extracted groundwater (the pump-and-treat process), can in fact provide an effective but interim barrier to waste transport. The process provides hydraulic containment that prevents the further migration of radioactive and/or
toxic materials in groundwater. When the amount of water to be treated is not large, and the waste material is not too dilute, the pump-and-treat process can provide a practical solution to many waste problems. The efficiency of the pump-and-treat process for restoration is greatly reduced when contaminants reside in (a) a heterogeneous medium with widely varying permeabilities and porosities, including fractures, (b) the unsaturated or vadose zone, or (c) a nonaqueous phase, especially dense nonaqueous phase liquids (DNAPL) (National Research Council, 1994c, 1999e).
Subsurface engineered barriers, such as slurry walls, are often used in conjunction with pump-and-treat systems to retard intrusion of uncontaminated water. Since pumping provides hydraulic containment, the barrier need not have a low permeability. The contribution of the slurry wall is to supplement hydraulic containment, thereby making containment easier and less costly to achieve.
The injection of water into a groundwater system to contain the migration of contaminants is another form of engineered barrier. In these cases, the injection of water is used to retard or change the local hydraulic gradient and in effect “contains” a contaminant plume. None of these technologies is particularly effective for managing the vadose zone, however (see Sidebar 4-1).
Waste and Contaminated Soil Stabilization
Stabilization approaches are often used to immobilize radionuclides and hazardous chemicals and thereby preclude their leaching and further migration from waste materials and contaminated soils. The approach is either to combine the waste with chemical additives such as lime, Portland cement, or fly ash to make a grout, or to provide electrical heating of the ground (e.g., in situ vitrification) to transform the waste or contaminated soil into a solid or glass from which radionuclides or hazardous chemicals (typically, metals are not easily leached) will not easily migrate (see Conner, 1990; Wilson and Clarke, 1994).
PERFORMANCE MONITORING OF ENGINEERED BARRIERS AND STABILIZED WASTES
Performance monitoring involves the continuous or periodic measurement of the effectiveness of the contaminant isolation system once it has been employed. The term “performance monitoring” often is associated only with the performance of a physical system (e.g., the reduced mobility of residual contaminants or the performance of technologies to isolate and/or clean up those contaminants). Monitoring is used to demonstrate the effectiveness of efforts to remove, treat, and contain contamination, but it is also used to support development of models of subsurface and contaminant behavior (National Research Council, 2000b). Approaches include groundwater measurement techniques available to conduct performance monitoring. These physical measurement techniques include groundwater monitoring (probably the most common), vadose zone monitoring, and cover and barrier monitoring (usually some form of vadose zone monitoring, but this can also include physical inspection). There is presently no well-established, reliable, and economic technology available to monitor effectively the vadose zone and heterogeneous media. This observation applies also to fractured subsurface media.
The Resource Conservation and Recovery Act of 1976, as amended (RCRA), the Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended (CERCLA), the Uranium Mill Tailings Remedial Action Program, and other waste disposal regulations require some type of post-remediation monitoring to be conducted at each site of residual contamination (see Appendix E). Monitoring requirements are usually spelled out in the site-specific documentation for each site. To date, it appears that relatively few site-specific post-remediation monitoring requirements have been defined at DOE sites. Numerous post-remediation issues remain to be resolved at many sites, including (1) What is to be monitored (e.g., soil, water, air)? (2) Where, how, and how often will monitoring be conducted? and (3) What conditions, if found, would necessitate further action (e.g., exceeding concentration limits or changes in hydraulic gradients)?
Monitoring serves two general purposes: to verify that the system being monitored is behaving as expected, and to compare monitoring results with pre-set limits for the purpose of standards verification. Monitoring will not be useful unless the results are examined critically, and conceptual modeling becomes a necessary adjunct to monitoring for establishing whether the system into which wastes have been emplaced is behaving as expected.
There are a number of reasons why monitoring and maintaining barriers, whether simple or complex, require effective institutional management. Near-surface disposal of chemical wastes and low-level radioactive waste usually requires the installation of a cap (i.e., a barrier above the waste to prevent the infiltration of water and subsequent transport of leached waste out of the disposal site, as well as to prevent human contact with the wastes). As previously noted, the cap may be composed of natural material such as clay, chosen for its resistance to infiltration, or man-made materials with greater expectations for performance. The cap may also be a multiple-barrier system. Whatever cap is chosen, its performance over time presents uncertainties. Settling can trap water, enabling greater infiltration through minor flaws in the cap. Undesirable vegetation may become established, with root systems that penetrate the cap. Careful attention to the cap and how wastes are emplaced can enhance confidence in continued good performance, but performance monitoring still will be needed. The monitoring systems associated with near-surface disposal of low-level radioactive waste are usually set up to monitor the water effluent from the burial cell. They should be designed to take into account uncertainty in the predicted performance of the barrier itself, such as through the use of systems to detect infiltration beneath the cap as supplements to monitoring intended to detect material leaking from the disposal cell.
Another form of barrier for waste disposal is waste stabilization —that is, fixing the waste in a form that enhances its resistance to percolation and leaching. One example is the containment of low-level radioactive waste in grout or cement or vitrified into glass logs before disposal. The performance of the stabilizing medium as an additional leaching barrier is typically quite difficult to predict, however. Credit for regulatory compliance cannot be taken for the barrier unless performance can be demonstrated.
Another example is the use of multiple grout and cement barriers to fill an emptied high-level radioactive waste tank when its residue has been reduced to low-activity waste, a practice that has now begun at the Savannah River Site. The grout and cement barriers are intended to isolate the residue from water infiltration and leaching and to serve as a barrier to intrusion by animals, plants, and humans. Predicting performance in resisting water infiltration can be difficult because of uncertainties that include the degree to which the first layers of grout take up the residue, the water pathway effects of the cold joints between successive pours of grout, and the effects of preferential corrosion of the tank metal and penetrating structures (thereby offering a partial bypass path). Moreover, waste tank residue is likely to be highly radioactive and not taken up in the grout, so there is substantial uncertainty associated with the volumetric classification and average concentration of the waste and prediction of the isolation performance of the system. Finally, a key challenge to disposition of the tank residues in this manner is to obtain a determination of waste incidental to reprocessing to allow it to be classified and handled as transuranic or low-level waste (for example, the Savannah River Site) (U.S. Nuclear Regulatory Commission, 1999).
CHARACTERISTICS OF IDEAL CONTAMINANT ISOLATION MEASURES
Much has been written about the ability of engineered barriers and waste stabilization measures to last for the required time period necessary to manage the risk associated with leaving wastes in place. It is difficult to project limited performance data that exist much beyond a few hundred years, and even these time periods are very controversial. Ongoing maintenance will be necessary, including, perhaps, replacement of the system itself. In order to be effective, contaminant isolation measures should have the following characteristics:
A design appropriate to the specific contaminant isolation requirements that provides the needed degree of protection and containment. The design should be developed with performance monitoring, maintenance, and repair needs in mind.
A well-designed performance monitoring approach that addresses how criteria for failure are determined for the system selected and the specific site environment.
A management and maintenance plan that specifies the types and frequencies of inspections and associated system repairs when needed, coupled with reasonable assurance that the plan will actually be carried out.
Incorporation of adequate quality assurance and quality control measures during the planning and implementation stages. These measures are critical to success since the best-designed barrier can, and most probably will, fail if the installation of the system is compromised.
CONSTRAINTS AND LIMITATIONS
Engineered barriers and waste stabilization approaches, while potentially providing solutions to some of the most difficult waste management problems, are not without potential shortcomings. They can be expensive to build and install. Experience with some of the more novel applications of barriers is limited. The retrieval of high-level waste from storage tanks may leave significant residues of “incidental waste” in the tank. That waste may be low-level waste or transuranic waste in the DOE classification system. If that waste is to be fixed in place it must be provided with sufficient isolating barriers to assure adequate protection. If further, more aggressive waste extraction is not to be attempted, a custom-designed barrier system may be installed to stabilize the residue for in situ disposal, but doing so runs the risk of reducing future options for using later, better techniques. Current thinking for such waste stabilization at the Savannah River Site calls for initial injection of chemically reducing grout to fix the residue, followed by the addition of other cementitious materials to accessible areas inside the tank vault and the tank. The objective is to achieve a barrier system that is as robust as reasonably achievable, given the limitations of working with an existing tank system design.
Incomplete Understanding of Long-Term Performance
Perhaps the most important consideration in the use of engineered barriers and waste stabilization approaches in waste management is the fact that there is limited experience with most, if not all, of the systems being considered. The lack of experience with barriers proposed for use in some of the more demanding applications raises particular concerns. Concrete barriers can degrade with time, as can chemical barriers, which by their nature will be altered through chemical reaction with the wastes whose chemical nature they are intended to change. Barriers made of synthetic materials can also deteriorate over long periods of time; unfortunately, data on their performance over time are especially limited.
Need for Institutional Management
The limited lifetimes and effectiveness of barriers for the long term lead to the conclusion that institutional management will be required to ensure the effective performance of barriers, except when they survive long enough that natural processes, such as radioactive decay or, in the case of toxic organic materials, biodegradation, reduce risk to acceptable levels. Regardless of the design life of a barrier or stabilized waste, there is a need to confirm and ensure its effectiveness and durability over time. This assurance needs to be provided by the use of institutional management measures such as sampling and/or monitoring to determine if the barrier is, in fact, functioning as designed. As subsequent chapters will discuss, the effectiveness of institutional management measures should not be assumed; most if not all barriers will require the use of some kind of institutional management to ensure their efficacy and durability. Necessary institutional management measures include, for example, maintenance of monitoring stations and, in some instances, periodic inspections. Data need to be evaluated with reasonable frequency by individuals with appropriate expertise and appropriate corrective measures implemented.
FUTURE DIRECTIONS FOR IMPROVEMENT
The evolution in the sophistication of barrier design and technology over the past few decades has been remarkable. It is to be hoped that this progress will continue as the scientific and engineering community seeks to refine and improve engineered barrier materials and design approaches. Nevertheless, given the contrast between
current contaminant isolation technologies and the characteristics of an ideal approach, there are several areas whose further research and development appear to have merit. New emphasis should be placed on the development of effective methods of performance monitoring. An adequate definition of “What constitutes failure?” should be determined, documented, and implemented. The procedure should also accommodate inevitable uncertainties that are inherent in the measurement process.
A corollary to the above is the realization that ongoing maintenance is critical to contaminant isolation effectiveness. Unfortunately, as will be discussed in later chapters, serious limitations in institutional management approaches may well mean that even careful planning will not be enough to ensure that maintenance will be carried out properly and that major failures will be remedied. Still, there is good reason to incorporate the need to accommodate repairs, including potential system replacement, into the initial design.
Finally, there will be an ongoing need for inspections, data collection and analysis, and decision making to determine whether the contaminant isolation technology is working or corrective action is needed. Monitoring approaches using trend analyses that accommodate uncertainties are emerging and their use is encouraged.
In summary, with few exceptions, contaminant isolation measures will be a necessary part of the overall remediation approach. Complete decontamination to avoid risk is rarely achievable. In some cases the contaminant may be moved into an isolation cell or impoundment designed specifically for the purpose (e.g., a uranium mill tailings impoundment). In other cases, the stabilization system design should be tailored to the existing configuration of the waste to be stabilized (e.g., waste residue in a buried high-level waste tank). In either case, the waste stabilization designer should take due account of the state of knowledge and uncertainties in waste isolation performance. Yet, good design may not be enough; it may be prudent to consider whether the waste should be as isolated as reasonably achievable (AIARA) (see Sidebar 4-2). In many cases, contaminant isolation measures cannot be relied upon to achieve their objectives without institutional management.
HOW CAN RADIATION EXPOSURES FROM WASTE DISPOSAL BE ALARA?
(by Robert M. Bernero)
For many years the international radiation protection community has advocated and followed three basic principles for protection against the potentially injurious effects of ionizing radiation, namely, (1) any practice that entails radiation exposures shall be justified, (2) strict limits shall be maintained for radiation exposures, and (3) radiation exposures shall be controlled to be as-low-as-reasonably-achievable (ALARA). Applying these principles in the control of nuclear activities results in setting a limit of 10 to 20 mSv/a (1-2 rem/yr) for radiation workers along with control practices that effectively keep these exposures ALARA, typically well below 5 mSv/a (500 mrem/yr). These radiation exposures are measurable.
For members of the public the limit is set at 1mSv/a (100 mrem/yr), a strict limit because the average member of the U.S. population is estimated to receive 3.6 mSv/a (360 mrem/yr)from background radiation. At these lower levels direct measurement is more difficult. Since public radiation exposure from nuclear facility operations is almost always due to effluents in the air or water pathway, controls to maintain public exposure ALARA are usually achieved by setting the point of compliance with the limit at a very close release point such as the exit of a ventilation stack. In this way there is substantial decrease in the actual radiation exposure at the site boundary. Reduction of the release source term is also available by filtration and other means to maintain releases and exposures ALARA.
In radioactive waste disposal a different situation is found. Decontamination or source removal can be conducted until measured residues have fallen to a level at which projected exposures of nearby populations are within protection limits and ALARA with respect to the cost or difficulty of further source reduction. For the deliberate disposal of large amounts of waste or for in situ disposal of wastes too difficult to remove, a system of barriers designed to inhibit waste migration from the site and consequent exposure of nearby populations is used. Typically, the barriers can be a system of packaging or stabilization of the waste itself to resist leaching, provision of caps or covers to divert water from coming into the waste, and liners or barriers to
prevent contaminated water from leaving the site. Performance of such a system cannot be directly measured, but only predicted through performance assessments. Down-gradient monitoring cannot be relied upon to detect in a timely way a system failure resulting in a release of radionuclides. The selection of a good low-level waste disposal system creates a tension between demonstrating compliance with disposal requirements using the uncertain predictions of a performance assessment and the lingering need to adhere to the third principle of radiation protection, to maintain exposures, or releases, ALARA. The compliance limit for waste disposal is typically a fraction of the public health exposure limit, while only about a factor of 10 less is a negligible exposure, evidently ALARA. The uncertainties of performance assessment are too great to enable discernment of that factor of 10 for ALARA demonstration. In the early 1980s the new U.S. regulations for disposal of commercial low-level radioactive waste (LLW) were released. Explicitly related to near-surface disposal or shallow land burial of waste, they set compliance requirements for the emplacement of stable waste forms in a well-covered and well-drained site. Further enhancements are welcomed, but the sections of the regulations reserved for engineered enhancement, “other than near surface disposal” (10 CFR Part 61.51[b] and Part 61.52[b]) were never completed. Many expressed a desire for such features for U.S. sites east of the Rocky Mountains, where the waste was likely to be in the saturated zone near the water table.
At about the same time that the U.S. was adopting its LLW regulations, France was modifying its own methods for disposal of LLW. There, the new methods for engineered, near-surface disposal were developed and partially implemented at the Centre de La Manche, west of Cherbourg (now closed), and fully implemented from the beginning at the Centre de L'Aube, currently operating east of Paris. In the French system the regulatory requirements also call for stable waste forms in a well-covered and well-drained site. Compliance is measured by performance assessment conducted as if the LLW were buried in trenches. Since French sites are similar to many U.S. sites east of the Rocky Mountains, the uncertainties in performance assessment are similar.
However, in France the waste management authority has added many enhancements to the isolation capability of the site based on their judgements of cost effectiveness and good management. To begin, all waste shipments are tracked, with their assay, using bar code markers to their records of generation, transport, and disposal. At the disposal site many of the waste packages are tightly compressed to fit together in concrete cylinders sealed with bitumen. A new combined bar code is assigned to this package. The waste packages are typically packed closely into concrete near surface vaults, with grout added to fill the interstitial space. The excavation and array of waste emplacements are covered by a very large canopy building mounted on rails, so that the waste emplacement area is kept dry during operations. After the waste is emplaced and the vaults closed, it is covered with soil, a synthetic membrane, and a clay layer to keep out moisture, and topped with soil cover and vegetation to retain stability. The emplaced waste has an underlying leachate collection system as a precaution, to be monitored for 300 years, although one would not expect any leachate from the array. Thus, the French system does not rely on subtle differences in uncertain performance assessments to determine whether public exposures are ALARA. Rather, it implicitly uses a different but equivalent principle of protection. The systems attempt to render the waste as-isolated-as-reasonably-achievable (AlARA). It is difficult to quantify the effects of these enhanced barriers to waste movement, but they are especially valuable in enhancement of intrusion barriers and in isolation of dominant waste isotopes with half-lives of about 30 years (e.g., cesium-137 and strontium-90). The enhancements compensate for uncertainties in compliance performance assessment. This AIARA concept also can be applied to the disposal of high-level waste (HLW) deep underground. Actually the concept is implicit in the waste disposal statutes where nearsurface disposal is authorized for LLW, but deep geologic disposal is required for HLW, providing a much greater degree of isolation.
Others have shown interest in the French approach for LLW isolation. In Spain, the El Cabril LLW disposal site is a near copy of the French Centre de L'Aube even though El Cabril is in a much drier, high elevation setting in southern Spain. In the U.S. no new “full service” LLW sites have been licensed since the Low-Level Radioactive Waste Policy Amendments Act of 1985, but most of the states east of the Rocky Mountains have indicated a desire to incorporate the engineered features of the French system, perhaps to render their waste AIARA. Nevertheless, the fallibility of bar code markers for maintaining records and identifying waste packages over periods of hundreds of years is recognized. In addition, the stability of governments over a period of 300 years is questionable. For example, a quick glance at the last 300 years of French history reveals a monarchy, five republics, a couple of empires, a war zone, and a hostile occupation. At each change in government, the survival of monitoring systems would be in extreme jeopardy.
While there is typically a tacit recognition that engineered barriers and waste stabilization approaches have limited periods of effectiveness, these technologies are often employed with inadequate understanding of, or attention to, the factors that are critical to their success. These include the need for well-conceived plans for performance monitoring that identify and correct potential failures and plans for maintenance and repair, including possible total system replacement.
The committee stresses the importance of building into planning and design approaches a recognition of, and allowance for, uncertainty and fallibility. Contaminant isolation systems should incorporate an effective means of performance monitoring as close to the waste and contaminated soils as possible without compromising system integrity. Monitoring within the containment system will provide useful information to vadose zone and groundwater monitoring and improve the ability to provide an early warning of potential failure.
It is now widely recognized that the subsurface is a complex, multiscale, spatially variable natural environment that can never be fully characterized. Hence the results of even the most thorough site characterization and
monitoring efforts are ambiguous and uncertain. The DOE should continue to sponsor targeted applied research efforts that not only address the critical knowledge gaps concerning contaminant isolation system design effectiveness and lifetimes, but that also incorporate improved ways of detecting and remedying potential failures. Specific examples of such research are found in two recent reports by the National Research Council (1999e, 2000b). With respect to performance monitoring there is a need for good data appropriate to the task, better and more affordable data collection technology to collect such data, and scientific research directed toward improved understanding of the factors that affect system performance.
The Hanford Barrier prototype (see Sidebar 4-3) represents an example of the difficulty of conducting a continuing program of relevant research projects until the real benefits (in this case, better understanding of performance over time) have been achieved. After its construction, the prototype barrier was monitored for only a few years before funding was terminated. Recently, some limited funding has been made available. While, the DOE should be commended for initiating the Hanford Barrier prototype research (and other similar efforts, such as research appropriate to caps for waste sites in humid eastern U.S. environments), a long-term commitment to research funding and priorities is needed to ensure that the resulting data are sufficient for projections of future performance and potential design improvements.