Environmental Monitoring and Adaptive Staging
Essential steps in an Adaptively Staged geologic repository program are: (1) monitoring the repository environment, including its component engineered barriers as well as its natural barriers; (2) scientific and programmatic evaluation of the new data, and (3) using this data in decisions to move the program forward or to modify operational strategies. Adaptive Staging cannot exist without adequate monitoring.
Figure E.1 shows the types of monitoring activities during various phases of a repository program. An early and ongoing monitoring program is one of the essential elements of Adaptive Staging. During early preoperational stages monitoring is essential to characterize the site and to provide background (or baseline) levels for key parameters. As the repository stages evolve over time, monitoring emphasis and priorities shift to near-field and in situ monitoring while far-field, surface, and borehole monitoring activities continue. During closure, monitoring measurements must provide assurance of the effectiveness of seals and the security of the waste. After closure, in situ monitoring may become infeasible, and more reliance will have to be given to indirect geophysical and remote-sensing methods.
Monitoring the repository environment will be most feasible and most valuable for Adaptive Staging during the operational phase, after requisite baseline measurements have been acquired. If the operational phase were to begin with a preliminary pilot-scale activity, intensive in situ monitoring of the pilot disposal rooms, their environments, and their engineered barriers would be essential for learning and scaling up to a fully operational activity. A pilot disposal operation would also serve as a testing ground for monitoring methodology.
An overview of monitoring needs and methods is presented in Table E.1. The lists in the last three columns relate to the environment in the first column, but there is no correlation implied between the order of the lists of columns 3 and 4 and the components listed in column 2. This table is not comprehensive and all inclusive; rather, it illustrates the complexity and varying needs of a monitoring program for a high-level radioactive waste repository. In addition, by the time repository monitoring for an operational phase actually occurs, these methodologies may well have advanced far from what is now available.
Monitoring to provide data for scientific input to decision-making may be either intrusive or noninvasive. Intrusive methods may offer the best approach to direct observation of engineered barriers, waste canisters, and surrounding near-field environments. However, intrusive methods pose a risk because the observation method itself might degrade the integrity of the repository. In addition, the measuring apparatus (e.g., drill hole or sensor) perturbs the local environment, often to an unknown extent.
Therefore, direct measurements of conditions within the repository and the near-field environment may be most feasible during the construction and operational phases of the repository, but would be impractical (with present technology) in the post-closure era (Figure E.1). One of the research priorities during the next few decades is to develop improved indirect methods to be used during the post-closure stage to monitor critical parameters within the repository.
Monitoring in a repository program can serve a variety of purposes (IAEA, 2001). These purposes may change as stages evolve. Pre-closure monitoring goals include: (1) providing baseline measurements of thermal, chemical, mechanical, geological, and biological conditions undisturbed by the repository system; (2) providing data for analyzing actual system (and component) performance and comparing the data with expectations, including model calculations (compliance); (3) providing data to be used at each stage in decision-making; (4) detecting any system behavior or failures that could harm the environment or human health (compliance); (5) safeguarding nuclear materials; (6) providing data that ensure responsibility and liability; (7) providing information for societal confidence in repository performance; (8) ensuring health of workers during the operational phase of the repository; and (9) assuring compliance with existing regulations.
TABLE E.1. Overview of Selected Monitoring Requirements for a Geologic Repository for High-level Radioactive Waste. This table is not meant to be all-inclusive.
Examples of Repository Components
Examples of Parameters and Processes
Examples of Methodologies
• Tunnels and Rooms (including coring walls, floors, and ceilings in host rock)
• Waste Packages/Containers
• Buffer Materials
• Backfill Materials
• Other Engineered Barriers
• Monitoring Components (e.g., sensors and wiring)
• 3D Dimensional Change
• Mineralogical Composition
• Air and Gas Composition
• Fluid Content and Composition
• Structural Integrity
• Vapor Saturation/Humidity
• Organic Activity (microbial)
• Direct Sampling with wells or cores (both continuous and discontinuous)
• Emplaced Meters and Sensors for Direct Nondestructive Measurement of Physical and Chemical Parameters
• Data Transmission Hardware (electrical cables and wires, fiber optic cables, wireless transmission)
• Remote Sensing and Surface Geophysics (including “time-lapse” sequencing)
• Tomographic Methods (including Gamma-scanning techniques)
• Acoustic methods
Natural (both near-field and far-field)
• Land Surface
• Surface Water (streams, lakes, wetlands, etc.)
• Vadose Zone (unsaturated zone)
• Saturated Zone
• Host Rocks (near-field and far-field)
• Deep Bedrock
• Microbial Environment
• Precipitation (and selected other meteorological and climatic factors)
• Temperature (air, water, rock)
• Chemical Composition (air, water, rock)
• Radionuclide Content (air, water, rock)
• Fluid Pressure (hydraulic head)
• Water Flux (surface water, vadose zone, saturated zone)
• Erosion and Sediment Transport
• State of Stress
• Land Surface Deformation and Erosion
• Biological activity
• Shallow & Deep Boreholes (variety of direct measurement options for hydraulic and compositional parameters and sample collection)
• Weather Stations (variety of instrumentation)
• Air Monitoring
• Particulate Collection
• Temperature Probes
• Surface Water Monitoring Stations (flow, stage, quality)
• Advanced GPS Surveys
• Strain Meters
• Borehole Geophysical Methods (including advanced tomographic methods)
• Surface Geophysical Methods (including “time-lapse” sequencing)
• Clinometers and Tilt Meters
• Remote Sensing (aerial and satellite surveys)
Post-closure monitoring goals also include 2, 4, 5, 6, and 7 above. A widely accepted “principle” of the international community is that long-term safety of the repository must not rely on institutional controls (i.e., the capability to monitor the repository after it has been sealed and closed [IAEA, 2001]). However, monitoring after closure should be continued for as long as any institutional control and memory persists to provide input for maintaining confidence in the performance of the system.
The monitoring methodology must not threaten the safety or security of the site or in any way compromise the safety and integrity of the repository by providing a potential pathway for release of radionuclides to the environment or intrusion into the repository. If wiring, pipes, holes, or tunnels are required to transfer information from the sensors to the recorders and observers, then those pathways might represent weaknesses in the multiple barrier approach to safety and would be unwise. Consequently, monitoring of conditions within the repository itself, including performance of the engineered barriers and waste canisters, will ultimately have to rely on either indirect noninvasive methodologies (such as remote sensing and geophysical methods) or long-term sustained wireless transmission of information from in situ sensors (which will require additional research and development). One of the grand challenges in this regard is placement of monitoring stations. While maintaining a secure repository, stations must be spatially arranged to provide the greatest probability to detect a leak if one occurs. Even though a monitoring system for a waste repository must be able to detect even small releases of radioactivity from waste containers into the repository environment, the strategy should be planned to detect precursors of possible releases, including deterioration or failure of engineered barriers. Within the repository this could mean monitoring the physical and chemical integrity of the waste containers and looking for degradation products of canister and engineered barrier materials. On a regional scale (far-field environment), monitoring must detect any environmental changes that could affect the fate and transport of any potentially leaked radioactive materials, or that would significantly alter the assumptions made during site characterization and licensing.
Before construction and operation, in the pre-closure stage, a comprehensive monitoring plan over the entire site must be developed to acquire a database of environmental information that characterizes the conditions, properties, and behavior at the site before any disturbance of background conditions. Establishment of baseline conditions should be essentially completed during the site-characterization stage. Examples for a well-characterized site are given in Table E.1.
Baseline measurements provide the foundation for formulating conceptual models and accessing future system behavior and are required for making decisions related to system performance. Reliable observations must be attained by techniques of varying sophistication, evaluated and synthesized with related data, and archived so that the previously gained information is readily available for comparison with future system measurements. Information-gathering of this type requires long-term institutional commitment so that baseline data are available to future generations. A monitoring strategy that provides information on poorly understood aspects of the systems is imperative. Heterogeneities in both time and space are inherent in natural systems. A strategy of measurements must be made that incorporates this inherent variability into the baseline. Because of these temporal and spatial variations, baseline monitoring must be ongoing and repeated at appropriate
frequencies so that future variations in response to waste emplacement can be distinguished from inherent background variability.
E.1 Remote monitoring
Geophysical and remote-sensing approaches are prominent in Table E.1’s list of example methodologies, and those items listed represent broad categories that encompass many specific tools. Two significant advantages of many of these methods are that they are noninvasive and that their signals can represent properties of relatively large areas or volumes of material (as opposed to point values). Geophysical methods would also be of particular value for the purpose of safeguarding nuclear materials in that they can detect undeclared or unapproved tunneling activities and movement of waste containers. The potentially strong reliance on geophysical methods suggests that a range of techniques can be applied and repeated throughout the site characterization, construction, and operational phases of a repository to provide a basis of comparison with surveying results after site closure. For example, Clark and Kleinberg (2002) illustrate the use of time-lapse seismic surveying to show how petroleum reservoir properties change over time; such an approach can be extended to environmental monitoring of a repository. Future developments in remote sensing may also provide breakthrough technologies with direct application.
Geophysical and remote-sensing approaches provide examples of significant advances in monitoring technology during the past few decades. This rate of progress will continue and it is difficult today to conceive of what future advances will bring. One recent example that may have direct use in a repository program, especially where it is important to detect regional changes in the water-table configuration, is the use of satellite microgravity surveys (Wahr and Molenaar, 1998; Parker and Pool, 1998). Continued future advances in monitoring is a major reason why a repository monitoring program must be Adaptive and allow incorporation of new and better technologies and methodologies.
E.2 Monitoring flow and transport
Some components and constitutive properties listed in Table E.1, such as water, chemicals, and microbes, are subject to migration. It is this potential for transport of radionuclides from the repository that is the primary concern for long-term safety. Prediction and detection of transport (rates and pathways) depend on how accurately and precisely the fluid flow field is defined and how well the site-specific flow process are understood conceptually. Mapping of the flow field is a key element in the establishment of baseline conditions and requires adequate monitoring of regional hydrologic conditions. Such data are largely obtained through direct measurements of hydraulic head (or fluid pressure or potential) in boreholes, and tracer tests, coupled with ongoing interpretation of the hydrostratigraphy.
Monitoring and sampling in the saturated zone are relatively straightforward, both conceptually and technically. Understanding flow directions in a saturated zone entails defining the hydrostratigraphy and mapping the water table, potentiometric heads in deeper aquifers, and the three-dimensional distribution of equipotential surfaces. The definition and monitoring of the flow field is itself a staged operation,
as the early data from initially drilled observation and test wells provide the basis locations of additional wells. A major monitoring problem at repository sites is the evaluation of the adequacy of the definition and understanding of the flow field relative to the need for additional monitoring wells. Because drilling is expensive, it is a difficult management decision to evaluate the tradeoffs between budgetary constraints and the desire for more precision. An Adaptive Staging approach is needed for such decisions. One basis for a decision might be the consideration of whether the level of understanding would make it likely that a potential release of radioactive contaminants from a repository failure would be detected by either a near-field or far-field monitoring network. For instance, at the Yucca Mountain site, the flow field is not yet adequately understood or defined with sufficient accuracy to have confidence in an early-warning monitoring network in the saturated zone.
Direct monitoring and sampling of the saturated zone use access through wells or boreholes. These are generally constructed with steel casing. Experience in the water well and petroleum industries indicates life expectancies for wells on the order of tens of years. This is very short relative to the expected life of a repository. Consideration should be given to the durability and longevity of typically constructed boreholes and materials and the need for improvement. One consequence of well collapse or other failure is the loss of continuity in the monitoring record, but a more serious outcome may be that a collapsed and abandoned well could provide a permeable pathway that acts as a short circuit for the release of contaminants to the accessible environment. That is, under certain circumstances abandoned or inaccessible monitoring wells may defeat the effectiveness of natural barriers.
In the unsaturated (or vadose) zone above the water table, fluid pressure is generally less than atmospheric, so water will not flow into an open borehole (or into repository drifts). Because water is under tension, the definition of fluid flux and monitoring in an unsaturated zone is much more complex and difficult to predict than for a saturated zone (e.g., NRC, 2001). The Yucca Mountain site is an exception to the general trend in site placement with respect to the water table for high-level repositories. That is, in this case the repository will be located in a thick unsaturated zone in an arid climatic zone. Here the unsaturated zone is heterogeneous and highly fractured. This and the low fluid flux through the unsaturated zone make the characterization and monitoring of infiltration and seepage extremely difficult.
Paradoxically, some of the characteristics that make characterization and monitoring difficult are the same characteristics that enhance the safety of the site. Because the unsaturated zone has high fracture permeability, air circulation can affect moisture movement and can also be a transport agent. Circulation of both air and water produces multiphase flow and transport processes, which are nonlinear and highly affected by temperature variations; this natural phenomenon would be greatly enhanced by emplacement of heat-generating wastes. Stephens (1995) includes a detailed discussion of the state of the art of monitoring the vadose zone and points out that “the field is rapidly evolving toward new and more sophisticated methods.…” Aside from repository considerations, Yucca Mountain constitutes a field laboratory for innovative new methodologies for characterizing and monitoring unsaturated zone flow. However, for repositories that would be situated deep in the saturated zone, as are most proposed sites throughout the world, knowledge of the details of flux through the unsaturated zone are not critical to the evaluation of site safety.
E.3 Application of monitoring data
A long-term environmental monitoring program is also critical for assessing extreme, and rare, hydrological and geologic events. Such random events occur so infrequently that establishing a historical basis for predicting their risk to the repository is extremely difficult. For example, large, intense storms with accompanying precipitation or high-magnitude seismic events might occur at the site only once in 100 to 500 years. These “rare” events occur at frequencies outside a human life-span and may not have yet been observed at the repository site. However, their impact is such that they may pose the greatest stress and danger to the repository during its regulatory lifetime (10,000 years).
Pre- and post-closure monitoring lies at the core of the early-warning strategy. Monitoring must be able to ascertain even minute changes in the thermal, chemical, mechanical, biological, hydrological, atmospheric, geological, and geophysical status of the entire repository environment that might reflect, or affect, the integrity of any aspect of the site. These changes may be recorded in the properties of the material or in the physical or biological processes that affect the site. A complicating factor is that processes are coupled such that the feedback among processes may have unknown effects on related processes. In addition, the relationship between the in situ processes and their external manifestations that allow for observation and detection may be uncertain. Detection limits may not be sufficiently low. Consequently, monitoring must be systematic and comprehensive both in spatial extent and temporal frequency.
E.4 Relationship between monitoring and other scientific programs
Adaptive Staging emphasizes learning; monitoring delivers some of the new information from which one can learn. The repository learning process should be linked firmly to the scientific analysis of data collected by the monitoring program; hence, monitoring should be firmly linked to a long-term science and technology program. Overall, the targets and design of the monitoring network must be based on the scientific and conceptual understanding (and conceptual uncertainty).
The performance confirmation program is a process to test and evaluate whether the repository system is working as expected and within the acceptable safety margin. The tests and evaluations (i.e. monitoring) must be based on observations of changes in natural and engineered systems and components. Within the context of Adaptive Staging, performance confirmation is an ongoing activity to maintain confidence in safety and to provide feedback to managers and engineers on the potential need for redesign of construction or operational details.
Monitoring provides one basis for performance confirmation, because it provides data representing direct and indirect observations of the natural and engineered systems that comprise the geologic repository. Scientific analysis of monitoring data provides the second basis for performance confirmation, as it is the process by which observed behavior will be compared with expected behavior and the significance of deviations evaluated. Therefore, with Adaptive Staging the monitoring program should be closely linked both to the performance confirmation program and to the long-term science and technology program.
Monitoring methodologies must be adapted during the long time spans of the repository to take advantage of advances in technology. Figure E.2 shows the com-
mittee’s perspective on the interdependence and interaction among the long-term science and technology program, the performance confirmation program, and the monitoring program. The science program must interact with the performance confirmation program, but each has many independent goals and functions. On the other hand the monitoring program provides data to both of these programs and must receive direction and guidance from the other two programs while its products and output feed data for their respective analyses; the monitoring program would likely have few functions independent of the needs of the other two programs.
Clark, B., and R.Kleinberg. 2002. Physics in oil exploration. Physics Today 55 (4):48–53.
EPRI (Electric Power Research Institute), 2001, Performance Confirmation for the Candidate Yucca Mountain High-Level Nuclear Waste Repository, Final Report-December. Palo Alto, Calif.: EPRI.
IAEA (International Atomic Energy Agency). 2001. Monitoring of Geological Repositories for High-Level Radioactive Waste. TECDOC-1208. Vienna, Austria: IAEA.
NRC (National Research Council), 2001. Conceptual Models for Flow and Transport in the Fractured Vadose Zone. Washington, D.C.: National Academy Press.
Parker, J.T.C., and D.R.Pool. 1998. Use of microgravity to assess the effects of El Niño on ground-water storage in southern Arizona: U.S. Geological Survey Fact Sheet 060–98.
Stephens, D.B. 1995. Vadose Zone Hydrology: Boca Raton, Fla.: Lewis Publishers.
Wahr, J., and M.Molenaar. 1998. Time variability of the Earth’s gravity field: Hydrological and oceanic effects and their possible detection using GRACE: Journal of Geophysical Research 103(B12):30,205–30,229.