FEATURE
A Testbed for Underground Nuclear Repository Design By simulating nuclear fuel disposal in a prototype repository, researchers are evaluating safety issues. STEVEN BANWART, PETER WIKBERG, A N D OLLE OLSSON
T
hroughout the world, nations are wrestling with the problem of how to safely dispose of spent nuclear fuel inventories (i). Approximately 10,000 metric tons of nuclear fuel is generated annually. At this rate, about 172,000 metric tons of spent fuel will accumulate by 2000 (see box; References 2-6). There is international consensus that, using available technology, deep geological disposal is the only option to meet necessary safety requirements without leaving potential risks and burdens to future generations (7). Crucial to this issue is a need for reliable information about the design and performance of underground nuclear waste repositories. The Aspo Hard Rock Laboratory in Oskarshamn, in southeastern Sweden, addresses this need with a prototype repository research program. Although the laboratory is not an actual repository—nuclear waste will never be put in the ground at this location—the facility comes as close as possible to study simulated emplacement of nuclear materials. The full-scale prototype repository provides opportunities for studying nuclear materials placement in a bedrock environment characteristic of future underground facilities. The Aspo research program is designed to establish site investigation methods, provide data on safetyrelated properties of rock at repository scale, develop and test the function of engineered barriers at full scale, and facilitate mathematical modeling of site performance and safety. Construction of the Aspo facility started in 1990, and it was put into regular use in 1995 (see figure on p. 511A). Activities under way include paleohydrological and solute transport studies, mathematical modeling of solute transport and groundwater flow, test and development of characterization methodology and models of the barrier function of rock, and technology demonstrations of important parts of the repository. Additional activities, including tests of backfill materials and operation of the prototype repository to simulate all steps in the deposition sequence, are planned to determine the effectiveness of future facility design, construction, and operation. 5 1 0 A • VOL. 3 1 , NO. 11, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
An international research effort The research program at Aspo is international. Information developed is relevant to policy makers in Finland, Switzerland, Japan, and France, where repository sites with similar geology may be built, and to other countries considering disposal in fractured rocks. Formal participant agreements have been established between the Swedish Nuclear Fuel and Waste Management Co., which operates the facility, and various partners, including Atomic Energy of Canada Ltd.; Power Reactor and Nuclear Fuel Development Corp., Japan; Central Research Institute of Electric Power Industry, Japan; Agence Nationale pour la Gestion des Dechets Radioactifs, France; POSIVA Oy, Finland; U.K. Nirex Ltd.; Nationale Genossenschaft fur die Lagerung von Radioaktiver Abfalle, Switzerland; Empresa Nacional de Residuos Radiactivos, Spain; and Bundesministerium fur Bildung, Wissensshaft, Forschung und Technologies, Germany. Regional and local site geology and hydrogeology at Aspo have been studied extensively since 1986. The bedrock protects engineered barriers isolating nuclear waste and provides long-term, stable chemical conditions favoring barrier longevity, low groundwater flow, and mechanical stability for excavation and construction. An important aim of site investigations at Aspo is to demonstrate that investigation methods provide needed data at repository scale about safety-related properties of rock. Collected field data are used to develop conceptual and mathematical models of site conditions (e.g., geological structure, groundwater flow, and hydrochemistry). Once developed, the models are used to assess future repository site performance and safety. Available results confirm that methods for preinvestigation of the Aspo site are reliable, particularly the ability to identify orientation, structural and hydraulic properties, and the hydrochemical character of major water-bearing fractures and fracture zones (8, 9). In developing the site, unexpected conditions requiring changes in layout or construction were not encountered. Geological barrier performance must be under0013-936X/97/0931-510A$14.00/0 © 1997 American Chemical Society
stood if contaminants, potentially transported by groundwater movement, are to be contained. Three-dimensional models for groundwater flow have been developed describing flow of groundwater with variable salinity within a volume large enough to represent a deep repository. Groundwater flow paths at the Aspo site are dominated by major fractures and fracture zones within the granite bedrock. Extensive hydraulic testing in boreholes drilled from the surface established parameters for groundwater flow modeling, the transmissivity of major water-bearing structures within the rock volume, and the hydraulic conductivity of the lowpermeability rock mass between them. Spatial variations in groundwater composition identified glacial meltwater, old Baltic Sea water, and modern recharge water within the rock volume. At depths greater than 500 meters (m), the groundwater's composition indicates that it has not been affected by the different stages of the Baltic Sea evolution since the last glaciation, an indication of the deep environment's stability over repository time scales (10,000-100,000 years).
The Hard Rock Laboratory The Aspo laboratory consists of a research complex situated on the surface and an access tunnel that extends to a depth of 450 meters. Research stations have been constructed at various points below ground.
Repository characteristics Most national repository concepts consider disposal in hydraulically saturated granite, clay, or salt formations. An exception is the U.S. Department of Energy project at Yucca Mountain in Nevada. The mountain rises 800 m above the surrounding valleys, with a depthto-water table reaching up to 700 m. The potential repository site is located approximately 300 m below ground surface in a volcanic tuff formation. At this depth, the site is still several hundred meters above the water table and is characterized by hydraulically unsaturated conditions. The repository concept minimizes groundwater transport as a migration pathway for radionuclides and prevents degradation of the waste form and engineered barriers under arid conditions. The Swedish concept involves disposal within a tunnel system located 500 m deep in granite bedrock. Disposal is based on containment of spent nuclear fuel within multiple engineered and natural barriers. The primary function of a repository is to maximally isolate wastes from the biosphere. The Swedish design concept buries spent nuclear fuel in copper canisters surrounded by a bentonite buffer within a stable geology at a depth of about one-half kilometer (km) without active operation and with no
plans for retrieval after closure (7,10). Safety assessment for geological disposal is made within a sourcepathway-target risk analysis framework. Scenarios for the evolution of site conditions are developed, processes that prevent radionuclide migration and determine barrier (engineered and geological) performance are modeled, and radiological exposure and distribution of radionuclides in the biosphere are evaluated. The ultimate aim is an integrated risk assessment of radiological consequences to humans and the environment over the lifetime of the planned repository. Engineered barriers must deter release of contaminants for as long as possible. A lengthy travel distance for groundwater flow between repository and surface ensures long time periods for radioactive decay before soluble radionuclides reach the biosphere. Stagnant or very slow groundwater flow prevents corrosive surface water that can degrade engineered barriers from reaching repository depths, VOL.31, NO. 11, 1997 /ENVIRONMENTAL SCIENCE Si TECHNOLOGY / N E W S " 5 1 1 A
High-level nuclear waste inventories worldwide Projected national inventories of high-level waste (HLW) produced by 2000 are substantial. Country
Belgium Canada Finland France Germany Japan Spain Sweden Switzerland United Kingdom United States Eastern Europe 3
Other a
HLW (metric tons)
'
3000 20,500 1700 19,000 10,100 18,000 2000 4700 2000 3250 35,000 30,000
22,750
Includes Armenia, Bulgaria, Czech Republic, Hungary, Kazakhstan, Lithuania, Romania, Russia, Slovak Republic, and Ukraine.
Source: Data are from References 2-6.
and if soluble radionuclides are released, it acts to slow their migration. Dilution by groundwater at shallower depths reduces any residual radioactivity dose to the biosphere. Before deposition in the repository, spent fuel assemblies are encapsulated in cylindrical copper canisters that are lined with inner steel containers to provide mechanical stability. The canisters isolate the spent fuel from groundwater. Special deposition holes (in which the canisters are placed) are bored in the tunnel floors. Following placement, each canister is surrounded by a compacted, sodium bentonite clay buffer. The spent fuel is a sparingly soluble ceramic material primarily composed of uraninite [U02(s)] that can degrade only if it comes in contact with groundwater. Bombardment of water by radiation causes radiolysis. The reaction splits water molecules, producing hydrogen gas and oxidants such as 0 2 . Subsequent corrosion by 0 2 converts U02(s) into U308(s), another sparingly soluble solid. If oxic conditions persist, U308(s) can be further oxidized to hexavalent uranium, which is soluble and can be transported with groundwater flow. Corrosion of the inner steel containers consumes oxidants and forms a passive layer of iron oxides, for example, magnetite and ferric hydroxide. Under anoxic conditions, the steel can corrode by reaction with water, producing hydrogen gas and iron oxides. The selected copper canister has a slow rate of corrosion. Considerable effort is being invested in developing quality assurance and control systems during fabrication trials {10). Copper can also corrode in the presence of hydrogen sulfide, although this process is slower and occurs uniformly over the metal surface. A conservative estimate of the composite corrosion rate is 5 millimeters (mm) in 105 years (10). The compacted buffer expands when wetted by groundwater, decreasing its hydraulic conductivity and slowing transport of soluble oxidants toward the 5 1 2 A • V O L . 3 1 , NO. 11, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
canister and dissolved radionuclides away from it. The buffer also provides an adsorption barrier for migration of radionuclides from the canister if it is breached. At high temperatures in excess of 100 °C, the backfill loses its swelling properties. Any process causing buffer alteration, and hence degradation of buffer performance, must be investigated and quantified. Studies of buffer performance at temperatures higher than the design temperature of 90 °C will provide information about the safety margin of the current buffer design. The geological barrier provides mechanical stability for excavation and construction, protection for the engineered barriers, and stable chemical conditions that favor engineered barrier longevity. It also reduces the impact of groundwater flow driven by topographical relief and increases the hydraulic residence time along flow paths to the surface. The reducing capacity associated with ferrous iron and sulfide found in fracture minerals and dissolved in groundwater provides a barrier to dissolved oxygen in the groundwater recharge. The reaction of dissolved 0 2 with ferrous iron (mineral and dissolved) is known to be rapid in groundwater and exhibits a characteristic half-life (up to a few days' duration) for 0 2 [11). Microbiological processes help maintain reducing conditions by catalyzing the consumption of oxidants in the groundwater. Establishment of reducing conditions is important because it prevents corrosion of engineered barriers. Moreover, several long-lived radionuclides—uranium, neptunium, and technetium—are much less soluble and mobile under reducing conditions.
Evaluating possible repository sites Feasibility studies are used to evaluate repository siting within four candidate municipalities in Sweden. These studies will identify possible sites for detailed investigations and outline possible consequences of siting a repository within each municipality and region. Detailed geotechnical and related performance assessment studies will be done for potentially suitable sites. Planning and research activities are underway for this phase of the program. Site investigations are scheduled to start after 1999. Continuous regulatory review and licensing preparation will occur in parallel with detailed site characterization, repository design, site-specific repository safety assessment, and environmental impact assessment. The target date for placement of the first spent fuel is 2010. Site characterization programs provide information required for adapting the repository layout to geological features and for making a safety assessment of a site. Aspo has been used as a test site for developing the site characterization program that will be used at a future candidate site. A summary of important results is presented below. Aspo is located within the Precambrian basement, which is dominated by Smaland granite (age about 1750 million years) in southeastern Sweden. Detailed geological mapping of topographical and geophysical (magnetic and seismic reflection profiling) lineaments, supported by data from three initial boreholes, provided the basis for detailed site
selection. Results from core logging and geophysical measurements (electrical, magnetic, and radiometric logs; borehole radar; and vertical seismic profiling) in additional boreholes, subsequently drilled in the selected area, characterized the rock mass at design depths. Collected data were used to develop a model predicting structural and lithological features within the rock volume to be excavated during construction of the laboratory. Extensive tests (pumping, interference, injection, and flowmeter logging) were systematically carried out on all cored boreholes. Water pressure was monitored before and during the tests and during tunnel construction. These measurements were used to evaluate hydraulic properties, identify hydraulic connections between major features, and monitor changes in water pressure during construction. The resultant geohydrological description of the rock volume was used to predict characteristic physical properties expected in the excavated tunnel: hydraulic conductivity along the tunnel, transmissivity of major conductive fractures, distance between fractures with a transmissivity greater than a specified value, and groundwater flow rate into the tunnel. Mean hydraulic conductivity at the approximate 15-m scale ranged from
