Repositories for low- and intermediate-level radioactive wastes in Finland
Published:January 01, 2008
The Finnish power companies TVO and FPH currently operate four nuclear reactors, two each at the Olkiluoto and Loviisa sites. Both companies are responsible for the safe management of nuclear wastes. The underground repository program for low- and intermediate-level waste was commissioned in the late 1970s. From the start, the design basis has been geological disposal, the safety of which rests on natural and engineered barriers. The disposal system should isolate the waste for a few hundred years. During this time, the radiotoxicity of the waste will decline significantly. The critical radionuclides are 14C, 239Pu/240Pu, 59Ni, 90Sr, and 137Cs. The Olkiluoto site consists mainly of micaceous gneiss intercalated with sparsely fractured tonalite. A tonalitic portion of the site was chosen for more detailed characterization. The Loviisa site consists entirely of Rapakivi granite, which is coarse grained and porphyritic. The repository layout was in both cases constrained by the local geology. At Olkiluoto, the shape of the tonalitic body bounded by fracture zones led to a vertical silo-type concept 60–100 m below the surface. At Loviisa, the subhorizontal fracture zones above the planned disposal depth favored horizontal drifts 120 m below the surface. Thorough safety assessments were conducted during the licensing process. The Olkiluoto repository was built between 1988 and 1992, and the Loviisa repository was built between 1993 and 1997. Operation of Olkiluoto was commissioned in 1992, and Loviisa was commissioned in 1998. In the future, the repositories will also be used for disposal of waste when the nuclear power plants are decommissioned.
The Finnish nuclear power plants at Olkiluoto and Loviisa have been in operation ∼25 yr. There are four power plant units, two boiling water reactors (BWRs) at Olkiluoto (2 × 840 MW) and two pressurized water reactors (PWRs) at Loviisa (2 × 488 MW). Teollisuuden Voima Oy (TVO) at Olkiluoto and Fortum Power and Heat Oy (FPH) in Loviisa are responsible for managing the nuclear waste they produce. The companies have established a joint company, Posiva Oy, to implement the disposal program for spent fuel, while other nuclear wastes are handled and disposed of by the power companies themselves. From a very early stage, the companies anticipated their responsibility for all measures relating to nuclear waste management and have been accumulating funds in advance for this purpose. The environmental policies of both companies highlight environmental responsibility, which includes management of the nuclear fuel cycle in accordance with environmental protection criteria.
The decision for geological disposal of low- and medium-level wastes was made in the early 1980s. As the result of research and development work, two underground repositories were constructed, one at each power plant site.
The repository at Olkiluoto was constructed between 1988 and 1992. Site characterization and technical design had been carried out between 1980 and 1987. Site characterization of the Loviisa repository was conducted at the same time as the work at Olkiluoto. After licensing, construction was postponed by five years and carried out between 1993 and 1997. The government granted the operating licenses for the repositories at Olkiluoto and Loviisa in 1992 and 1998, respectively. In the future, these repositories will be expanded to accommodate the radioactive wastes resulting from dismantling of the power plant units. Their final closure and sealing will take place after several decades.
Waste Types and Handling
Nuclear waste is defined in the Nuclear Energy Act as radioactive waste in the form of spent fuel, or in some other form, that is generated in connection with or as a result of the use of nuclear energy. The solid and liquid wastes arising from the controlled area of a nuclear power plant, which contain mainly short-lived beta and gamma emitters, are grouped into the following two activity categories:
Low-level waste (LLW) contains so little radioactivity that it can be handled at the nuclear power plant without any special radiation protection arrangements. The activity concentration in this waste is, as a rule, not more than 1 MBq/kg.
Intermediate-level waste (ILW) contains radioactivity to the extent that it requires effective radiation protection arrangements during handling. The activity concentration in this waste ranges from 1 MBq/kg to 10 GBq/kg.
The regulations allow conditional (case-by-case) or unconditional removal of control (clearance), provided that certain criteria are met. Mass- and surface-specific limits for unconditional removal of radiation control are given in the regulations. The limits can be applied to small waste quantities that do not exceed 100 t/yr per nuclear power plant or other nuclear installation.
The radionuclides most relevant to the long-term safety of the repository are 14C, 60Co, 59Ni, 63Ni, 90Sr, 99Tc, 129I, 135Cs, 137Cs, 238Pu, 239Pu, 240Pu, 241Pu, 241Am, and 244Cm (Vieno and Nordman, 1998).
Operating waste from the nuclear power plant at Olkiluoto includes both low-level waste and intermediate-level waste. Such waste derives from maintenance and repair activities and from various water-treatment systems. The annual accumulation of operating waste is 100–150 m3 (Posiva, 2003).
The main part of the operating waste is directly packaged for handling, storing, and disposal. Compressible low-level waste is packaged into disposal drums with a hydraulic compactor, after which the drums are compacted to half of their original height, while maintaining their initial diameter. Uncompressible low-level waste is packaged in stainless-steel or concrete containers and disposal drums. Metallic waste can be compacted before packaging.
Ion-exchange resins from water-purification systems are first dried and then mixed with heated bitumen and packed into disposal drums. Miscellaneous liquids and sludges from tank bottoms, drains, and evaporators are solidified with concrete or special solidification media by mixing them inside the disposal drums.
The Loviisa Power Plant produces intermediate-level waste and low-level waste, including plant-operating waste such as ion-exchange resins, evaporator concentrates, and maintenance waste. Intermediate-level waste and low-level waste is conditioned and stored at the plant site. Spent ion-exchange resins and evaporator concentrates are stored temporarily in liquid waste storage tanks (Kallonen, 2002; Posiva, 2003). A cementation-based solidification plant has been constructed and the licensing process is under way. Waste disposal will commence in 2008.
Dry maintenance waste from power plant service and repair work is packed into 200 L steel drums. Compressible waste is compacted into drums with a hydraulic press, thereby reducing the volume of waste by a factor of three to four.
Repository Concepts and Safety
Safety of geological disposal is based on both engineered barriers and the geological environment, which is used as a natural barrier. Safety does not depend on technical supervision or monitoring of the geological or biological environment. The role of engineered barriers is most significant for the first few hundred years. During this time, the radioactivity of the waste will diminish to a small fraction of its original level. Safety in the long-term is based on the performance of the natural barrier. Safety of the repositories has been assessed as part of the licensing process with the aid of safety analysis.
Engineered barriers include the solid form of the waste, steel drums, concrete packages, and reinforced concrete structures. Backfilling materials isolate the concrete structures from the bedrock. The sealing material for the tunnels and vaults will give support to the rock, obstruct groundwater flow, and discourage inadvertent human intrusion into the repository.
During the development of repository concepts and technical designs, the safety principles and regulatory requirements mentioned previously have been applied. The two repositories differ significantly from each other in their layout. This is mainly due to the local geological conditions and differences in waste types.
TVO's Olkiluoto repository consists of two vertical silos that are 60–100 m deep, one for intermediate-level waste and the other for low-level waste (Fig. 1). The repository has the capacity to accommodate the volumes of waste produced during the 40 yr of expected operation of the existing plants at Olkiluoto. The silo for intermediate-level waste is lined with concrete. The silos are 34 m high and 24 m in diameter. A 665-m-long inclined transport tunnel leads to the repository level, and a 400-m-long construction tunnel leads to the bottom level of the vault. A vertical shaft from the −60 m level is equipped with an elevator and provides a conduit for operating systems and ventilation. The facility is controlled from the control room at the entrance of the access tunnel. The excavated volume of the repository is 90,000 m3.
The excavated volume of Loviisa repository is currently ∼110,000 m3. The 1170-m-long transport tunnel leads to repository level −110 m. Two vertical shafts lead to the ground surface. Both shafts provide conduits for operating systems and ventilation. One of the shafts is equipped with a personnel cage. The actual repository consists of two tunnels for solid low-level waste (106 m long, with a cross section of 30 m2) and a cavern for solidified intermediate-level waste (84 m long, with a cross section of 300 m2) (Fig. 2).
At Olkiluoto, the main rock type is mica-rich gneiss. Within this banded rock mass, more homogeneous gray gneisses and granites occur. These are typically quite narrow west-east–oriented bands. The site for the repository at Olkiluoto was located in the western part of Olkiluoto Island. The geology of the area is exposed in many outcrops. The rock type is gray gneiss with a tonalitic composition. Several holes were drilled for site characterization and evaluation. The rock is rather homogeneous and sparsely fractured in comparison to the adjacent mica-rich gneiss. The tonalitic gray gneiss is subvertical and wedge-shaped downward. Due to the homogeneous nature of the rock, which means also good predictability, the decision was made to locate the repository within the tonalitic rock body. Figure 3 illustrates the bedrock structure and layout (äikäs, 1986).
The bedrock of the Loviisa site is composed of Rapakivi granite. This porphyritic granite has several variations with typical ovoid texture. Drill-hole investigations indicated gently dipping fracture zones at depth (Fig. 4). These zones are hydraulically more conductive than the average of the surrounding rock. The zones dip gently east, and their distance from each other varies. In developing the design of the repository, it was found that the structure of the bedrock favors horizontal galleries. From the hydrogeological standpoint, it was judged beneficial to locate these galleries between the two subhorizontal zones. The hydraulically conductive zone tends to mitigate the effects of the hydraulic gradient and thus reduce the flow below the zone. One supporting piece of evidence for this behavior is the salinity of the groundwater, which increases significantly below the zone (Anttila, 1988).
Finland made an early commitment to geological disposal of nuclear wastes. Research and development for selecting sites for repositories for low-level and intermediate-level waste began in the early 1980s, together with the development of repository concepts. The power companies decided to build separate repositories because land was available and the geologic characteristics were good at both power plant sites, and because significant differences occur in the waste types and in the accumulation rates of waste.
Site characterization was conducted in parallel at both sites. Site evaluation was conducted in conjunction with safety assessment as part of the licensing process. Geological models were drafted based on the information obtained from site investigations. They in turn were used for repository design and hydrogeological modeling. Radionuclides may be released from the repository to the bedrock after the engineered barriers have degraded; however, their concentrations are kept, in all cases studied, below regulatory criteria. The most important radionuclide in most calculated scenarios is 14C.
Excavation and construction of both repositories proceeded without problems. The bedrock quality was largely as predicted, based on site characterization. At Olkiluoto, the rock conditions were excellent, and the inflow of the groundwater was very small, only ∼40 L/min. At Loviisa, the gently dipping fracture zones required some grouting and reinforcement.
The repositories have successfully operated for more than 10 yr. They can be expanded in the future in case a higher capacity is needed, for example, due to the construction of new reactors in Finland.
Deep Geologic Repositories
Deep Geologic Repositories reviews the success stories of underground waste isolation. It focuses on repositories that did, do, and will permanently and safely isolate dangerous materials from the near-surface biosphere. Complementary topics address the isolation capability of average crustal rock, investigations at one representative underground research laboratory, and the geologic preservation of fission products from Precambrian nuclear reactors. An international cast of contributors presents proven practical solutions to a formerly confounding issue in environmental and engineering geology: What do we do with wastes that retain their dangerous characteristics in human terms forever? The principal answer: Recycling into the lithosphere by “reverse” mining.
- engineering properties
- environmental analysis
- fracture zones
- metamorphic rocks
- power plants
- radioactive isotopes
- radioactive waste
- rock mechanics
- site exploration
- underground installations
- underground storage
- waste disposal
- Western Europe
- Loviisa Finland