What Is Nuclear Waste?
When electricity is produced using a nuclear power plant, this generates nuclear waste, which – due to its radioactivity – demands special treatment.
Low-level and intermediate-level waste, also known as reactor waste, is generated during the operation and maintenance of a nuclear power plant. Some structures of the plant, such as the pressure vessel and the associated piping, become radioactive or are contaminated with radioactive substances. When the plant is closed down, all radiation-emitting waste must be disposed of. Both reactor waste and decommissioning waste are disposed of in the reactor waste repository. The nuclear power companies Teollisuuden Voima Oyj and Fortum Power and Heat Oy take care of their reactor waste and decommissioning waste themselves.
Nuclear power plants use uranium as their fuel. Fission products from uranium are extremely radioactive at first. Teollisuuden Voima Oyj and Fortum Power and Heat Oy established Posiva Oy in order to take care of their spent nuclear fuel. This fuel will be disposed of in the bedrock of the Olkiluoto island in the municipality of Eurajoki.
Spent Nuclear Fuel
Uranium, the fuel used at nuclear power plants, generates radioactive fission products. Spent nuclear fuel is high-level nuclear waste, and shall be taken care of in such a manner that it does not harm the biosphere.
Composition of Spent Nuclear Fuel
Nuclear power plant fuel is fabricated from uranium ore excavated from the bedrock. In the early stages of the fuel’s life cycle the ore is processed into nuclear fuel assemblies. The uranium fuel coming into the plant is packed as ceramic pellets in metal tubes about 1 cm wide and 3 to 5 meters long, also known as fuel rods, that are then joined together into fuel assemblies containing 60 to 265 fuel rods.
Spent and fresh nuclear fuel looks similar on the outside, but in spent fuel some of the uranium has been converted into radioactive fission products and transuranium elements that are heavier than uranium. 96% of spent fuel is still uranium.
Radioactivity Decreases Quickly
The uranium, fission products and transuranium elements contained in the spent fuel are radioactive, which means they slowly decay into other substances and, finally, into permanently non-radioactive substances. Some substances only take a few fractions of a second to decay, others need billions of years. Decaying radioactive substances emit radiation.
Spent nuclear fuel is very radioactive immediately after use, but within a year the activity is already reduced to one-hundredth of the original level. In the beginning of the final disposal, which happens approximately 40 years after removal from the reactor, only 1/1,000 of the original radioactivity is left in the fuel. The radioactivity of the substances that emit the highest doses of radiation is slowly reduced, after which only the long-lived, slowly decaying substances remain.
Fuel Cycle and Uranium Content of the Fuel Assemblies
Each fuel assembly in the Olkiluoto 1 and 2 plant units contain about 180 kg of uranium. In the Loviisa 1 and 2 plant units, the amount is 120 kg. In Olkiluoto, the fuel assemblies are placed in the reactor for about four years; in Loviisa, for about three years. Each year, some of the spent fuel is replaced with fresh fuel, which, at Olkiluoto, means more than 100 and at Loviisa about 120 new assemblies each year.
Fuel Assemblies Removed from Reactors
Fuel assemblies removed from reactors are first cooled down in the water basins inside the reactor building. After a few years they are transferred to an interim storage for spent nuclear fuel, located at the power plant site, where the assemblies are submerged in water for dozens of years to wait for final disposal. During this time the radioactivity and heat generation of the spent fuel is reduced to a level where final disposal is possible.
Final Disposal of Spent Fuel
The nuclear power companies Teollisuuden Voima Oyj and Fortum Power and Heat Oy established Posiva Oy in 1995 in order to take care of the research and practical measures relating to the final disposal of the spent fuel generated in their nuclear power plants.
Low-level and intermediate-level waste, also known as reactor waste, is generated during the operation and maintenance of a nuclear power plant. The nuclear power companies Teollisuuden Voima Oyj and Fortum Power and Heat Oy take care of their reactor waste themselves.
How Reactor Waste is Generated
Low-level waste is maintenance waste that has been contaminated with radioactive substances. Low-level waste includes, among other objects, protective plastic sheets, tools, protective clothing and towels used during maintenance work, as well as engine parts and piping removed from the plant. The compound used in cleaning the circulating water is intermediate-level waste. The annual amount of reactor waste generated in Olkiluoto is from 150 to 200 m³ and in Loviisa from 100 to 150 m³.
Treatment of Reactor Waste
Low-level waste is compressed and packed into 200-litre barrels. Non-compressible waste is packed into steel or concrete boxes. In Olkiluoto, the intermediate level ion-exchange resins is solidified into bitumen, packed into steel barrels and placed in the waste repository. In Loviisa, the waste is solidified into cement and packed into concrete barrels.
Final Disposal of Reactor Waste
Reactor waste is first stored inside the power plant and then in separate interim storage facilities for low and intermediate waste. From there, the waste is transferred to a repository facility (RW repository) excavated at the plant site.
The reactor waste repository at Olkiluoto was commissioned in 1992 and the one in Loviisa in 1997. The RW repositories have been excavated to a depth of 60 to 100 meters inside the bedrock. The waste is transferred to the repository through a tunnel with a special-purpose car. When the repository is no longer being used, all connections to the repository are sealed off. After that, the repository does not need any monitoring. Over time, the radioactive substances contained in the waste lose their harmful effects on the environment. The waste poses no threat to the biosphere.
Some nuclear power plant structures become radioactive during the operation, and these structures have to be disposed of when the plant is closed down. The most important plant structure is the reactor pressure vessel. Later, the radioactive structures and equipment of the encapsulation plant yet to be commissioned will also be disposed of.
The actions after a nuclear power plant is closed down, including the final disposal of decommissioning waste, are called decommissioning. At both Loviisa and Olkiluoto, the decommissioning waste will most likely be disposed of in a reactor waste repository.
Treatment of Decommissioning Waste at Olkiluoto
The decommissioning in Olkiluoto has been planned to take place 30 years after closing down the power plant units. The advantage with deferred dismantling is that the quantity of short-lived radionuclides is decayed to a half and the amount of radioactive waste is diminished.
Treatment of Decommissioning Waste at Loviisa
At Loviisa, it has been planned that some of the structures that are not necessary for the other activities taking place at the site (such as storage of spent fuel, solidification of waste, and final disposal of low-level and intermediate-level waste) will be decommissioned immediately after the plant operation is terminated.
Nuclear Waste is Generated in Nuclear Power Plants
There are a total of four reactor units in the two Finnish nuclear power plants in Eurajoki and Loviisa. The fifth is under construction. Some of the waste generated in nuclear electricity production is radioactive, demanding careful attention.
The Olkiluoto units 1 and 2, belonging to Teollisuuden Voima Oyj, have a net capacity of 860 MW for each plant unit. The Loviisa units 1 and 2, belonging to Fortum Power and Heat Oy, have a net capacity of 488 MW for each plant unit. Once finished, OL3, the fifth Finnish nuclear power plant unit to be constructed in Olkiluoto, will have a net capacity of more than 1600 MW.
Operation of Plant Units
The first Loviisa unit was finished in 1977 and the second in 1980. The Olkiluoto units were finished in 1978 and 1980.