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.