Nuclear power has various environmental impacts, both positive and negative, including the construction and operation of the plant, the nuclear fuel cycle, and the effects of nuclear accidents. Nuclear power plants do not burn fossil fuels and so do not directly emit carbon dioxide. The carbon dioxide emitted during mining, enrichment, fabrication and transport of fuel is small when compared with the carbon dioxide emitted by fossil fuels of similar energy yield, however, these plants still produce other environmentally damaging wastes. Nuclear energy and renewable energy have reduced environmental costs by decreasing emissions resulting from energy consumption.
There is a catastrophic risk potential if containment fails, which in nuclear reactors can be brought about by overheated fuels melting and releasing large quantities of fission products into the environment. In normal operation, nuclear power plants release less radioactive material than coal power plants whose fly ash contains significant amounts of thorium, uranium and their daughter nuclides.
A large nuclear power plant may reject waste heat to a natural body of water; this can result in undesirable increase of the water temperature with adverse effect on aquatic life. Alternatives include cooling towers. As most commercial nuclear power plants are incapable of online refueling and need periodic shutdowns to exchange spent fuel elements for fresh fuel, many operators schedule this unavoidable downtime for the peak of summer when rivers tend to run lower and the issue of waste heat potentially harming the fluvial environment is most acute.
Mining of uranium ore can disrupt the environment around the mine. However, with modern in-situ leaching technology this impact can be reduced compared to "classical" underground or open-pit mining. Disposal of spent nuclear fuel is controversial, with many proposed long-term storage schemes under intense review and criticism. Diversion of fresh- or low-burnup spent fuel to weapons production presents a risk of nuclear proliferation, however all nuclear weapons states derived the material for their first nuclear weapon from (non-power) research reactors or dedicated "production reactors" and/or uranium enrichment. Finally, some parts the structure of the reactor itself becomes radioactive through neutron activation and will require decades of storage before it can be economically dismantled and in turn disposed of as waste. Measures like reducing the cobalt content in steel to decrease the amount of cobalt-60 produced by neutron capture can reduce the amount of radioactive material produced and the radiotoxicity that originates from this material. However, part of the issue is not radiological but regulatory as most countries assume any given object that originates from the "hot" (radioactive) area of a nuclear power plant or a facility in the nuclear fuel cycle is ipso facto radioactive, even if no contamination or neutron irradiation induced radioactivity is detectable.
Waste streams
Nuclear power has at least three waste streams that may impact the environment: Spent nuclear fuel at the reactor site (including fission products and plutonium waste) Tailings and waste rock at uranium mining mills Releases of ill-defined quantities of radioactive materials during accidents Nuclear reprocessing and breeder reactors which can decrease the need for storage of spent fuel in a deep geological repository have faced economic and political hurdles but are in some use in Russia, India, China, Japan and France, which are among the countries with the highest nuclear energy production outside the United States. However, the U.S. has not undertaken significant efforts towards either reprocessing or breeder reactors since the 1970s instead relying on the once through fuel cycle.
Radioactive waste
High-level waste
The spent nuclear fuel from uranium-235 and plutonium-239 nuclear fission contains a wide variety of carcinogenic radionuclide isotopes such as strontium-90, iodine-131, and caesium-137. Such waste includes some of the most long-lived transuranic elements such as americium-241 and isotopes of plutonium. The most long-lived radioactive wastes, including spent nuclear fuel, usually must be contained and isolated from the environment for a long period of time. Spent nuclear fuel storage is mostly a problem in the United States, following a 1977 prohibition by then-President Jimmy Carter on nuclear fuel recycling. France, The United Kingdom, and Japan are some of the countries that have rejected the repository solution. Spent nuclear fuel produced by some types of reactors is a valuable asset, not simply waste.