There is a "catastrophic risk" potential if containment fails,[2] which in nuclear reactors can be brought about by overheated fuels melting and releasing large quantities of fission products into the environment. The most long-lived radioactive wastes, including spent nuclear fuel, must be contained and isolated for a long period of time. However, spent nuclear fuel can sometimes be reused, reducing the amount of waste. Emission of radioactivity from a nuclear plant is controlled by regulations. Abnormal operation may result in release of radioactive material on scales ranging from minor to severe, although these scenarios are very rare.[3] 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.[4]

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.[5] 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.[6] This is especially pronounced in France, which produces some 70% of electricity with nuclear power plants and where electric home heating is widespread. However, in regions with high heating, ventilation, and air conditioning power use, the summer season, rather than imposing lower power demands, may be the peak season of electricity demand, complicating scheduled summer shutdowns

The Onagawa Nuclear Power Plant – a plant that cools by direct use of ocean water, not requiring a cooling tower 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. 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. 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.[7] 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.

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.

https://en.m.wikipedia.org/wiki/List_of_nuclear_power_accidents_by_country