Top 10 Worst Nuclear Incidents

A magnitude-9.0 earthquake and 14-meter tsunami knocked out backup cooling at three reactors, causing triple meltdowns and hydrogen explosions that released radioactive cesium and iodine into the Pacific. Over 154,000 residents were evacuated, and the cleanup is projected to take 40 years and cost over $200 billion.

A failed cooling system at the Mayak plutonium production facility near Chelyabinsk caused a chemical explosion in a radioactive waste tank, releasing 20 million curies of contamination. The Soviet government secretly evacuated 10,000 people and suppressed all information about the accident for decades.

A partial meltdown of Reactor 2 in Pennsylvania, caused by a stuck-open relief valve and confusing control room indicators, released small amounts of radioactive gas. While health effects were minimal, the incident effectively halted new nuclear plant construction in the United States for over 30 years.

A graphite fire in an air-cooled plutonium production reactor in Cumberland (now Sellafield) released radioactive iodine-131 and polonium-210 across the UK and northern Europe. An estimated 240 cancer cases resulted, and 2 million liters of contaminated milk were dumped.

Scrap metal scavengers in Goiânia broke open an abandoned radiotherapy source containing cesium-137, fascinated by its blue glow. They distributed the powder to family and friends, resulting in 4 deaths, 249 contaminated individuals, and the demolition of several homes in one of the worst radiological contamination events outside the nuclear industry.

A manual control rod withdrawal at a small Army reactor in Idaho caused an instantaneous criticality excursion that killed all three operators — one was impaled on the ceiling by a shield plug. It remains the only fatal nuclear reactor accident in U.S. history and was initially shrouded in Cold War secrecy.

Workers at a uranium processing plant in Ibaraki Prefecture manually poured enriched uranium solution into a precipitation tank using buckets, bypassing safety protocols. The resulting uncontrolled nuclear chain reaction killed two workers over agonizing months and exposed 667 people to radiation.

The United States' largest thermonuclear detonation at Bikini Atoll yielded 15 megatons — 2.5 times the predicted yield — showering the crew of the Japanese fishing vessel Daigo Fukuryū Maru and Marshall Islanders with lethal radioactive fallout, causing international outrage and galvanizing the global nuclear disarmament movement.

Two separate incidents at Canada's NRX and NRU research reactors involved partial fuel meltdowns and significant radioactive contamination. Future U.S. President Jimmy Carter, then a Navy nuclear engineer, led a cleanup team in 1952. The events drove major improvements in reactor safety design that influenced the global nuclear industry.

A magnitude-9.0 earthquake and 14-meter tsunami knocked out backup cooling at three reactors, causing triple meltdowns and hydrogen explosions that released radioactive cesium and iodine into the Pacific. Over 154,000 residents were evacuated, and the cleanup is projected to take 40 years and cost over $200 billion.

A failed cooling system at the Mayak plutonium production facility near Chelyabinsk caused a chemical explosion in a radioactive waste tank, releasing 20 million curies of contamination. The Soviet government secretly evacuated 10,000 people and suppressed all information about the accident for decades.

A partial meltdown of Reactor 2 in Pennsylvania, caused by a stuck-open relief valve and confusing control room indicators, released small amounts of radioactive gas. While health effects were minimal, the incident effectively halted new nuclear plant construction in the United States for over 30 years.

A graphite fire in an air-cooled plutonium production reactor in Cumberland (now Sellafield) released radioactive iodine-131 and polonium-210 across the UK and northern Europe. An estimated 240 cancer cases resulted, and 2 million liters of contaminated milk were dumped.

Scrap metal scavengers in Goiânia broke open an abandoned radiotherapy source containing cesium-137, fascinated by its blue glow. They distributed the powder to family and friends, resulting in 4 deaths, 249 contaminated individuals, and the demolition of several homes in one of the worst radiological contamination events outside the nuclear industry.

A manual control rod withdrawal at a small Army reactor in Idaho caused an instantaneous criticality excursion that killed all three operators — one was impaled on the ceiling by a shield plug. It remains the only fatal nuclear reactor accident in U.S. history and was initially shrouded in Cold War secrecy.

Workers at a uranium processing plant in Ibaraki Prefecture manually poured enriched uranium solution into a precipitation tank using buckets, bypassing safety protocols. The resulting uncontrolled nuclear chain reaction killed two workers over agonizing months and exposed 667 people to radiation.

The United States' largest thermonuclear detonation at Bikini Atoll yielded 15 megatons — 2.5 times the predicted yield — showering the crew of the Japanese fishing vessel Daigo Fukuryū Maru and Marshall Islanders with lethal radioactive fallout, causing international outrage and galvanizing the global nuclear disarmament movement.

Two separate incidents at Canada's NRX and NRU research reactors involved partial fuel meltdowns and significant radioactive contamination. Future U.S. President Jimmy Carter, then a Navy nuclear engineer, led a cleanup team in 1952. The events drove major improvements in reactor safety design that influenced the global nuclear industry.