This is a list of notable accidents involving nuclear material. In some cases, these incidents involve people being injured or killed due to the release of radioactive contamination. Most incidents involve accidental releases that have caused contamination, but had no other immediate effects. Some incidents only had the potential to release radioactive material, and are included because of the tensions such incidents caused (collisions between nuclear-powered submarines, for instance). Due to government and business secrecy, it is difficult to determine with certainty the extent of some events listed below or, occasionally, whether they happened at all.
An accidental detonation of a nuclear weapon has never happened. For an implosion assembly weapon this risk is lower, because it would require the precisely synchronised simultaneous detonation of its numerous explosive lenses. For a gun-assembly weapon the risk is higher.
- May 21, 1946 – Canadian physicist Louis Slotin manually assembled a critical mass of plutonium while demonstrating his technique to visiting scientists at Los Alamos and suffered a fatal criticality accident. The device consisted of two half-spheres of beryllium-covered plutonium, which can be moved together slowly to measure the criticality. Normally the device would be operated by machinery, but Slotin distrusted the devices and manually operated it by holding the upper sphere with his thumb inserted in a hole in the top like a bowling ball. In most experiments, a number of washers would be arranged to prevent the two hemispheres from falling together completely, but he had removed them. In order to slowly bring the two pieces together, he rested one edge on the lower sphere and rotated a slot screwdriver between the other edge to control the separation. At one point, the screwdriver slipped and the assembly went critical while he was still holding onto it. None of the seven observers received a lethal dose, but Slotin died on the 30th from massive radiation poisoning, with an estimated dose of 1000 rads (rad), or 10 grays (Gy). In the movie Fat Man and Little Boy John Cusack played a combination of Harry K. Daghlian and Louis Slotin.  (http://web.ncf.ca/lavitt/louisslotin/2trnt10.html)
- February 13, 1950 – A B-36 en route from Alaska to perform a simulated bombing run on Californian cities developed multiple engine fires due to carburetor icing in the extreme cold. The crew dumped the single Mark IV bomb (carrying the depleted uranium tamper but not its plutonium core) off British Columbia then abandoned ship. The high explosives detonated on impact. (http://www.lutins.org/nukes.html)
- November 10, 1950 – A B-50 returning one of several US Mark IV bombs secretly deployed in Canada had engine trouble and jettisoned the weapon at 10,500 feet (3,200 m). The bomb, carrying the depleted uranium tamper but not its plutonium core ("pit"), was set to self-destruct at 2500' (750 m) and dropped over the St. Lawrence River off Rivière du Loup, Quebec. The explosion shook area residents and scattered nearly 100 pounds (45 kg) of uranium. (http://www.bullatomsci.org/issues/1999/nd99/nd99norris.html)
- April 26, 1953 – A class of radiochemistry students at Rensselaer Polytechnic Institute in Troy, New York observed high levels of radiation. Ground radiation averaged about 50 Ci/km² (2 becquerel|kBq]]/km³); some puddles register 270 nCi/L (10 Bq/m³), nearly 3000 times the United States Atomic Energy Commission limit. The radiation is traced to fallout from the Simon test, which had occurred two days previously. (http://ratical.org/radiation/SecretFallout/SFchp1.html) There is an even worse rainout in June. (http://ratical.org/radiation/SecretFallout/SFchp4.html)
- May 19, 1953 – The United States government detonated the 32-kiloton of TNT (130 TJ) bomb "Harry" at the Nevada test site. The bomb later gained the name "Dirty Harry" because of the tremendous amount of offsite fallout generated by the bomb.  (http://www.commondreams.org/views03/0526-05.htm) Winds carried fallout 135 miles (220 km) to St. George, Utah, where residents reported "an oddly metallic sort of taste in the air."  (http://www.ratical.org/radiation/KillingOurOwn/KOO3.html) A 1962 AEC report found that "children living in St. George, Utah may have received doses to the thyroid of radioiodine as high as 120 to 440 rads" (1.2 to 4.4 Gy). (http://www.thebulletin.org/issues/1997/nd97/nd96ortmeyer.html)
- March 1, 1954 – During the early morning of March 1st, a Japanese Fishing boat, the Fukuryu Maru, or "Lucky Dragon," and its crew witnessed what they believed to be the sun rising to the west of them as they sailed in the Pacific Ocean. In fact, they were witnessing the 15 megaton of TNT (63 PJ) detonation of the hydrogen bomb "Bravo" at the Bikini Atoll, 85 miles (140 km) away. Four hours later, white ash began to fall like snow onto the boat. Many of the crew members gather the ash into bags as souvenirs. Before the evening was over, the entire crew had become ill. The 23 crew members were hospitalized in Japan and one later died of kidney failure due to radiation exposure. The incident brought a rift in relations between Japan and the United States because the US did not warn Japan or any other country of the bomb's testing, leaving the Lucky Dragon exposed to the fallout. (In partial mitigation, the device yielded about 2½ times what was predicted because of an overlooked reaction; the US expanded its exclusion zones in later tests.) Fallout was enhanced by debris from coral dispersed by the explosion. The US issued an apology and paid 2 million US dollars in compensation.  (http://www.ratical.org/radiation/KillingOurOwn/KOO4.html) Additionally, in the same incident, 64 natives of Rongelap Atoll were exposed for 50 hours to fallout that produced a whole-body radiation dose of 1.75 Sv, 28 residents of Rongerik atoll were exposed to doses of about 780 mSv before being permanently evacuated, 18 residents of Alininae atoll were exposed to 680 mSv for about 50 hours, and 157 residents of Utirik atoll were exposed to 140 mSv for about 55 to 75 hours.
- First notice of radioactivity in the fallout was raised seven hours after the detonation when fallout reached Rongerik atoll. A group of 28 service members working at the weather station on Rongerik, 160 miles (260 km) east of Bikini, began evacuating about 30 hours after the explosion.
- 1955 – Unexpected wind shift dropped test fallout on Las Vegas, Nevada (http://www.ratical.org/radiation/KillingOurOwn/KOO3.html)
- November 29, 1955 – An operator's error destroyed a three-year-old experimental breeder reactor EBR-I. (http://mt.sopris.net/mpc/industrial/nuclear.operations.html) (http://www.citizen.org/cmep/foodsafety/food_irradiation/articles.cfm?ID=1461)
- July 2, 1956 – Nine individuals were injured after two explosions destroyed a portion of Sylvania Electric Products' Metallurgy Atomic Research Center in Bayside, Queens, New York.
- July 26, 1956 – A US B-47 practising landings at Lakenheath Air Base in Suffolk, England skidded into a nuclear storage mound with three Mark VI bombs inside. The resulting fire was extinguished without sparking explosions, although a secret cable by US 7th Air Division General James Walsh in Britain remarked that the bombs were "knocked about," and, "Preliminary exam by bomb disposal officer says a miracle that one Mark Six with exposed detonators sheared didn't go." He was presumably referring to the possibility of a high explosive detonation and possible radiological contamination of the area rather than a nuclear explosion. Accidental ignition of the explosives in a nuclear weapon is insufficient to trigger the nuclear explosion of an implosion assembly weapon such as those involved in the accident because this requires the precisely synchronised simultaneous detonation of its numerous explosive lenses (although it could detonate a gun-assembly weapon).
- March, 1957– Employees of a Houston company licensed by the Atomic Energy Commission to encapsulate sources for radiographic cameras opened a can containing 10 pellets of Iridium192. Using a jeweler's lathe isolated inside a Plexiglas box and 33 inches (840 mm) of concrete, the two operators discovered that two of the pellets were powderized. Some of the dust escaped the containment facility. One of the workers, dressed in street clothes, left the area while another remained, working in lab clothes and wearing a respirator. The contamination was not discovered by company personnel for a month and not by the AEC for about five weeks. The incident was reported in Look Magazine in 1961. By then, at least eight private homes and seven automobiles had been contaminated by the spreading dust. Only the two workers were found to have suffered radiation burns. The widely reported incident, in the early days of AEC civilian licensing administration, reportedly led to families of the workers being alienated from neighbors who feared contamination. Reports released by the Mayo Clinic four years after the accident found few of the radiological injuries claimed in widespread press reports, but failed to assuage public fears that followed publicity of the accident.
- May 22, 1957 – Land grants of University of New Mexico, near Albuquerque, New Mexico: A bomber accidentally dropped a 10-megaton of TNT (40 PJ) hydrogen bomb. The trigger explosive detonated, creating a 12 foot (4 m) deep crater 25 feet (8 m) across. Some radiation was detected.
- July 28, 1957 – A C-124 Globemaster with 3 nuclear weapons and a nuclear capsule from Dover Air Force Base lost power in two engines. Two weapons were jettisoned somewhere off Rehobeth, Delaware and Cape May, New Jersey/Wildwood, New Jersey; they were reportedly never found.
- 1957 – Keleket Co.: A capsule of radium salt burst leading to a five-month decontamination that cost US$250,000. The capsule was used to calibrate the radiation-measuring devices produced there.
- September 11, 1957 – A major fire at Rocky Flats weapon mill 27 km from Denver began in a glove box and spread through the ventilation system into the stack filters. Plutonium (among lesser evils) was released, but no one was sure how much; estimates ranged from 25 mg to 250 kg. (http://www.energy-net.org/00NUKE/nhistory.htm) (http://www.cdphe.state.co.us/rf/1957fire.htm) (http://www.racteam.com/Experience/Publications/RF_1957_Fire_Risk.htm) (http://www.ratical.org/radiation/KillingOurOwn/KOO8.html)
- September 29, 1957 – Cooling system failure results in a nuclear waste storage tank steam explosion at Mayak, a spent nuclear fuel reprocessing facility near Chelyabinsk, Russia. The explosion, estimated to have the same energy as 75 tons of TNT (310 GJ), releases some 20 MCi (700 PBq) and subjecting (by various estimates) 124,000 to 270,000 people to dangerously high levels of radiation. (http://www.american.edu/projects/mandala/TED/ural.htm) Of these, only 7,500 were evacuated, most of them too late to prevent dangerous levels of exposure. Inadequate medical records mean that number of people that died as a result is unknown, but probably numbers hundreds. A series of less prominent accidents preceded and followed this meltdown, in addition to a polluted water supply for people remaining in the area. More than 500,000 inhabitants of the region have been exposed to radiation as a result. Approximately 41,000 acres (166 km²) of the worst contamination region has been designated a 'nature reserve', where scientists study the effects of it on wildlife. The US government learnt of the accident but kept it secret to avoid turning public opinion against the fledgling US nuclear industry. The accident was revealed by the US government in 1977 as a result of the Freedom of Information Act, and only admitted by the Russian government in 1992.
- October 8–12, 1957 – Windscale Pile No. 1 at Sellafield north of Liverpool, England began an annealing process to release Wigner energy from graphite portions of the reactor. The reactor that burned was one of two air-cooled graphite-moderated natural uranium reactors at the site used for production of plutonium. Technicians mistakenly overheated the reactor pile because poorly placed temperature sensors indicated the reactor was cooling rather than heating, leading to failure of a nuclear cartridge, which allowed uranium and irradiated graphite to react with air. The nuclear fire burned four days, melting and consuming a significant portion of the reactor core. About 150 burning fuel cells could not be lifted from the reactor core, but operators succeeded in creating a fire break by removing nearby fuel cells. A risky effort to cool the graphite core with water eventually quenched the fire. The air-cooled reactor had spewed radioactive gases throughout the surrounding countryside. Milk distribution was banned in a 200 mile² (520 km²) area around the reactor. Over the following years, Pile No. 1 and neighboring Pile No. 2 were shut down, although nuclear decommission work resumed in 1990 and continued at least through 1999. The incident, similar in scale to the Three Mile Island meltdown, was later blamed for dozens of cancer deaths. (http://www.nucleartourist.com/events/windscal.htm) (http://www.lakestay.co.uk/1957.htm) (http://www.british-energy.com/media/factfiles/mn_item57.html) (http://www.bellona.no/en/energy/nuclear/sellafield/wp_5-2001/21663.html)
- January 31, 1958 – A B-47 with a fully armed nuclear weapon crashes and burns for 7 hours at a US Air Force base, 90 miles (145 km) N.E. of Rabat, Morocco. The Air Force evacuates everyone within 1 mile (1.6 km) of the base. Many vehicles and aircraft are contaminated. However, Moroccan officials are not notified.
- February 5, 1958 – A damaged B-47 off the coast of the US state of Georgia, flying near Tybee Island, jettisons a weapon lacking its nuclear core from 7200 feet (2,200 m) after attempting to land three times at Hunter Air Force Base. The plane had suffered a collision with an F-86 during simulated combat near Savannah, Georgia, and could not land safely with the heavy bomb on board. The bomb is never recovered. See Tybee Bomb for further information.
- February 28, 1958 – At the US airbase at Greenham Common, England, a B-47E of the 310th Bomb Wing developed problems shortly after takeoff and jettisoned its two 1,700 gallon external fuel tanks. They missed their designated safe impact area and one hit a hanger whilst the other struck the ground 65 feet (20 m) behind a parked B-47E. The parked B-47E, which was fuelled with a pilot onboard and carrying a 1.1 megaton of TNT (4.6 PJ) B28 thermonuclear free fall bomb, was engulfed by flames. The conflagration took sixteen hours and over a million gallons of water to extinguish, partly because of the magnesium alloys used in the aircraft. The fire detonated the high explosives in the nuclear weapon and convection spread plutonium and uranium oxides over a wide area — foliage up to 13 kilometres away was contaminated with uranium-235. Although two men were killed and eight injured, the US and UK governments kept the accident secret — as late as 1985, the British Government claimed that a taxiing aircraft had struck a parked one and that no fire was involved. However two scientists, F.H. Cripps and A. Stimson, working for the Atomic Weapons Research Establishment at Aldermaston independently discovered high concentrations of radioactive contamination around the base in 1960. Their secret report referring to the accident was declassified in 1996.
- 1958 – Unexpected wind shift drops test fallout on Los Angeles, California (http://www.ratical.org/radiation/KillingOurOwn/KOO3.html)
- 1958 – Soviet military reactor near Chelyabinsk releases radioactive dust. 12 villages evacuated.
- 1958 – In the NRU reactor in Chalk River, Canada, several metallic uranium fuel rods overheat and rupture inside the core. One of the damaged rods catches fire and is torn in two while it is being removed from the core by a robotic crane. As the remote-controlled crane passes overhead carrying the larger portion of the damaged rod, a three foot (1 m) length of burning uranium fuel breaks off and falls into a shallow maintenance pit. The ventilation system is jammed in the "open" position, thereby contaminating the accessible areas of the building as well as a sizable area downwind from the reactor site. A relay team of scientists and technicians eventually extinguishes the fire by running past the pit at top speed while wearing full protective gear, dumping buckets of wet sand on the burning uranium fuel.
- March 11, 1958 – A B-47 from Hunter Air Force Base in Georgia en route to an overseas base drops an unarmed nuclear weapon into the yard of Walter Gregg and his family in Mars Bluff, near Florence, South Carolina. The trigger explodes and destroys Gregg's house, injuring six members of his family. The blast forms a crater 60 feet (20 m) wide and 30 feet (10 m) deep. Five houses and a church are also damaged. Residents carry away radioactive pieces of the bomb for souvenirs, which have to be retrieved by an Air Force cleanup crew. Five months later the Air Force pays the Greggs $54,000 of his estimated $300,000 loss.
- December 30, 1958 – A critical mass of plutonium solution is accidentally assembled during chemical purification at Los Alamos. The crane operator dies of acute radiation sickness. The March 1961 Journal of Occupational Medicine prints a special supplement medically analyzing this accident. Hand-manipulations of critical assemblies are abandoned as a matter of policy in U.S. federal facilities after this accident.
- October 1959 – One killed and 3 seriously burned in explosion and fire of prototype reactor for the USS Triton (SSRN/SSN-586) at the United States Navy's training center in West Milton, New York. The Navy stated "The explosion was completely unrelated to the reactor or any of its principal auxiliary systems," but sources familiar with the operation claim that the high-pressure air flask that exploded was to feed a crucial reactor-problem backup system.
- November 20, 1959: A chemical explosion occurred in the radio-chemical processing plant at Oak Ridge National Laboratory in Tennessee during decontamination of processing machinery. The explosion caused extensive plutonium contamination to the building, to adjacent streets and to nearby building exteriors. The explosion was theorized to have occurred after hot nitric acid was exposed to decontamination fluids containing phenol that had been left in an evaporator after operators failed to water-wash the equipment clean of decontamination fluids. (Report ORNL-2989, Oak Ridge National Laboratory). Areas that could not be effectively cleaned in the following weeks were painted with bright warning paint or with concrete. Oak Ridge officials began using secondary containment structures for radio-chemical processing facilities following the accident, which resulted in no reported injuries to personnel. The accident resulted in the release of about 15 grams of plutonium 239.
- 1961 –The USS Theodore Roosevelt (SSBN-600) attempts to dump the depleted resin from its demineralization system (used to remove dissolved radioactive minerals and particles from the primary coolant loops of submarines). The ship is contaminated when wind blows resin back onto the ship.
SL-1 reactor being removed from the National Reactor Testing Station
- At the National Reactor Testing Station in Idaho Falls, Idaho, the experimental SL-1 reactor had a critical incident with a steam explosion and a severe dispersal of radioactive material, killing three workers at the installation. With the exception of Iodine131, most of the radiation was contained within about a three-acre (12,000 m²) area. Vegetation was contaminated with I131 at levels as high as 100 times background levels as far as 20 miles (30 km) from the reactor. Radio-iodine contaminated vegetation at more than double background levels more than 50 miles (80 km) from the reactor, including about a 50 mile (80 km) stretch along the Snake River near Burley and American Falls. The portable reactor had manually actuated control rods. Moving a single rod could cause the criticality incident. The rods were known to jam in the lightweight aluminum housing. Some investigators believe that a rod stuck and then suddenly released, causing the criticality incident. Investigators never concluded why the rod had been removed. One worker was found pinned to the ceiling by a control rod, apparently driven by the steam. The accident was discovered by those outside the reactor building when radiation and thermal alarms alerted fire crews and health physicists, who discovered radiation levels exceeding 200 mR/h (2 mSv/h) hundreds of feet or meters from the reactor building. Emergency crews were at first unable to find either a fire or the workers, but encountered radiation levels as high as 1000 mR/h (10 mSv/h) inside the reactor building. One of the three workers was removed from the building but died a few hours later. The other two bodies remained in the building for several days while hundreds of rescue workers initiated recovery operations. Of those recovery personnel, 22 received radiation exposures in the range of 30 to 270 mSv, according to 1961 Atomic Energy Commission reports. The reactor was dismantled and the 13 short ton (12 metric ton) core and pressure vessel was removed several months later.
- January 24, 1961 – A B-52 bomber suffered a fire caused by a major leak in a wing fuel cell and exploded in mid-air 12 miles (20 km) north of Seymour Johnson Air Force Base, Goldsboro, North Carolina. The incident released the bomber's two Mark 39 hydrogen bombs. Five crewmen parachuted to safety, but three died—two in the aircraft and one on landing. Three of the four arming devices on one of the bombs activated causing it to carry out many of the steps needed to arm itself, such as the charging of the firing capacitors and critically the deployment of a 100-foot (30 m) diameter retardation parachute. The parachute allowed the bomb to hit the ground with little damage. The fourth arming device — the pilot's safe/arm switch — was not activated and so the weapon did not detonate. The other bomb plunged into a muddy field at around 700 miles per hour (300 m/s) and disintegrated. Its tail was discovered about 20 feet (7 m) down and much of the bomb recovered including the tritium bottle and the plutonium. However excavation was abandoned because of uncontrollable flooding by ground water and the most of the thermonuclear stage, containing uranium, was left in situ. It was estimated to lie at around 180 feet (55 m). The Air Force purchased the land and fenced it off to prevent its disturbance and it is tested regularly for contamination, although none has so far been found. See: [Broken Arrow: Goldsboro, NC http://www.ibiblio.org/bomb/].
- March 1961 – A B-52 with nuclear weapons crash-lands near Yuba City, California. (The string of such accidents over the past few years prompts President John F. Kennedy to have the weapons' safety interlocks improved.)
- July 4, 1961 – The Soviet Hotel-class K-19 submarine experiences a major accident after a reactor cooling system fails off the coast of Norway. The incident contaminates the crew, parts of the ship, and some of the ballistic missiles carried onboard, and several fatalities result. Reactor core temperatures reach 800 °C, nearly enough to melt the fuel rods, although the crew is able to regain temperature control by using emergency procedures. The movie K-19: The Widowmaker, starring Harrison Ford and Liam Neeson, tells a controversially fictionalized story of these events.
- October, 1961 On four occasions between mid-October 1961 and August 1962, United States Air Force Jupiter IRBM mobile missiles carrying 1.4 megaton (5.9 PJ) nuclear warheads were struck by lightning at their launch sites near the Gioia Del Colle Air Base, Italy. In each case, thermal batteries were activated, and on two occasions, tritium-deuterium "boost" gas was injected into the warhead pits, partially arming them. After the fourth lightning strike on a Jupiter IRBM, the U.S. Air Force placed protective lightning strike-diversion tower arrays at all of the Italian and Turkish Jupiter IRBM missiles sites. These Jupiter missiles are sometimes called "The Other Missiles Of October". Their deployment in Italy and Turkey prompted the Soviet Union to place its missiles in Cuba in 1962 causing the Cuban Missile Crisis.
- December 10, 1961 – An underground nuclear test explosion unexpectedly releases clouds of radioactive steam, causing several New Mexico highways to be closed.
- July 26, 1962 – Nuclear Test attempt Bluegill Prime from Johnston Island in the Pacific Ocean. Second failure to launch a nuclear weapon using the Thor IRBM missile. The payload consisted of two re-entry vehicles, one with an instrument pod, the other with the warhead. The missile engine malfunctioned immediately on ignition. Range safety fired the destruct system while the missile was still on the launch pad. The Johnston Island launch complex was heavily damaged and contaminated with plutonium. Three months of repairs and decontamination were necessary before tests could resume.
- April 10, 1963 – The nuclear submarine USS Thresher (SSN-593) sinks east of Boston, Massachusetts with 129 men onboard during sea trials. A year earlier, just before the end of its refit interval, the boat had been abused in a munitions test where it literally tried to approach explosions as closely as possible. The boat was refitted afterward, and sank on its sea trials. In a show of poor planning, the sea trial was conducted where the bottom was below the hull's crush depth. In the yard, destructive tests of a few silver-soldered pipe connections had failed. At the time, nondestructive testing was unknown, and no test records were available. The investigators believed that the sinking was caused by the failure of a major through-hull silver-soldered connection, such as a secondary-loop cooling inlet, and that the reactor and its design were not responsible. The reactor was not recovered.
- May 1963 – Mandan, North Dakota records the highest ever recorded concentration of strontium-90 in milk in the US, as of 2003. It probably originated at the highly secret Hanford Site.
- January 13, 1964 – A B-52 with two nuclear weapons crashes near Cumberland, Maryland.
- April 21, 1964 – A US nuclear-powered navigational satellite failed to reach orbital velocity and reentered the atmosphere 150,000 feet (46 km) above the Indian Ocean. The satellite's SNAP generator contained 17 kCi (630 TBq) of plutonium-238, which at least partially burned upon reentry. Increased levels of Pu238 were first documented in the stratosphere four months later. About 16 kCi (600 TBq) of Pu238 was estimated to have settled into the atmosphere by 1970. The EPA estimated the abortive launch resulted in far less Pu238 contamination to human lungs (0.06 mrem or 0.6 µSv) compared to fallout from weapons tests in the 1950s (0.35 mrem or 3.5 µSv).
- July 24, 1964 – An accident at a commercial nuclear fuel fabrication facility in Charlestown, Rhode Island leaves one person dead.