nuclear and radiation accident is defined by the International Atomic Energy Agency as “an event that has led to significant consequences to people, the environment or the facility. Examples include lethal effects to individuals,large radioactivity release to the environment, or reactor core melt.”[1] The prime example of a “major nuclear accident” is one in which a reactor core is damaged and large amounts of radiation are released, such as in the Chernobyl Disaster in 1986.

The likelihood and potential impact of nuclear accidents has been a topic of debate practically since the first nuclear reactors were constructed. It has also been a key factor in public concern about nuclear facilities.[2] Many technical measures to reduce the risk of accidents or (should one occur) to minimize the amount of radioactivity released to the environment have been adopted. Despite the use of such measures, “there have been many accidents with varying impacts as well near misses and incidents”.[2]

Stuart Arm states, “apart from Chernobyl, no nuclear workers or members of the public have ever died as a result of exposure to radiation due to a commercial nuclear reactor incident.”[3]

Benjamin K. Sovacool has reported that worldwide there have been 99 accidents at nuclear power plants.[4] Fifty-seven accidents have occurred since the Chernobyl disaster, and 57% (56 out of 99) of all nuclear-related accidents have occurred in the USA. Relatively few accidents involved fatalities.[4]

Serious radiation accidents include the radiotherapy accident in Costa Rica,[5] radiotherapy accident in Zaragoza,[6] radiation accident in Morocco,[7] Goiania accident,[8] radiation accident in Mexico City, radiotherapy unit accident in Thailand,[9] and the Mayapuri radiological accident in India.[9]

The International Atomic Energy Agency maintains a website reporting recent accidents: [14].

Pathways from airborne radioactive contamination to man

Accident types

[edit]Loss of coolant accident

Main article: Loss of coolant

[edit]Criticality accidents

criticality accident (also sometimes referred to as an “excursion” or “power excursion”) occurs when a nuclear chain reaction is accidentally allowed to occur in fissile material, such as enriched uranium or plutonium. The Chernobyl accident is an example of a criticality accident. This accident destroyed a reactor at the plant and left a large geographic area uninhabitable. In a smaller scale accident at Sarov a technician working with highly enriched uranium was irradiated while preparing an experiment involving a sphere of fissile material. The Sarov accident is interesting because the system remained critical for many days before it could be stopped, though safely located in a shielded experimental hall.[25] This is an example of a limited scope accident where only a few people can be harmed, while no release of radioactivity into the environment occurred. A criticality accident with limited off site release of both radiation (gamma and neutron) and a very small release of radioactivity occurred at Tokaimura in 1999 during the production of enriched uranium fuel.[26] Two workers died, a third was permanently injured, and 350 citizens were exposed to radiation.

[edit]Decay heat

Decay heat accidents are where the heat generated by the radioactive decay causes harm. In a large nuclear reactor, a loss of coolant accident can damage the core: for example, at Three Mile Island a recently shutdown (SCRAMedPWR reactor was left for a length of time without cooling water. As a result the nuclear fuel was damaged, and the core partially melted. The removal of the decay heat is a significant reactor safety concern, especially shortly after shutdown. Failure to remove decay heat may cause the reactor core temperature to rise to dangerous levels and has caused nuclear accidents. The heat removal is usually achieved through several redundant and diverse systems, and the heat is often dissipated to an ‘ultimate heat sink’ which has a large capacity and requires no active power, though this method is typically used after decay heat has reduced to a very small value. However, the main cause of release of radioactivity in the Three Mile Island accident was a pilot-operated relief valve on the primary loop which stuck in the open position. This caused the overflow tank into which it drained to rupture and release large amounts of radioactive cooling water into the containment building.

In 2011, an earthquake and tsunami caused a loss of power to two plants in Fukushima, Japan, crippling the reactor as decay heat caused 90% of the fuel rods in the core of the Daiichi Unit 3 reactor to become uncovered.[27] As of March 20, 2011, the removal of decay heat is still a cause for concern.


Transport accidents can cause a release of radioactivity resulting in contamination or shielding to be damaged resulting in direct irradiation. In Cochabamba a defective gamma radiography set was transported in a passenger bus as cargo. The gamma source was outside the shielding, and it irradiated some bus passengers.

In the United Kingdom, it was revealed in a court case that in March 2002 a radiotherapy source was transported from Leeds to Sellafield with defective shielding. The shielding had a gap on the underside. It is thought that no human has been seriously harmed by the escaping radiation.[28]

[edit]Equipment failure

Equipment failure is one possible type of accident, recently at Białystok in Poland the electronics associated with a particle accelerator used for the treatment of cancer suffered a malfunction.[29] This then led to the overexposure of at least one patient. While the initial failure was the simple failure of a semiconductor diode, it set in motion a series of events which led to a radiation injury.

A related cause of accidents is failure of control software, as in the cases involving the Therac-25 medical radiotherapy equipment: the elimination of a hardware safety interlock in a new design model exposed a previously undetected bug in the control software, which could lead to patients receiving massive overdoses under a specific set of conditions.

[edit]Human error

A sketch used by doctors to determine the amount of radiation to which each person had been exposed during the Slotin excursion.

An assessment conducted by the Commissariat à l’Énergie Atomique (CEA) in France concluded that no amount of technical innovation can eliminate the risk of human-induced errors associated with the operation of nuclear power plants. Two types of mistakes were deemed most serious: errors committed during field operations, such as maintenance and testing, that can cause an accident; and human errors made during small accidents that cascade to complete failure.[30]

In 1946 Canadian Manhattan Project physicist Louis Slotin performed a risky experiment known as “tickling the dragon’s tail”[31] which involved two hemispheres of neutron-reflective beryllium being brought together around a plutonium core to bring it to criticality. Against operating procedures, the hemispheres were separated only by a screwdriver. The screwdriver slipped and set off a chain reaction criticality accident filling the room with harmful radiation and a flash of blue light (caused by excited, ionized air particles returning to their unexcited states). Slotin reflexively separated the hemispheres in reaction to the heat flash and blue light, preventing further irradiation of several co-workers present in the room. However Slotin absorbed a lethal dose of the radiation and died nine days afterwards. The infamous plutonium mass used in the experiment was referred to as the demon core.

[edit]Lost source

Lost source accidents,[32][33] also referred to as an orphan source are incidents in which a radioactive source is lost, stolen or abandoned. The source then might cause harm to humans. For example, in 1996 sources were left behind by theSoviet army in Lilo, Georgia.[34] Another case occurred at Yanango where a radiography source was lost, also at Samut Prakarn a cobalt-60 teletherapy source was lost[35] and at Gilan in Iran a radiography source harmed a welder.[36] The best known example of this type of event is the Goiânia accident which occurred in Brazil.

The International Atomic Energy Agency has provided guides for scrap metal collectors on what a sealed source might look like.[37][38] The scrap metal industry is the one where lost sources are most likely to be found.[39]


Some accidents defy classification. These accidents happen when the unexpected occurs with a radioactive source. For instance if a bird were to grab a radioactive source containing radium from a window sill and then fly away with it, return to its nest and then die shortly afterwards from directirradiation then a minor radiation accident would have occurred. As the hypothetical act of placing the source on a window sill by a human permitted the bird access to the source, it is unclear how such an event should be classified, as a lost source event or a something else. Radium lost and found[40][41] describes a tale of a pig walking about with a radium source inside; this was a radium source lost from a hospital. There are also accidents which are “normal” industrial accidents that involve radioactive material. For instance a runaway reaction at Tomsk involving red oil caused radioactive material to be spread around the site.

For a list of many of the most important accidents see the International Atomic Energy Agency site.[42

Japanese nuclear incidents

Date Incident level Location Type
Description Notes
6 August 1945 Nuclear bombing Hiroshima 13kt explosion
9 August 1945 Nuclear bombing Nagasaki 21kt explosion
5 December 1965 Broken arrow coast of Japan Loss of a nuclear bomb
A USN aircraft with one B43 nuclear bomb fell off the aircraft carrier Ticonderoga into 16,200 feet (4,900 m) of water while the ship was underway from Vietnam to Yokosuka, Japan. The weapon was never recovered. Navydocuments show it happened about 80 miles (130 km) from the Ryukyu Islands and 200 miles (320 km) from Okinawa.[1]
March 1981 INES Level 2 Tsuruga Overexposure of workers
More than 100 workers were exposed to doses of up to 155 millirem per day radiation during repairs of a nuclear power plant.[2]
June 1999 INES Level 2[3] Shika plantIshikawa Prefecture Control rod malfunction
Operators attempting to insert one control rod accidentally withdrew three causing a 15 minute uncontrolled sustained reaction at the number 1 reactor of Shika Nuclear Power Plant[4]
30 September 1999 INES Level 4 Ibaraki Prefecture Accidental criticality

During preparation of a uranyl nitrate solution, uranium in solution exceeded the critical mass, at a uranium reprocessing facility in Tokai-mura northeast of Tokyo, Japan. Three workers were exposed to (neutron) radiation doses in excess of allowable limits. Two of these workers died. 116 other workers received lesser doses of 1 mSv or greater though not in excess of the allowable limit.[5][6][7][8]

11-15 March 2011 INES Level 3[citation needed] Fukushima II Nuclear Power PlantFukushima Prefecture Overheating, possible radioactivity emergency

After the 2011 Tōhoku earthquake and tsunami of March 11, the cooling systems for three reactors (numbers 1, 2 and 4) of the Fukushima II (Fukushima Dai-ni) nuclear power plant were compromised due to damage from the tsunami.[9] Nuclear Engineering International reported that all four units were successfully automatically shut down, but emergency diesel generators at the site were out of order.[10] People were evacuated around 10 kilometres (6.2 mi) from the plant, due to possible radioactive contamination.[11][12] By 15 March, all four reactors at Daini were reported shutdown, cold and safe.[13]

11 March 2011 – onwards INES Level 7[14] Fukushima I Nuclear Power PlantFukushima Prefecture Multiple partial meltdowns, core breaches, explosions, radiological releases, cooling failures

After the 2011 Tōhoku earthquake and tsunami of March 11, the cooling systems for multiple reactors (units 1, 2, 3) and spent fuel cooling ponds (all 6 units and central pool) of the Fukushima I (Fukushima Dai-ichi) nuclear power plant were compromised due to damage from the tsunami.

Atomic bombings of Hiroshima and Nagasaki

During the final stages of World War II in 1945, the United States conducted two atomic bombings against the cities of Hiroshima and Nagasaki in Japan, the first on August 6, 1945 and the second on August 9, 1945. These two events are the only active deployments of nuclear weapons in war to date.[2]

For six months before the atomic bombings, the United States intensely fire-bombed 67 Japanese cities. Together with the United Kingdom and the Republic of China, the United States called for a surrender of Japan in the Potsdam Declaration on July 26, 1945. The Japanese government ignored this ultimatum. By executive order of President Harry S. Truman, the U.S. dropped the nuclear weapon “Little Boy” on the city of Hiroshima on Monday, August 6, 1945,[3][4]followed by the detonation of “Fat Man” over Nagasaki on August 9.

Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in Nagasaki,[1] with roughly half of the deaths in each city occurring on the first day. The Hiroshima prefectural health department estimates that, of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. In a US estimate of the total immediate and short term cause of death, 15–20% died from radiation sickness, 20–30% from flash burns, and 50–60% from other injuries, compounded by illness.[5] In both cities, most of the dead were civilians.[6][7][8]

Six days after the detonation over Nagasaki, on August 15, Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2, officially ending the Pacific War and therefore World War II. Germany had signed its Instrument of Surrender on May 7, ending the war in Europe. The bombings led, in part, to post-war Japan‘s adopting Three Non-Nuclear Principles, forbidding the nation from nuclear armament.[9] The role of the bombings in Japan’s surrender and the U.S.’s ethical justification for them, as well as their strategical importance, is still debated.[10][11]

Atomic bombings of Hiroshima and Nagasaki
Part of the Pacific WarWorld War II
Two photos of atomic bomb mushroom clouds, over two Japanese cities in 1945.
Atomic bomb mushroom clouds over Hiroshima (left)and Nagasaki (right)
Date August 6 and 9, 1945
Location Hiroshima and NagasakiJapan
Result Debated; possibly crucial in thesurrender of Japan
United States United States Japan Empire of Japan
Units involved
509th Composite Group
Casualties and losses
none 90,000–166,000 killed in Hiroshima[1]
60,000–80,000 killed in Nagasaki[1]

The bombing

Seizo Yamada’s ground level photo taken from approximately 7 km northeast of Hiroshima.

For the composition of the USAAF mission, see 509th Operations Group#Components.

Hiroshima was the primary target of the first nuclear bombing mission on August 6, with Kokura and Nagasaki being alternative targets. August 6 was chosen because clouds had previously obscured the target. The 393d Bombardment Squadron B-29Enola Gay, piloted and commanded by 509th Composite Group commander Colonel Paul Tibbets, was launched from North Field airbase on Tinian in the West Pacific, about six hours flight time from Japan. The Enola Gay (named after Colonel Tibbets’ mother) was accompanied by two other B-29s. The Great Artiste, commanded by Major Charles W. Sweeney, carried instrumentation; and a then-nameless aircraft later called Necessary Evil (the photography aircraft) was commanded by Captain George Marquardt.[25]

After leaving Tinian the aircraft made their way separately to Iwo Jima where they rendezvoused at 2,440 meters (8,010 ft) and set course for Japan. The aircraft arrived over the target in clear visibility at 9,855 meters (32,333 ft). During the journey, Navy CaptainWilliam Parsons had armed the bomb, which had been left unarmed to minimize the risks during takeoff. His assistant, 2nd Lt. Morris Jeppson, removed the safety devices 30 minutes before reaching the target area.[26]

The dark portions of the garments this victim wore during the flash caused burns on their skin.[27]

About an hour before the bombing, Japanese early warning radar detected the approach of some American aircraft headed for the southern part of Japan. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small—probably not more than three—and the air raid alert was lifted. To conserve fuel and aircraft, the Japanese had decided not to intercept small formations. The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29s were actually sighted. However a reconnaissance mission was assumed because at 07.31 the first B29 to fly over Hiroshima at 32,000 feet (9,800 m) had been the weather observation aircraft Straight Flush that sent a morse code message to the Enola Gay indicating that the weather was good over the primary target and because it then turned out to sea the ‘all clear’ was sounded in the city. At 08.09 Colonel Tibbets started his bomb run and handed control over to his bombardier.[28]

The release at 08:15 (Hiroshima time) went as planned, and the gravity bomb known as “Little Boy“, a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 43 seconds to fall from the aircraft flying at 31,060 feet (9,470 m)[29] to the predetermined detonation height about 1,900 feet (580 m) above the city. The Enola Gay had traveled 11.5 miles away before it felt the shock waves from the blast.[30]

Due to crosswind, it missed the aiming point, the Aioi Bridge, by almost 800 feet (240 m) and detonated directly over Shima Surgical Clinic.[31] It created a blast equivalent to about 13 kilotons of TNT (54 TJ). (The U-235 weapon was considered very inefficient, with only 1.38% of its material fissioning.)[32] The radius of total destruction was about one mile (1.6 km), with resulting fires across 4.4 square miles (11 km2).[33] Americans estimated that 4.7 square miles (12 km2) of the city were destroyed. Japanese officials determined that 69% of Hiroshima’s buildings were destroyed and another 6–7% damaged.[34]

70,000–80,000 people, or some 30%[35] of the population of Hiroshima were killed immediately, and another 70,000 injured.[36] Over 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured—most had been in the downtown area which received the greatest damage.[37]

Although the U.S. had previously dropped leaflets warning civilians of air raids on 35 Japanese cities, including Hiroshima and Nagasaki,[38] the residents of Hiroshima were given no notice of the atomic bomb.[39][40][41]

Japanese realization of the bombing

Hiroshima before the bombing.
Hiroshima after the bombing.

The Tokyo control operator of the Broadcasting Corporation of Japan noticed that the Hiroshima station had gone off the air. He tried to re-establish his program by using another telephone line, but it too had failed.[42]About 20 minutes later the Tokyo railroad telegraph center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within 16 kilometers (10 mi) of the city came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the headquarters of the Imperial Japanese Army General Staff.

Military bases repeatedly tried to call the Army Control Station in Hiroshima. The complete silence from that city puzzled the men at headquarters; they knew that no large enemy raid had occurred and that no sizeable store of explosives was in Hiroshima at that time. A young officer of the Japanese General Staff was instructed to fly immediately to Hiroshima, to land, survey the damage, and return to Tokyo with reliable information for the staff. It was generally felt at headquarters that nothing serious had taken place and that the explosion was just a rumor.

The staff officer went to the airport and took off for the southwest. After flying for about three hours, while still nearly 100 miles (160 km) from Hiroshima, he and his pilot saw a great cloud of smoke from the bomb. In the bright afternoon, the remains of Hiroshima were burning. Their plane soon reached the city, around which they circled in disbelief. A great scar on the land still burning and covered by a heavy cloud of smoke was all that was left. They landed south of the city, and the staff officer, after reporting to Tokyo, immediately began to organize relief measures.

By August 8, 1945, newspapers in the U.S. were reporting that broadcasts from Radio Tokyo had described the destruction observed in Hiroshima. “Practically all living things, human and animal, were literally seared to death,” Japanese radio announcers said in a broadcast received by Allied sources.[43]

Post-attack casualties

A victim with severe burns.

According to the U.S. Department of Energy the immediate effects of the blast killed approximately 70,000 people in Hiroshima.[44] Estimates of total deaths by the end of 1945 from burns, radiation and related disease, the effects of which were aggravated by lack of medical resources, range from 90,000 to 166,000.[1][45] Some estimates state up to 200,000 had died by 1950, due to cancer and other long-term effects.[3][6][46] Another study states that from 1950 to 2000, 46% of leukemia deaths and 11% of solid cancer deaths among bomb survivors were due to radiation from the bombs, the statistical excess being estimated to 94 leukemia and 848 solid cancers.[47] At least eleven known prisoners of war died from the bombing.[48]

Survival of some structures

Small-scale recreation of the Nakajima area around ground zero.

Some of the reinforced concrete buildings in Hiroshima had been very strongly constructed because of the earthquake danger in Japan, and their framework did not collapse even though they were fairly close to the blast center. Eizo Nomura (野村 英三 Nomura Eizō?) was the closest known survivor, who was in the basement of a reinforced concrete building (it remained as the Rest House after the war) only 170 m (560 ft) from ground zero (the hypocenter) at the time of the attack.[49][50] Akiko Takakura (高蔵 信子 Takakura Akiko?) was among the closest survivors to the hypocenter of the blast. She had been in the solidly built Bank of Hiroshima only 300 meters (980 ft) from ground-zero at the time of the attack.[51] Since the bomb detonated in the air, the blast was directed more downward than sideways, which was largely responsible for the survival of the Prefectural Industrial Promotional Hall, now commonly known as the Genbaku, or A-bomb Dome. This building was designed and built by the Czech architect Jan Letzel, and was only 150 m (490 ft) from ground zero. The ruin was named Hiroshima Peace Memorial and was made a UNESCO World Heritage site in 1996 over the objections of the U.S. and China.[52] The Memorial monument for Hiroshima was built in Hiroshima for bombing victims.[53][54][55]