radiotherapy equipment

Nuclear and radiation accidents

This article covers notable accidents involving nuclear devices and radioactive materials. In some cases, a release of radioactive contamination occurs, but in many cases the accident involves a sealed source or the release of radioactivity is small while the direct irradiation is large. Due to government and business secrecy, it is not always possible to determine with certainty the frequency or the extent of some events in the early days of the radiation industries. Modern misadventures, accidents, and incidents, which result in injury, death, or serious environmental contamination, tend to be well documented by the International Atomic Energy Agency

Because of the different nature of the events it is best to divide the list into nuclear and radiation accidents. An example of nuclear accident might be one in which a reactor core is damaged such as in the Three Mile Island accident, while an example of a radiation accident might be some event such as a radiography accident where a worker drops the source into a river. These radiation accidents such as those involving the radiography sources often have as much or even greater ability to cause serious harm to both workers and the public than the well known nuclear accidents.

Radiation accidents are more common than nuclear accidents, and are often limited in scale. For instance at Soreq, a worker suffered a dose which was similar to one of the highest doses suffered by a worker on site at Chernobyl on day one. However, because the gamma source was never able to leave the 2-metre thick concrete enclosure, it was not able to harm many others.

The web page at the International Atomic Energy Agency, which deals with recent accidents is The safety significance of nuclear accidents can be assessed and conveyed using the International Atomic Energy Agency International Nuclear Event Scale.

Nuclear Regulatory CommissionHeadquarters and Regional staff members typically participate in four full-scale and emergency response exercises each year, selected from among the list of full-scale Federal Emergency Management Agency (FEMA)-graded exercises required of nuclear facilities. Regional staff members and selected Headquarters staff also participate in post-plume, ingestion phase response exercises. On-scene participants include the NRC licensee, and State, county, and local emergency response agencies.( This allows for Federal Interagency participation that will provide increased preparedness during the potential for an event at an operating nuclear reactor.

The US Nuclear Regulatory Commission (NRC) collects reports of incidents occurring at regulated facilities. The agency currently (2006) uses a 4 level taxonomy to classify reported incidents:

  • Notification of Unusual Event
  • Alert
  • Site Area Emergency
  • General Emergency

Not all reportable events constitute accidents. Incidents which threaten the normal operation or security of a facility may be reportable but not result in any release of radioactivity.

The US Department of Energy uses a similar classification system for events occurring at fuel cycle plants and facilities owned by the US government which are therefore regulated by the DOE instead of the NRC.

Lists of accidents


In dividing up accidents to create the list nuclear accidents, the following criteria have been followed:

  1. There must be well-attested and substantial health damage, property damage or contamination for an event to be listed.
  2. To qualify as "civilian", the nuclear operation/material must be principally for non-military purposes, "military" accidents include all other accidents. (Main article: List of military nuclear accidents)
  3. For a "nuclear" accident the event should involve fissile material, fission or a reactor, all other accidents are considered radiation accidents as they involve radioactive not nuclear materials (accidents with non-radioactive X-ray and electron beam generators are also included in this class). (Main article: List of civilian radiation accidents)
  4. The damage must be related directly to radioactive/nuclear material, not merely (for example) at a nuclear power plant. Hypothetical examples of nonradiation/nonnuclear accidents occurring at nuclear/radiation facilities would be:
    • A nuclear worker crashing his private car in the car park of a nuclear power plant into a lamp post or even a truck carrying a spent fuel cask, property damage has occurred but no release of radiation or contamination will have occurred so it is a simple road traffic accident.
    • A veterinarian, while preparing a frightened dog for radiography, is bitten by the animal. While the bite is an injury which occurred while a radiation worker was at work (and was performing a task related to radiation work), the accident did not involve exposure of a human (or canine) to radiation so it is a simple dog bite.


The worst nuclear accident in history is the Chernobyl disaster. Other examples of serious accidents include the Three Mile Island accident, the Windscale fire, the SL-1 accident, and the Mayak accident.

Accident types

Loss of coolant accident

Criticality accidents

A 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. 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 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

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 (SCRAMed) PWR reactor was left for a length of time without cooling water. As a result the nuclear fuel was damaged, and the core partly melted. 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.


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 recent court case that 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.

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 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.

Human error

Human error has been responsible for some accidents, such as when a person miscalculated the activity of a teletherapy source. This then led to patients being given the wrong dose of gamma rays. In the case of radiotherapy accidents, an underexposure is as much an accident as an overexposure as the patients may not get the full benefit of the prescribed treatment. Also, humans have made errors while attempting to service plants and equipment which has resulted in overdoses of radiation, such as the Nevvizh and Soreq irradiator accidents. In Japan two minor millennium bugs came to light

In 1946 Canadian Manhattan Project physicist Louis Slotin performed a risky experiment known as "tickling the dragon's tail" 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 radiation of several co-workers present in the room. However Slotin absorbed a lethal dose of the radiation and died during the following week.

Lost source

Lost source accidents are ones in which a radioactive source is lost, stolen or abandoned. The source then might cause harm to humans or the environment. For example, see the event in Lilo where sources were left behind by the Soviet army. Another case occurred at Yanango where a radiography source was lost, also at Samut Prakarn a cobalt-60 teletherapy source was lost and at Gilan in Iran a radiography source harmed a welder The best known example of this type of event is the Goiânia accident which occurred in Brazil.

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


Some accidents defy classification. These accidents happen when the unexpected occurs with a radioactive source. For instance if a bird grabs a radioactive source containing radium from a window sill and then was to fly away with it, returning to its nest and then the bird dies shortly afterwards from direct irradiation then it is the case that a minor radiation accident has occurred. As the act of placing the source on a window sill by a human was the event which permitted the bird access to the source, it is unclear how such an event should be classified (if is a lost source event or a something else). Radium lost and found describes a tale of a pig walking about with a radium source inside; this was a radium source lost from a hospital.

Also some accidents are "normal" industrial accidents which happen to involve radioactive material, for instance a runaway reaction at Tomsk (see 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 .

Analyses of nuclear power plant accidents

The Nuclear Regulatory Commission (NRC) now requires each nuclear power plant in the U.S. to have a probabilistic risk assessment (PRA) performed upon it. The two types of such plants in the US (as of 2007) are boiling water reactors and pressurized water reactors, and a study based on two early such PRAs was done (NUREG-1150) and released to the public. However, those early PRAs made unrealistically conservative assumptions, and the NRC is now generating a new study.

NRC Incident Reports

NRC Alerts

NRC Site Area Emergencies

NRC General Emergencies

NRC ASP Analysis Program

The NRC established the Accident Sequence Precursor (ASP) analysis program in 1979 in response to the Risk Assessment Review Group report (see NUREG/CR-0400, dated September 1978). The primary objective of the ASP Program is to systematically evaluate U.S. nuclear power plant operating experience to identify, document, and rank the operating events that were most likely to lead to inadequate core cooling and severe core damage (precursors), if additional failures had occurred. To identify potential precursors, NRC staff reviews plant events from licensee event reports (LERs), inspection reports, and special requests from NRC staff. The staff then analyzes any identified potential precursors by calculating a probability of an event leading to a core damage state.

(ref NRC Commission Document SECY-05-0192 Attachment 2 )

A "significant precursor" is an event that leads to a conditional core damage probability (CCDP) or increase in core damage probability (CDP) that is greater than or equal to . In other words given that the precursor event has occurred, the probability that a subsequent failure will cause core damage is ≥ 0.001.

As of 24-Oct-2005 the "significant" precursor events (i.e. the worst category) were (listed from highest probability of occurrence 1 to lowest probability of occurrence 0.1%):

Date CDP Plant Notes
1979-03-28 1.000 Three Mile Island Unit 2
1975-03-22 0.200 Browns Ferry Unit 1 (ref NRC IE BULLETIN NO. - 75-04A)
1978-03-20 0.100 Rancho Seco (ref LER 312/78-001)
1977-09-24 0.070 Davis-Besse
1974-05-08 0.020 Turkey Point Unit 3
1985-06-09 0.010 Davis-Besse
1978-11-27 0.010 Salem Unit 1
1976-07-20 0.010 Millstone Unit 2
1975-04-29 0.009 Brunswick Unit 2
1981-04-19 0.007 Brunswick Unit 1
2002-02-27 0.006 Davis-Besse
1991-04-03 0.006 Harris Unit 1
1983-02-25 0.005 Salem Unit 1
1981-01-02 0.005 Millstone Unit 2
1980-02-26 0.005 Crystal River Unit 3
1978-03-25 0.005 Farley Unit 1
1977-12-11 0.005 Davis-Besse
1975-11-05 0.005 Kewaunee
1974-04-07 0.005 Point Beach Unit 1
1994-09-17 0.003 Wolf Creek Unit 1
1986-06-13 0.003 Catawba Unit 1
1978-04-13 0.003 Calvert Cliffs Unit 1
1985-05-15 0.002 Hatch Unit 1
1984-09-21 0.002 Lasalle Unit 1
1981-06-24 0.002 Davis-Besse
1979-05-02 0.002 Oyster Creek
1977-07-12 0.002 Zion Unit 2
1986-12-27 0.001 Turkey Point Unit 3
1980-06-11 0.001 St. Lucie Unit 1
1980-04-19 0.001 Davis-Besse
1979-06-03 0.001 Hatch Unit 2
1977-08-31 0.001 Cooper
1971-01-12 0.001 Point Beach Unit 1

See also


External links

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