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Loss of coolant accident Criticality accidents Criticality accidents and power excursions in nuclear reactors, for example the Chernobyl 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 it was 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. Transport 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 lead 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
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 IAEA 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.* Others 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 windowsill 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 IAEA site *. Civilian nuclear accidents Main article: List of civilian nuclear accidents Civilian radiation accidents Main article: List of civilian radiation accidents Military nuclear accidents Main article: List of military nuclear accidents 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 1×10-3. 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 0.001): 1) Three Mile Island Unit 2, CDP = 1.000, (28-Mar-1979) 2) Browns Ferry Unit 1, CDP = 0.200, (22-Mar-1975) (ref NRC IE BULLETIN NO. - 75-04A) 3) Rancho Seco, CDP = 0.100, (20-Mar-1978) 4) Davis-Besse, CDP = 0.070, (24-Sep-1977) 5) Turkey Point Unit 3, CDP = 0.020, (8-May-1974) 6) Davis-Besse, CDP = 0.010, (9-Jun-1985) 7) Salem Unit 1, CDP = 0.010, (27-Nov-1978) 8) Millstone Unit 2, CDP = 0.010, (20-Jul-1976) 9) Brunswick Unit 2, CDP = 0.009, (29-Apr-1975) 10) Brunswick Unit 1, CDP = 0.007, (19-Apr-1981) 11) Davis-Besse, CDP = 0.006, (27-Feb-2002) 12) Harris Unit 1, CDP = 0.006, (3-Apr-1991) 13) Salem Unit 1, CDP = 0.005, (25-Feb-1983) 14) Millstone Unit 2, CDP = 0.005, (2-Jan-1981) 15) Crystal River Unit 3, CDP = 0.005, (26-Feb-1980) 16) Farley Unit 1, CDP = 0.005, (25-Mar-1978) 17) Davis-Besse, CDP = 0.005, (11-Dec-1977) 18) Kewaunee, CDP = 0.005, (5-Nov-1975) 19) Point Beach Unit 1, CDP = 0.005, (7-Apr-1974) 20) Wolf Creek Unit 1, CDP = 0.003, (17-Sep-1994) 21) Catawba Unit 1, CDP = 0.003, (13-Jun-1986) 22) Calvert Cliffs Unit 1, CDP = 0.003, (13-Apr-1978) 23) Hatch Unit 1, CDP = 0.002, (15-May-1985) 24) Lasalle Unit 1, CDP = 0.002, (21-Sep-1984) 25) Davis-Besse, CDP = 0.002, (24-Jun-1981) 26) Oyster Creek, CDP = 0.002, (2-May-1979) 27) Zion Unit 2, CDP = 0.002, (12-Jul-1977) 28) Turkey Point Unit 3, CDP = 0.001, (27-Dec-1986) 29) St. Lucie Unit 1, CDP = 0.001, (11-Jun-1980) 30) Davis-Besse, CDP = 0.001, (19-Apr-1980) 31) Hatch Unit 2, CDP = 0.001, (3-Jun-1979) 32) Cooper, CDP = 0.001, (31-Aug-1977) 33) Point Beach Unit 1, CDP = 0.001, (12-Jan-1971) See also | |||||||||||
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