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Nuclear Accident Types
6:07 AM
Nuclear Accident Types
Loss of coolant accident
A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's Emergency Core Cooling System (ECCS) exists specifically to deal with a LOCA.
Nuclear reactors generate heat internally; to remove this heat and convert it into useful electrical power, a coolant system is used. If this coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown system is designed to stop the fission chain reaction. However, due to radioactive decay the nuclear fuel will continue to generate a significant amount of heat. The decay heat produced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of the thermal rating of the reactor. If all of the independent cooling trains of the ECCS fail to operate as designed, this heat can increase the fuel temperature to the point of damaging the reactor.
- If water is present, it may boil, bursting out of its pipes. (For this reason, nuclear power plants are equipped with pressure-operated relief valves and backup supplies of cooling water.)
- If graphite and air are present, the graphite may catch fire, spreading radioactive contamination. This situation exists only in AGRs, RBMKs, Magnox and weapons-production reactors, which use graphite as a neutron moderator. (see Chernobyl disaster.)
- The fuel and reactor internals may melt; if the melted configuration remains critical, the molten mass will continue to generate heat, possibly melting its way down through the bottom of the reactor. Such an event is called a nuclear meltdown and can have severe consequences. The so-called "China syndrome" would be this process taken to an extreme: the molten mass working its way down through the soil to the water table (and below) - however, current understanding and experience of nuclear fission reactions suggests that the molten mass would become too disrupted to carry on heat generation before descending very far; for example, in the Chernobyl accident the reactor core melted and core material was found in the basement, too widely dispersed to carry on a chain reaction (but still dangerously radioactive).
- Some reactor designs have passive safety features that prevent meltdowns from occurring in these extreme circumstances. The Pebble Bed Reactor, for instance, can withstand extreme temperature transients in its fuel. Another example is the CANDU reactor, which has two large masses of relatively cool, low-pressure water (first is the heavy-water moderator; second is the light-water-filled shield tank) that act as heat sinks.
Under operating conditions, a reactor may passively (that is, in the absence of any control systems) increase or decrease its power output in the event of a LOCA or of voids appearing in its coolant system (by water boiling, for example). This is measured by the coolant void coefficient. Most modern nuclear power plants have a negative void coefficient, indicating that as water turns to steam, power instantly decreases. Two exceptions are the Russian RBMK and the Canadian CANDU (in the latter case, for reasons outlined at the site Nuclearfaq, which also describes the safety systems designed to reliably handle this feature of the design). Boiling water reactors, on the other hand, are designed to have steam voids inside the reactor vessel.
Modern reactors are designed to prevent and withstand loss of coolant, regardless of their void coefficient, using various techniques. Some, such as the pebble bed reactor, passively slow down the chain reaction when coolant is lost; others have extensive safety systems to rapidly shut down the chain reaction, and may have extensive passive safety systems (such as a large thermal heat sink around the reactor core, passively-activated backup cooling/condensing systems, or a passively cooled containment structure) that mitigate the risk of further damage.
Critical accidents
A critical 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. 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. Two workers died, a third was permanently injured, and 350 citizens were exposed to radiation.
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 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.
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 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.
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
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.
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 irradiation of several co-workers present in the room. However Slotin absorbed a lethal dose of the radiation and died nine days afterwards.
Lost source
Lost source accidents, 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 the Soviet army in Lilo, Georgia. 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 has 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 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 direct irradiation 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 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.
The Worst Nuclear Accident
6:05 AM
The worst nuclear accident to date was the Chernobyl Nuclear Accident which occurred in 1986 in Ukraine. That accident killed 56 people directly, and caused an estimated 4,000 additional cases of fatal cancer, as well as damaging approximately $7 billion of property. Radioactive fallout from the accident was concentrated in areas of Belarus, Ukraine and Russia. Approximately 350,000 people were forcibly resettled away from these areas soon after the incident.
Comparing the historical safety record of civilian nuclear energy with other forms of electrical generation, Ball, Roberts, and Simpson, the IAEA, and the Paul Scherrer Institute found in separate studies that during the period from 1970 - 1992, there were just 39 on-the-job deaths of nuclear power plant workers worldwide, while during the same time period, there were 6,400 on-the-job deaths of coal power plant workers, 1,200 on-the-job deaths of natural gas power plant workers and members of the general public caused by natural gas power plants, and 4,000 deaths of members of the general public caused by hydroelectric power plants. In particular, coal power plants are estimated to kill 24,000 Americans per year, due to lung disease as well as causing 40,000 heart attacks per year in the United States. According to Scientific American, the average coal power plant emits more than 100 times as much radiation per year than a comparatively sized nuclear power plant in the form of toxic coal waste known as fly ash.List of Nuclear Power Plants Accidents
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99 accidents at nuclear power plants from 1952 to 2009 (defined as incidents that either resulted in the loss of human life or more than US$50,000 of property damage, the amount the US federal government uses to define major energy accidents that must be reported), totaling US$20.5 billion in property damages. Fifty-seven accidents have occurred since the Chernobyl disaster, and almost two-thirds (56 out of 99) of all nuclear-related accidents have occurred in the USA. There have been comparatively few fatalities associated with nuclear power plant accidents.
| Date | Location | Description | Deaths | I-131 Release in 1,000 Ci | Cost (in millions 2006 $US) | INES level |
|---|---|---|---|---|---|---|
| January 3, 1961 | Idaho Falls, Idaho, US | Explosion at National Reactor Testing Station | 3 | 0.08 | 22 | |
| February 22, 1977 | Jaslovské Bohunice, Czechoslovakia | Severe corrosion of reactor and release of radioactivity into the plant area, necessitating total decommission | 0 | 1,700 | 4 | |
| March 28, 1979 | Middletown, Pennsylvania, US | Loss of coolant and partial core meltdown, see Three Mile Island accident and Three Mile Island accident health effects | 0 | 0.017 | 2,400 | 5 |
| September 15, 1984 | Athens, Alabama, US | Safety violations, operator error, and design problems force six year outage at Browns Ferry Unit 2 | 0 | 110 | ||
| March 9, 1985 | Athens, Alabama, US | Instrumentation systems malfunction during startup, which led to suspension of operations at all three Browns Ferry Units | 0 | 1,830 | ||
| April 11, 1986 | Plymouth, Massachusetts, US | Recurring equipment problems force emergency shutdown of Boston Edison’s Pilgrim Nuclear Power Plant | 0 | 1,001 | ||
| April 26, 1986 | Pripyat, Ukraine | Steam explosion and meltdown (see Chernobyl disaster) necessitating the evacuation of 300,000 people from Kiev and dispersing radioactive material across Europe (see Chernobyl disaster effects) | 53 | 7000 | 6,700 | 7 |
| May 4, 1986 | Hamm-Uentrop, Germany | Experimental THTR-300 reactor releases small amounts of fission products (0.1 GBq Co-60, Cs-137, Pa-233) to surrounding area | 0 | 0 | 267 | |
| March 31, 1987 | Delta, Pennsylvania, US | Peach Bottom units 2 and 3 shutdown due to cooling malfunctions and unexplained equipment problems | 0 | 400 | ||
| December 19, 1987 | Lycoming, New York, US | Malfunctions force Niagara Mohawk Power Corporation to shut down Nine Mile Point Unit 1 | 0 | 150 | ||
| March 17, 1989 | Lusby, Maryland, US | Inspections at Calvert Cliff Units 1 and 2 reveal cracks at pressurized heater sleeves, forcing extended shutdowns | 0 | 120 | ||
| November 24, 1989 | Greifswald, East Germany | Electrical error causes fire in the main trough that destroys control lines and five main coolant pumps | 0 | 443 | ||
| February 20, 1996 | Waterford, Connecticut, US | Leaking valve forces shutdown Millstone Nuclear Power Plant Units 1 and 2, multiple equipment failures found | 0 | 254 | ||
| September 2, 1996 | Crystal River, Florida, US | Balance-of-plant equipment malfunction forces shutdown and extensive repairs at Crystal River Unit 3 | 0 | 384 | ||
| September 30, 1999 | Ibaraki Prefecture, Japan | Workers at the Tokaimura uranium processing facility try to save time by mixing uranium in buckets, killing two and exposing one more to radiation levels above permissible limits | 2 | 54 | 4 | |
| February 16, 2002 | Oak Harbor, Ohio, US | Severe corrosion of control rod forces 24-month outage of Davis-Besse reactor | 0 | 143 | 3 | |
| August 9, 2004 | Fukui Prefecture, Japan | Steam explosion at Mihama Nuclear Power Plant kills 5 workers and injures dozens more | 5 | 9 | 1 |
Susquehanna Nuclear Power Plant
5:56 AM
The Susquehanna Nuclear Power Plant located in Luzerne County, Pennsylvania just south of Shickshinny, in Salem Township, Pennsylvania, United States. It is operated by PPL and has two General Electric boiling water reactors on a site of 1,075 acres (4.4 km²), with 1,130 employees working on site and another 180 employees in Allentown, Pennsylvania. While PPL operates the facility, Harrisburg-based Allegheny Electric Cooperative purchased 10% of the plant in 1977.
Susquehanna Steam Electric Plant produces 60 million kilowatt hours per day. It has been in operation since 1983.
The prime builder was Bechtel Power Corporation of San Francisco, Ca.
In November 2009, the Nuclear Regulatory Commission (NRC) extended the operation licenses of the reactors for an additional 20 years.
In 2008, PPL filed an application with the U.S. Nuclear Regulatory Commission for a license to build and operate a new nuclear plant under consideration near Berwick, Pa. The Bell Bend nuclear plant would be built near the company’s existing two-unit Susquehanna nuclear power plant. A decision by PPL on whether to move forward with the Bell Bend plant will not be made for several years.
| Susquehanna Steam Electric Station | |
|---|---|
| Country | United States |
| Locale | Berwick, Pennsylvania |
| Status | Operational |
| Commission date | Unit 1: November 12, 1982 Unit 2: June 27, 1984 |
| Licence expiration | Unit 1: July 17, 2042 Unit 2: March 23, 2044 |
| Owner(s) | Pennsylvania Power & Light (90%) Allegheny Electric Cooperative (10%) |
| Architect(s) | Bechtel |
| Reactor information | |
| Reactors operational | 1 x 1185 1 x 1140 MW |
| Reactor type(s) | BWR-4 |
| Reactor supplier(s) | General Electric |
| Power generation information | |
| Installed capacity | Unit 1: 1,340 MWe Unit 2: 1,250 |
| Annual generation | Unit 1: 9,456 GW·h Unit 2: 8,781 |
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