Design Basis Accident
Encyclopedia
A design basis accident or maximum credible accident (MCA) is a postulated accident that a nuclear facility
must be designed and built to withstand without loss to the systems, structures, and components necessary to assure public health and safety.
Subtypes of DBAs are:
Circumstances like the 2011 Tōhoku earthquake and tsunami
were considered unlikely or impossible, and so the resulting Fukushima I nuclear accidents are described using this terminology as "beyond design basis accidents" or "non-design-basis accidents".
Accidents caused by poor design, failure to follow listed safety procedures or other forms of human error
are not considered to be beyond design basis accidents. The terminology can be unclear, however, because a poorly handled design basis accident can result in conditions beyond what was considered likely, causing a beyond design basis accident. For this reason, some industry experts have criticized the use of design basis terminology. The Three Mile Island accident
and the Chernobyl disaster
are examples of design basis accidents becoming non design basis accidents largely due to failures to follow the written operating procedures.
Forecasting the location of the next earthquake or the size of the next tsunami is an imperfect art. Nuclear plants situated outside known geological danger zones "could pose greater accident threats in the event of an earthquake than those inside, as the former could have weaker protection built in". The Fukushima I plant, for example, was "located in an area designated, on Japan's seismic risk map, as having a relatively low chance of a large earthquake and tsunami; when the 2011 tsunami arrived, it was in excess of anything its engineers had planned for".
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
must be designed and built to withstand without loss to the systems, structures, and components necessary to assure public health and safety.
Subtypes of DBAs are:
- design-basis criticalityCriticalityCriticality may refer to:* Critical thinking in education* Critical reflection in adult education* Critical mass in nuclear reactions* Criticality accident* Criticality matrix* Nuclear Criticality Safety...
: "A criticality accident that is the most severe design basis accident of that type applicable to the area under consideration." - design-basis earthquakeEarthquakeAn earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
(DBE): "That earthquake for which the safety systems are designed to remain functional both during and after the event, thus assuring the ability to shut down and maintain a safe configuration." - design-basis event (DBE): "A postulated event used in the design to establish the acceptable performance requirements of the structures, systems, and components."
- design-basis explosionExplosionAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
: "An explosion that is the most severe design basis accident of that type applicable to the area under consideration." - design-basis fireFireFire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
: "A fire that is the most severe design basis accident of this type. In postulating such a fire, failure of automatic and manual fire suppression provisions shall be assumed except for those safety class items or systems that are specifically designed to remain available (structurally or functionally) through the event." - design-basis floodFloodA flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
: "A flood that is the most severe design basis accident of that type applicable to the area under consideration." - design-basis tornadoTornadoA tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
(DBT): "A tornado that is the most severe design basis accident of that type applicable to the area under consideration."
Circumstances like the 2011 Tōhoku earthquake and tsunami
2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
were considered unlikely or impossible, and so the resulting Fukushima I nuclear accidents are described using this terminology as "beyond design basis accidents" or "non-design-basis accidents".
Accidents caused by poor design, failure to follow listed safety procedures or other forms of human error
Human Error
Human Error is the stage name of Rafał Kuczynski , a polish electronic musician, working mostly in the ambient music genre, produced only with a computer...
are not considered to be beyond design basis accidents. The terminology can be unclear, however, because a poorly handled design basis accident can result in conditions beyond what was considered likely, causing a beyond design basis accident. For this reason, some industry experts have criticized the use of design basis terminology. The Three Mile Island accident
Three Mile Island accident
The Three Mile Island accident was a core meltdown in Unit 2 of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg, United States in 1979....
and the Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
are examples of design basis accidents becoming non design basis accidents largely due to failures to follow the written operating procedures.
Beyond design basis events
As the Fukushima I nuclear accidents showed, external threats — such as earthquakes, tsunamis, fires, flooding, tornadoes and terrorist attacks — are some of the greatest risk factors for a serious nuclear accident. Some accident sequences are considered so unlikely that complete safeguards against the accident sequence have not been built in.Forecasting the location of the next earthquake or the size of the next tsunami is an imperfect art. Nuclear plants situated outside known geological danger zones "could pose greater accident threats in the event of an earthquake than those inside, as the former could have weaker protection built in". The Fukushima I plant, for example, was "located in an area designated, on Japan's seismic risk map, as having a relatively low chance of a large earthquake and tsunami; when the 2011 tsunami arrived, it was in excess of anything its engineers had planned for".