Design basis accident
Design accidents in a nuclear power plant (nuclear power plant, NPP) are accidents that the safety systems must still be designed to control. Outside the facility, applicable accident limit values for radiation exposure must not be exceeded when they occur.
Terms
Amount of damage
Every conceivable accident is basically characterized by two different variables, which one tries to describe at least approximately by numbers. The first variable is the severity of the consequences of the accident, the so-called amount of damage .
Probability of occurrence
The second key variable is the probability of occurrence , i.e. the frequency with which the event occurs. Estimated probability of occurrence of design accidents is - depending on the estimation method and reactor type - between 1/3625 and 1 millionth per reactor year.
risk
The product of the amount of damage and the probability of occurrence is called the risk . It is used, for example, in the insurance industry as a basis for calculating tariffs.
Design basis accident
Design accidents are accidents that are to be assumed when designing a nuclear facility . Their controllability must be proven in the course of the approval process. The most important means of doing this is redundancy . The basis for the proof are technical-physical models for the control of the accident. Even more serious accidents, the risk of which is below the risk-acceptability threshold, are called "beyond design basis accidents". For them, measures are developed to exclude or limit their effects , which must also be proven in the approval process.
GAU
A design basis accident is often called a GAU . This designation, an abbreviation for the largest accident to be assumed, goes back to the concept of the maximum credible accident developed in the early days of the use of nuclear energy in the USA . This restricted the design of systems to the control of a certain major accident type, namely the complete demolition of a main coolant line, while the emergency cooling remains partially functional. In today's new buildings like the EPR , this concept is no longer used, so that the term GAU loses its meaning here.
Super GAU
In the case of beyond-design-basis accidents, one often speaks of a worst-case scenario . While the consequences of a disaster for people and the environment are not noticeable if the safety measures are functioning, a major disaster leads to contamination of the environment. (The prefix “super” corresponds to the original Latin word meaning “beyond ... beyond”.)
Design basis accidents
A condition for the approval of nuclear facilities is the proof that the limit values for the release of radioactive material into the environment are not exceeded in any design basis accident. When planning a nuclear facility, different scenarios must therefore be taken into account. Various incidents are conceivable which would lead to the release of radiating material if the system were not designed to withstand such an accident. In German nuclear power plants with pressurized water reactors , a design accident is, for example, a break in the main coolant line with massive loss of coolant . The entirety of the applicable design basis accidents is specified in the safety criteria for nuclear power plants .
Retrofitting obligation
Since new knowledge about possible accident sequences is gained over time, it may be necessary to postulate new design basis accidents or to tighten existing ones. This can make it necessary to retrofit additional safety devices and possibly lead to the withdrawal of the operating license. An example of this are the conclusions from the accident at Three Mile Island in 1979. There, a chemical reaction of water with the hot material of the melted reactor core (see core meltdown ) produced a large amount of hydrogen gas within a few hours . This gas development had not been taken into account in the design of the nuclear power plants until then. A few years after the accident, the operators of German nuclear power plants were obliged to take precautions against this risk. This was done by retrofitting the systems with valves to be operated in an emergency for pressure relief ( Wallmann valve ) and recombiners ( Töpfer candles ).
State of the art
The operator of a nuclear power plant must implement suitable and appropriate safety precautions in accordance with the advancing state of science and technology in order to take additional precautions against risks to the general public beyond the requirements for the approval. The operators are helped by technical and scientific research and expert organizations, for example GRS in Germany , the Institut de Radioprotection et de Sûreté Nucléaire in France, and the Japan Nuclear Energy Safety Organization in Japan .
Beyond design basis accidents
Beyond the design basis accidents are defined as accidents in which higher loads occur than in the design basis accident defined above. In the event of a release of radioactivity beyond the legally stipulated limit values, the scope of the design-basis accident is exceeded by definition; it is an over-design basis accident.
Strictly speaking, an accident from INES level 5 meets this requirement. A beyond design basis accident with INES level 5 occurred, for example, in 1957 in the British nuclear complex Sellafield (formerly Windscale, see Windscale brand ) and also in the American nuclear power plant Three Mile Island (1979). However, it is customary in politics and the press to describe serious and catastrophic accidents as “super-GAU” (INES 6 and INES 7). The best-known examples of worst-case scenarios are the disasters in Fukushima (2011) and Chernobyl (1986). In some cases, the site and its surroundings became uninhabitable for a long time, for example the city of Prypiat, four kilometers from the Chernobyl nuclear power plant .
Measures for the case of beyond design basis accidents are specified in the emergency manual of the respective power plant. Possible beyond design basis accidents are also included in the civil protection plans of the authorities. By stress testing the safety margins can also be identified that are in design-wide incidents still available to any impact to reduce the environment.
An beyond-design-basis incident usually completely loses the investment in the nuclear facility concerned. Costs for emergency measures, for the elimination of accidental damage (as far as possible) and economic costs (for example due to additional cancer) can exceed the operating profits generated up to that point many times over. No insurance company insures these risks; the major part is borne by the states or their taxpayers.
criticism
In the 1960s, the term GAU emerged in specialist committees. Until 1965, it was believed that at least a partial meltdown could be tolerated. The GAU at that time was more of a “bureaucratic fiction”, wrote the Bielefeld historian Joachim Radkau in 1983, and the danger was not taken seriously. When more and more reactors were connected to the grid in the 1970s and - after the first oil crisis - more than a hundred nuclear power plants were planned in West Germany, the safety debate took a turn: the likelihood of accidents now became the focus of the discussion.
See also
- List of accidents in nuclear facilities
- List of reportable events in German nuclear facilities
- List of accidents in European nuclear facilities
Individual evidence
- ↑ New calculations: A nuclear disaster threatens every ten to 20 years. In: welt.de . Retrieved February 8, 2017 .
- ↑ New Reactor Build EDF / AREVA EPR Step 2 PSA Assessment. (PDF) In: onr.org.uk. P. 3 , accessed February 8, 2017 .
- ^ Hansjörg Seiler: Law and technical risks: Basics of technical safety law. vdf Hochschulverlag AG, 1997, ISBN 3-7281-2442-7 , page 214 .
- ↑ § 7 Paragraph 2 No. 3 AtG; Section 3 (1) no. 1 letter c AtVfV
- ↑ Federal Office for Radiation Protection : Nuclear Technology - Possible Safety Disruptions: What does the abbreviation GAU stand for? ( Memento from April 26, 2016 in the Internet Archive )
- ↑ Valentin Crastan , Electrical Energy Supply 2 , Berlin - Heidelberg 2012, p. 343.
- ↑ AtG §7d Further precautions against risks: The holder of a license to operate a facility for the splitting of nuclear fuels for the commercial generation of electricity must ensure that the safety precautions that are developed and suitable are implemented in accordance with the advancing state of science and technology and are appropriate to, in addition to the requirements of Section 7 (2) number 3, make a not only minor contribution to further precaution against risks for the general public.
- ↑ Dietrich Schwarz: 3.3 Modern nuclear fission power plants . In: Eckhard Rebhan (Ed.): Energy Handbook. Generation, conversion and use of energy . Springer, Berlin, Heidelberg 2002, ISBN 3-540-41259-X , pp. 260 ( limited preview in Google Book Search).
- ↑ Limited Liability Disaster
- ↑ Manfred Kriener , zeit.de of April 17, 2011: One GAU per year doesn't do any harm
