Design-basis event explained

A design-basis event (DBE) is a postulated event used to establish the acceptable performance requirements of the structures, systems, and components, such that a nuclear power plant can withstand the event and not endanger the health or safety of the plant operators or the wider public. Similar terms are design-basis accident (DBA) and maximum credible accident.^{[1] [2]}

Subtypes of DBEs are:^[3]

• design-basis criticality: "A criticality accident that is the most severe design-basis accident of that type applicable to the area under consideration."
• design-basis earthquake (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 explosion: "An explosion that is the most severe design-basis accident of that type applicable to the area under consideration."
• design-basis fire: "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 flood: "A flood that is the most severe design-basis accident of that type applicable to the area under consideration."
• design-basis tornado (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 were not considered within the design basis of the plant, and so the resulting Fukushima I nuclear accidents were described using this terminology as "beyond design basis" or "non-design-basis".^[4] However, some have claimed that the design basis for tsunami events at Fukushima was incorrect.^[5]

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.^[6] 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 because of design deficiencies, inadequate training, procedures inadequate for the conditions (TMI), failure to follow operating procedures (Chernobyl), and control room design shortfalls.^[7]

Beyond-design-basis events

Beyond-design-basis events can reduce or eliminate the margin of safety of the structures, systems and components, possibly resulting in a catastrophic failure.^[8]

The Fukushima Daiichi nuclear disaster was caused by a "beyond-design-basis event": the tsunami and associated earthquakes were more powerful than the plant was designed to accommodate. The plant withstood the earthquake but the tsunami overflowed the seawall.^[9] Since then, the possibility of unforeseen beyond design basis events has been a major concern for plant operators.^[10]

See also

• Lists of nuclear disasters and radioactive incidents

Notes and References

1. Web site: NRC: Glossary -- Design-basis accident . 7 February 2017 . United States Nuclear Regulatory Commission .
2. Web site: Design basis accident . 7 February 2017 . European Nuclear Society.
3. Web site: An Exceptional Nuclear Glossary . dead . https://web.archive.org/web/20170907022408/http://www.nuclearglossary.com/abcs/nuclearglossary_D.html#design_basis_accident . 7 September 2017 . 7 February 2017 . nuclearglossary.com.
4. Web site: NRC: Glossary -- Beyond design-basis accidents. 7 February 2017.
5. Web site: Acton . James M. . Hibbs . Mark . March 2012 . Why Fukushima Was Preventable . Carnegie Endowment for International Peace.
6. Web site: Ungethuem . J. . Stokes . M. D. . Overview of Beyond-design Basis Accident Management with Particular Reference to Severe Accident Scenarios in German Pressurized Water Reactors . dead . https://web.archive.org/web/20110826140336/http://sacre.web.psi.ch/ISAMM2009/oecd-sami2001/Papers/p15-Ungetuem/BDB%20Acc.Management.pdf . 26 August 2011 . 7 February 2017 . Paul Scherrer Institut .
7. Book: Rogovin . Mitchell . Three Mile Island: A Report to the Commissioners and to the Public . . 1980 . 1 . Washington, D.C. . 3 . Long before the accident at Three Mile Island, there was a high bravado quotient widespread throughout the industry and its regulators. Licensing procedures were not entirely adequate, giving rise to deficiencies in some plant designs. Operator training was totally inadequate for emergencies, and poorly monitored. Control rooms were often designed with precious little attention to the operators' needs. The lessons learned from malfunctions and mistakes at nuclear plants both here and abroad were never effectively shared within the industry. . 17 October 2021.
8. Book: Todreas, Neil E. . Nuclear Systems: Elements of Thermal Hydraulic Design . CRC Press . 1992 . 2 . 347.
9. News: Report Finds Japan Underestimated Tsunami Danger. Fackler. Martin. 1 June 2011. The New York Times. 18 August 2019.
10. Reactors, residents and risk . Declan Butler . 21 April 2011 . Nature . 472 . 7344 . 400–401 . 10.1038/472400a . 21525903 . 4371109 .