A design basis accident (DBA) 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:
- 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 event (DBE): "A postulated event used in the design to establish the acceptable performance requirements of the structures, systems, and components."
- 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 considered unlikely or impossible, and so the resulting Fukushima I nuclear accidents were 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.
Beyond design basis events 
The Fukushima I nuclear accident was caused by a "beyond design basis event," the tsunami and associated earthquakes were more powerful than the plant was designed to accommodate, and the accident is directly due to the tsunami overflowing the too-low seawall. Since then, the possibility of unforeseen beyond design basis events has been a major concern for plant operators.
See also 
- Declan Butler (21 April 2011). "Reactors, residents and risk". Nature.
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