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:
Circumstances like the 2011 TÅhoku 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 accidents" or "non-design-basis accidents". However, some have claimed that the design basis for tsunami events at Fukushima was incorrect.
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.