[1][2] Subtypes of DBEs are:[3] 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".
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 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.
[9] Since then, the possibility of unforeseen beyond design basis events has been a major concern for plant operators.