Beyond design-basis are accidents that are outside the realm of what the plant was designed to withstand. The classic example is the Fukushima nuclear accident.
Beyond design-basis events are difficult to predict either because they have never occurred or have a low probability of occurring. This is closely related to the idea of black swan theory.
Events that occur due to human error, poor design and negligence in construction are not considered beyond basis-design events, since these are predictable and preventable.
|Example||Design Basis Specifications||Beyond Design Basis Accident|
|Car Crash Testing||Car bumpers are designed to withstand speeds of up to 5 MPH with minimal damage.||Front or rear end collisions at speeds higher than 5 MPH result in major damage to the car and/or passengers.|
|Fukushima Daiichi 2011||Plant was built to withstand tsunamis with a maximum height of 5.7m and earthquakes with a maximum magnitude of 7.5.||The plant was subject to a 10m tall tsunami and 9.0 magnitude earthquake, it was unable to remain safe resulting in substantial radiation leakage into the Pacific Ocean|
Machinery such as vehicles and power plants are designed to withstand varying degrees of accidents. For example, a power plant would need to withstand external threats common to the area and natural disasters. If a speculated accident occurs, based on its design, the plant should be able to remain safe and secure. A power plant located in a politically volatile region might be built to withstand plane crashes—these types of considerations are within the scope of design-basis accidents as opposed to the rigour beyond design-basis accidents.
It is difficult to predict these events and prepare for them due to their extreme rarity. Under these infrequent circumstances the plant may be unable to operate safely. Reduction in the safety margin of a plant can cause a catastrophic failures such as meltdowns.