[3][4] An assessment of permanent or temporary changes in a nuclear power plant is performed to evaluate if such changes are within risk criteria.
This risk analysis allows decision making of any changes within a nuclear power plant in accordance with legislation, safety margins, and performance strategies.
During the 2011 earthquake and resultant 15+ meter tsunami on the east coast of Japan, the Fukushima I nuclear power plant suffered core damages at three of its six reactors after the emergency core cooling systems failed due to the extreme beyond design basis conditions.
However, all of these types of plants have varying designs due to regulations, individual utility preferences, and construction location.
In 1995, Sandia National Laboratories estimated that the individual BWR-3 and BWR-4 reactors in the United States have a core damage frequency between 10−4 and 10−7.