Core damage frequency
Core damage frequency (CDF) is a term used in probabilistic risk assessment (PRA) that indicates the likelihood of an accident that would cause damage to a nuclear reactor core. Core damage accidents are considered serious because damage to the core may prevent control of the nuclear reaction, which can lead to a nuclear meltdown. Some sources on CDF consider core damage and core meltdown to be the same thing, and different methods of measurement are used between industries and nations, so the primary value of the CDF number is in managing the risk of core accidents within a system and not necessarily to provide large-scale statistics.
An assessment of permanent or temporary changes in a nuclear power plant is performed to evaluate if such changes are within risk criteria. For example, the probability of core damage may increase while replacing a component, but the probability would be even higher if that component were to fail because it wasn't replaced. Risk measures, such as core damage frequency and large early release frequency (LERF), determine the risk criteria for such changes.
This risk analysis allows decision making of any changes within a nuclear power plant in accordance with legislation, safety margins and performance strategies.
A 2003 study commissioned by the European Commission remarked that "core damage frequencies of 5 × 10−5 [per reactor-year] are a common result" or in other words, one core damage incident in 20,000 reactor years. A 2008 study performed by the Electric Power Research Institute, the estimated core damage frequency for the United States nuclear industry is estimated at once in 50,000 reactor years, or 2 × 10−5.
Assuming there are 500 reactors in use in the world, the above numbers mean that, statistically, one core damage incident would be expected to occur somewhere in the world every 40 or 100 years, respectively.
Nuclear power was first achieved about 60 years ago. The 582 operational reactors that have been built since account for about 14,400 reactor years. The numbers above imply that this should have resulted in less than one accident. However, of these 582 reactors, 11 have suffered from serious core damage, resulting in a historical accident rate of 1 in every 1,309 reactor years. In five of these accidents, the damage was light enough that the reactor was repaired and restarted.
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. The reactors were General Electric BWR-3 and BWR-4 types inside Mark I containments. These had been estimated to have a core damage frequency between 10−4 and 10−7
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