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 CDF estimates mean that, statistically, one core damage incident would be expected to occur somewhere in the world every 40 years for the 2003 European Commission estimated average accident rate or every 100 years for the 2008 Electric Power Research Institute estimated average accident rate.
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