The main objective of the Working Group on Risk Assessment (WGRISK) is to advance Probabilistic Safety Assessment (PSA) understanding and to enhance its use for improving the safety of nuclear installations, for improving the operation and the design of nuclear installations, and for increasing regulatory effectiveness through risk-informed approaches.
The loss of electrical power sources1 is generally recognised as an important contributor to nuclear plant risk. In particular, the impact of external hazards leading to a loss of electrical sources (external and/or internal to the nuclear plant) has been further underscored by the Fukushima Daiichi accident.
Generally, losses of functional capability can be assessed by design analysis as well as analysis of operating experience. PSA can augment this understanding by providing insights related to the potential consequences of the loss of a safety function and other provisions (defences) that can prevent or mitigate these consequences. These consequences usually include core damage (CD) or large releases of fission products occurring shortly after an initiating event. The use of frequencies and conditional probabilities for these consequences (e.g. core damage frequency, large early release frequency, conditional probabilities of core damage, or large early release given an initiator), can provide insights into the level of defence in depth in the event of loss of a safety function.