DEPENDENT FAILURES DEVELOPMENTS

Dependent failures, or multiple related failures, destroy the assumption of independence of component failures which is used in the synthesis of system reliability. To obtain an understanding of the impact of dependencies on the reliability of a system, the analyst needs to have knowledge of the root causes of related failures, the coupling mechanism that allows single failures to combine to create a dependent event, and the potential defences that could be introduced to mitigate against a dependency. An assessment of dependencies also benefits from the application of a structured analysis, combining engineering knowledge of a system, together with generic and plant-specific data into a model which will lead to the quantification of the impact of dependencies on system reliability. This paper describes some of the developments that have taken place in recent years with the objective of improving the modelling of dependencies and the classification and coding of multiple related failure data. The development of procedures to provide a structured assessment of dependent failures within a probabilistic safety assessment is also described, together with the development of a database for the storage and retrieval of multiple related failure data.