Modeling Safety Barriers and Defense in Depth with Mulitlevel Flow Modeling

The barrier concept plays a central role in designand operation of safety critical processes. In plant design barriers are provided as means of prevention to avoid critical process conditions which may be harmful to the environment. In plant operations barriers may beestablished and maintained through control actions in order to limit the consequences of critical plant events.

The barrier concept has had a significant practical value for industry by guiding the design thinking of safety engineers. The provision of material barriers preventing the release of radioactive materials from the reactor core to the environment is accordingly a basic principle of nuclear safety design.

The application of barriers is furthermore an integral part of the defence in depth principle applied by nuclear industry. Here several barriers are combined with reliability techniques such as redundancy and diversity to create systems with a high level of safety. Chemical industries apply similar techniques for protection of the environment against the release of toxic materials.

The paper exploresdifferent ways barriers can be represented in Multilevel Flow Modeling (MFM).One of the existing flow functions in MFM is a barrier function. It is shown that other barrier types can be represented andthat their combination into barrier chains may be used to analyze and design levels of safety in automated processes.Suggestion for further research on barrier modeling with MFM are included.