The first (recommended step) for defining a scenario is to define PIEs. It is recommended that a list of PIEs is created for each safety assessment. One way of doing this is, for each included PIE type, to add PIEs that can be potentially relevant. For example, if the PIE type “Lightening resulting in tree fall” has been included, then one can include several PIEs, like “Lightening resulting in tree falling on facility 1”, ““Lightening resulting in tree falling on facility 2”, etc. These PIEs can be linked to the corresponding facilities, this way defining the origin of the scenario. Some of these PIEs might be defined as irrelevant. In this case, they are included for information purpose only and will not generate scenarios. A PIEs would be classified as irrelevant if they are possible, but the probability is extremely low. For relevant PIEs both qualitative or quantitative probability values should be defined.