In the light of the TEPCO Fukushima accident, this report documents a study performed on a set of common-cause failure (CCF) events due to external factors, meaning that not only storms and hurricanes are included but also high outdoor temperatures and excessive algae growth. The events were derived from the International CCF Data Exchange (ICDE) database, where a brainstorming exercise performed by the OA (Operating Agent) on how to identify interesting events resulted in finding 52 events related to the topic out of 1 600 ICDE events in total. The study is based on a workshop performed during an ICDE Steering Group meeting in April 2012, where the scope of events due to external factors was analysed in work groups. During the workshop 9 events were identified to not have resulted from external factors and therefore outside the workshop scope, i.e. this report includes the assessment of 43 ICDE events.