Defence in depth (DiD) is a concept that has been used for many years alongside tools to optimise nuclear safety in reactor design, assessment and regulation. The 2011 Fukushima Daiichi nuclear power plant accident provided unique insight into nuclear safety issues and raised questions about the tools used at nuclear power plants, including the effectiveness of the DiD concept, and whether DiD can be enhanced and its implementation improved. This report is intended primarily for nuclear regulatory bodies, although information included herein is expected to be of interest to licensees, nuclear industry organisations and the general public.