The publication discusses the issues that must be dealt with when preparing the facility for safe enclosure, or safely maintaining it for a long time. It provides details of lessons learned from deferred decommissioning of nuclear facilities following planned shutdown. These lessons have been learned from a variety of facilities, with a variety of hazards, configurations and decommissioning programmes. While some of the considerations addressed may apply to facilities involved in an operating incident or accident, they are not specifically addressed by this publication as the individual nature of their hazards and decommissioning challenges precludes their use as exemplars. The publication concerns the preparation for, and the steady state part of the safe enclosure phase; it should be understood that in a later part of that phase the on- and off-site requirements and arrangements will change as plans and infrastructure are prepared for the next phase, which is the final dismantling, remediation and site release.