A circular outage cover of light-weight composite material is normally situated in a standby position adjacent to the opening at the outer end of the equipment access tunnel of a nuclear reactor containment vessel. In case of an emergency during an outage, the outage cover can be quickly rolled into position and engaged with a transition ring provided as an extension of the tunnel by retractable retaining shoes on the retaining ring and an inflatable seal carried by the outage cover. The ability of the outage cover to be closed quickly makes it possible to keep the operational cover open throughout most of the outage, and reduces the overall outage time. Hydraulic, pneumatic and electrical services pass radially through the transition ring and do not interfere with closure of the outage cover. An auxiliary diaphragm cooperable with the transition ring is used for pressure testing of the outage cover.