A reliability analysis using the GO-FLOW methodology is given for the emergency core cooling system (ECCS) of a marine reactor experiencing either a collision or a grounding accident. The analysis is an example of a phased mission problem, and the system is a relatively large system with 90 components. An overview of the GO-FLOW methodology, a description of the ECCS, and the analysis procedure are given. Time-dependent mission unreliabilities under three accident conditions are obtained by one GOFLOW chart with one computer run. The GO-FLOW methodology has proved to be a useful tool for probabilistic safety assessments of actual systems.