[en] The availability of a fusion power plant is dependent on the frequency and duration of both planned and unplanned outages. Planned outages result from a maintenance plan for major component replacement, and are entirely predictable once divertor and blanket lifetimes are established. Unplanned outages result from failures of one or more of many diverse components, and can be estimated only by a probabilistic approach taking into account distributions of failure frequencies and repair times. A Monte Carlo model, PAMPAS, has been developed which will enable studies of new conceptual power plant designs once sufficient design detail and component reliability data are available. It can already be used for more generic studies and results are shown which give insight into the magnitude of the problem of achieving adequate plant availability. A simplified model of plant availability has been incorporated into the PROCESS systems code, in order to assess the economic impact of plant unavailability. Initial results show a strong influence on optimum plant parameters dependent on the speed of maintenance operations. This work is continuing as part of preparations for a European fusion power plant conceptual study