All components and systems that are important to safety are tested at regular intervals, with the time between tests depending on the specific characteristics of the component. (In practice, essentially all plant systems are important to safety to some degree.) These data are added to the exist­ing probabilistic safety assessment model to keep it up to date and to provide a current estimate of the component, subsystem, and overall plant systems reliability to respond to operational and safety demands. In a very real sense, the testing and maintenance activities help to keep the whole plant in ‘good as new’ condition over its whole operating life.