During the lifetime of a large engineering plant, the criteria against which the safety of the plant is judged, the technology upon which the design is based and the state of the plant itself will all change. Each impinges upon the safety case for the plant. If a plant is not to be shut down prematurely, have limitations imposed upon its operation, or be subject to expensive retrofitting, the design must be robust against such changes. The designer has the difficult task of anticipating these changes, possibly 30 or 40 years before they occur, without making the plant too expensive to build or too difficult to operate. This Paper examines how well the design safety principles established for the PFR in the late sixties have withstood the test of time.