Natural methodology for establishing safety goal compliance requirements on a plant specific basis

Defining reliability assurance criteria on a plant specific basis requires: 1) additional consideration be given to an appropriate process by which overall design objectives are translated into reliability requirements; 2) considerable flexibility be maintained in the process by which reliabilities are forecast and assured; and 3) reasonably fast and interactive techniques be developed for evaluating plant events for potential importance as contributors to a higher than acceptable level of plant safety function failures. An approach which implements these recommendations and which is cost effective for plant specific applications is advocated. This approach is based upon the following technical elements: 1) development of simplified plant risk models called macromodels which can be used to define appropriate levels of detail for reliability assurance requirements, 2) performance of calculations using the plant macromodel which assess the importance of a given set of equipment (usually a system) as a contributor to plant risk measures. 3) performance of sensitivity and uncertainty calculations which identify the potential for variability in risk measures and in system importances given that ranges or distributions of system and subsystem reliabilities are used in the macromodel; 4) the identification of high availability contributors in highly critical systems by means of system reviews of plant systems (or reviews of operating data if available)