Use of PRA techniques to optimize the design of the IRIS nuclear power plant

True design optimization of a plant's inherent safety and performance characteristics results when a probabilistic risk assessment (PRA) is integrated-with the plantlevel design process. This is the approach being used throughout the design of the International Reactor Innovative and Secure (IRIS) nuclear power plant to maximize safety. A risk-based design optimization tool employing a "one-button" architecture is being developed by the Oak Ridge National Laboratory to evaluate design changes; new modeling approaches, methods, or theories; modeling uncertainties and completeness; physical assumptions; and data changes on component, cabinet, train, and system bases. Unlike current PRAs, the one-button architecture allows components, modules, and data to be interchanged at will with the probabilistic effect immediately apparent. Because all of the current and previous design, modeling, and data sets are available via the one-button architecture, the safety ramifications of design options are evaluated, feedback on design alternatives is immediate, and true optimization and understanding can be achieved. Thus, for the first time, PRA analysts and designers can easily determine the probabilistic implications of different design configurations and operating conditions in various combinations for the entire range of initiating events. The power of the one-button architecture becomes evident by the number of design alternatives that can be evaluated-11 component choices yielded 160 design alternatives. Surprisingly, the lessons learned can be counter-intuitive and significant. For example, one of the alternative designs for IRIS evaluated via this architecture revealed that because of common-cause failure probabilities, using the most reliable components actually decreased systems ' reliability.