15.20. Probabilistic risk analysis was used extensively in the design proc­ess to select the design options that would minimize the predicted core melt frequency. This approach contrasts with PR A studies of already built plants for which remedial options are limited. Defense in depth is provided by various redundant safety systems which would be effective before the passive features are called upon to control an accident. The response of all safety systems to every known transient and accident scenario has re­ceived consideration during the design process.

15.21. The design includes the MSHIM load-following strategy, which utilizes “gray” rods, thus avoiding the need for boron concentration ad­justment (§14.21). An advanced instrumentation and control system using multiplexing reduces the amount of wiring needed (§14.30). Plant layout has been planned to provide good access for construction and maintenance. Also, extensive use of prefabricated modules is expected to reduce con­struction time and improve economics.

15.22. The AP600 is designed for a 60-year operating life. Cost reduc­tion is achieved by the use of passive safety features, general design sim­plicity, and complete plant standardization, which requires 100 percent engineering prior to the start of construction. Such savings tend to com­pensate for the unfavorable economy of scale of the 600-MW(el) size com­pared with large plants. An estimate of $1370 per kilowatt (1990 dollars) is within the $1500/kW EPRI goal for advanced plants.