This criteria document provides guidelines for the designer in interpreting the requirements of Title 10, Code of Federal Regulations, Part 50, "Licensing of Production and Utilization Facilities," Appendix A, "General Design Criteria for Nuclear Power Plants," with respect to design against single failures in PWR fluid process systems. Means of treating both active and passive failures are addressed for safety-related fluid process systems following various initiating events. Current acceptable practice is used as a basis for the criteria. Failure criteria for the electric power supply and control systems and the protection systems are provided in: IEEE-308-I974 "IEEE Standard Criteria for Class IE Power Systems for Nuclear Power Generating Stations," IEEE-279-I971 "IEEE Standard Criteria for Protection Systems for Nuclear Power Generating Stations," (ANSI N42.7-I972); and IEEE-379-I977 (Revision of IEEE-379-I972/ANSI N41.2) "IEEE Standard for Application of the Single-Failure Criterion to Nuclear Power Generating Station Class IE Systems." Structural failures of components, such as braces, supports, or restraints, as well as occurrences involving common mode failures, are excluded.