Safety Instrumented Functions as Criticality Defenses

摘要

The objective of this paper is to share the SRS methodology for identifying the reliability requirements and documenting the expected performance of Safety Instrumented Functions (SIFs) used as criticality defenses. Nuclear Criticality SIFs are comprised of sensors, logic solvers, and final control elements, which may be either automatic or manual, to detect a process hazard and respond to prevent a criticality. The Savannah River Site (SRS) has invoked the chemical process industry safety standard (ANSI/ISA 84.00.01)1 for the design of safety significant instrumented systems. The ISA standard provides a graded approach to design based on the amount of risk reduction that is required of an SIF. SRS is embarking on application of this standard to nuclear criticality defenses, thus integrating criticality safety requirements with verifiable design methodology. Per the DOE G 421.1-12 discussion of the double contingency principle, guidance for a single contingency barrier includes, The estimated probability that the control will fail (when called upon for protection) is not greater than 1 in 100 demands. The application of this standard to nuclear criticality SIFs will provide clear requirements in terms of safety availability and testing to assure that the instrumented criticality system as designed, installed, and maintained will meet is performance requirements. The paper identifies the numerous challenges presented by this initiative and the benefits of this approach.