Reliability Allocation to I&C Sub-Systems: A Practical Methodology

As part of probabilistic safety assessment for a nuclear plant, regulators around the globe specify global limits for frequency of nuclear incidents like core damage frequency (CDF), lethal dose to the public, etc. Further allocation of individual targets to subsystems of the plant is left to plant designers normally. In this paper, a workable methodology for reliability allocation is illustrated with Instrumentation and Control (I&C) sub-systems of a Fast Breeder Reactor (FBR). Here, plant I&C is modeled as a set of systems grouped into four categories namely 1) control systems 2) mitigating systems 3) support systems 4) Plant protection systems. From the global limit on core damage frequency, the reliability allocation for I&C subsystems are done based on the hypothesized demand from the subsystems for plant protection. It is illustrated that all I&C sub-systems with varying functions can be linked to the global target directly or indirectly. A case study with entire I&C subsystems which are designed based on functional consideration in Prototype Fast Breeder Reactor is shown as a proof of concept. While it is widely agreed that probabilistic approach is supplementary to deterministic approaches for enhanced safety, the paper argues that probabilistic targets have to be plant wide to have maximum benefits.