  Digital instrumentation and control (I&C) upgrades are a vital research area
for nuclear industry. Despite their performance benefits, deployment of digital
I&C in nuclear power plants (NPPs) has been limited. Digital I&C systems
exhibit complex failure modes including common cause failures (CCFs) which can
be difficult to identify. This paper describes the development of a
redundancy-guided application of the Systems-Theoretic Process Analysis (STPA)
and Fault Tree Analysis (FTA) for the hazard analysis of digital I&C in
advanced NPPs. The resulting Redundancy-guided System-theoretic Hazard Analysis
(RESHA) is applied for the case study of a representative state-of-the-art
digital reactor trip system. The analysis qualitatively and systematically
identifies the most critical CCFs and other hazards of digital I&C systems.
Ultimately, RESHA can help researchers make informed decisions for how, and to
what degree, defensive measures such as redundancy, diversity, and
defense-in-depth can be used to mitigate or eliminate the potential hazards of
digital I&C systems.
