  Upgrading the existing analog instrumentation and control (IC) systems to
state-of-the-art digital IC (DIC) systems will greatly benefit existing
light-water reactors (LWRs). However, the issue of software common cause
failure (CCF) remains an obstacle in terms of qualification for digital
technologies. Existing analyses of CCFs in I&C systems mainly focus on hardware
failures. With the application and upgrading of new DIC systems, design flaws
could cause software CCFs to become a potential threat to plant safety,
considering that most redundancy designs use similar digital platforms or
software in their operating and application systems. With complex multi-layer
redundancy designs to meet the single failure criterion, these IC safety
systems are of particular concern in U.S. Nuclear Regulatory Commission (NRC)
licensing procedures. In Fiscal Year 2019, the Risk-Informed Systems Analysis
(RISA) Pathway of the U.S. Department of Energy (DOE) Light Water Reactor
Sustainability (LWRS) Program initiated a project to develop a risk assessment
strategy for delivering a strong technical basis to support effective,
licensable, and secure DIC technologies for digital upgrades and designs. An
integrated risk assessment for the DIC (IRADIC) process was proposed for this
strategy to identify potential key digital-induced failures, implement
reliability analyses of related digital safety IC systems, and evaluate the
unanalyzed sequences introduced by these failures (particularly software CCFs)
at the plant level. This paper summarizes these RISA efforts in the risk
analysis of safety-related DIC systems at Idaho National Laboratory.
