Digital instrumentation and control systems upgrades in current generation nuclear power plants

Over the past 20 years, the nuclear power industry in the United States (U.S.) has been slowly replacing old, obsolete, and difficult-to-maintain analog technology for its nuclear power plant protection, control, and instrumentation systems with digital systems. The advantages of digital technology, including more accurate and stable measurements and the ability to improve diagnostics capability and system reliability, have led to an ever increasing move to complete these upgrades. Because of the difficulties with establishing digital systems safety based on analysis or tests, the safety demonstration for these systems relies heavily on establishing the quality of the design and development of the hardware and software. In the United States, the U.S. Nuclear Regulatory Commission (NRC) has established detailed guidelines for establishing and documenting an appropriate safety demonstration for digital systems in NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition," Chapter 7, "Instrumentation and Controls," Revision 5, issued March 2007 [1], and in a number of regulatory guides and interim staff guidance documents. However, despite the fact that the United States has a well-defined review process, a number of significant challenges associated with the design, licensing, and implementation of upgrades to digital systems for U.S. plants have emerged. Among these challenges have been problems with the quality of the systems and the supporting software verification and validation (V&V) processes, challenges with determining the optimum balance between the enhanced capabilities for the new systems and the desire to maintain system simplicity, challenges with cyber security, and challenges with developing the information needed to support the review of new systems for regulatory compliance.