MODERNIZING APPROCHES TO ADDRESS COMMON CAUSE FAILURE IN DIGITAL INSTRUMENTATION AND CONTROL SYSTEMS

摘要

As microprocessor-based safety systems were first introduced in nuclear power plants in theUS in the 1980s, the U.S. Nuclear Regulatory Commission (NRC) recognized that digitalinstrumentation and control (DI&C) can provide advantages in reliability and functionality, but thatit also creates the potential for a new vulnerability to a common cause failures (CCFs) amongsystems in which functions are performed by identical software executed in identical hardware.Specifically, the staff recognized that a latent, systemic fault in design or implementation of softwarecould result in the concurrent failure of essential safety or compensating systems. The potential forpervasive and latent systemic faults resulting in a CCF could be more significant for DI&C systemsbecause of increased resource sharing and the potential for unspecified interactions or unanalyzedconditions. In SECY-93-087, “Policy, Technical, and Licensing Issues Pertaining to Evolutionaryand Advanced Light-Water Reactor (ALWR) Designs,” dated April 2, 1993 [1], the NRC staffidentified policy, technical, and licensing issues pertaining to evolutionary and advanced light waterreactor designs, one of which was defense against CCF in DI&C systems. The staff presentedposition recommendations for addressing the potential for CCFs in DI&C safety systems. In theStaff Requirements Memorandum (SRM) to SECY SECY-93-087 [2], the Commission approved,in part, and disapproved, in part, the staff’s recommendation. The NRC staff implemented theCommission direction into staff guidance for the review of digital I&C systems in a BranchTechnical Position (BTP) 7-19, “Guidance for Evaluation of Defense-in-Depth and Diversity inDigital Computer-Based Instrumentation and Control Systems,” [3] in the Standard Review Planused for new digital systems for new reactors and operating reactors. In the SRM to SECY 16-0070[4], “Integrated Strategy to Modernize the Nuclear Regulatory Commission’s DigitalInstrumentation and Control Regulatory Infrastructure”, dated October 25, 2016, the Commissionapproved implementation of the staff’s integrated action plan (IAP) to modernize the NRC’s digitalinstrumentation and control regulatory infrastructure. As part the work outlined in the IAP NRCstaff has reviewed the current NRC position on defense against CCF in digital I&C systems. Thispaper outlines the high level principles the staff will be using to update the CCF guidance based oncurrent Commission direction.