Models for the Reliability Analysis of Digital Instrumentation and Control Systems for Nuclear Power Plants

10 The objective of this chapter is to discuss two approaches for reliability analysis of digital 11 instrumentation and control systems in nuclear power plants taking into account the 12 regulatory side. Dynamic Flowgraph Methodology (DFM) and Markov/Cell-to-Cell 13 Mapping Technique (CCMT) are discussed and case studies developed are presented. 14 These case studies involve simplified control systems for a steam generator and a 15 pressurizer of a Pressurized Water Reactor (PWR) plant for the purpose of evaluating 16 each method. Advantages and limitations of each approach are addressed. For the DFM 17 approach, three concerns in the literature are addressed: modeling of the system itself, 18 incorporation of the methodology results into existing Probabilistic Safety Assessments 19 (PSA), and identification of software failures. The Markov/CCMT, which has been used 20 in dynamic probabilistic safety assessments, is approached by means of a simplified 21 digitally controlled water volume control system. The Markov/CCMT methodology 22 results in detailed data of the system reliability behavior in relation to time. However, 23 it demands a higher computational effort than usual as the complexity (i.e., number of 24 components and failure states) of the system increases. As a regulatory research 25 conclusion, the methodologies presented can be used on PSA risk informed assessment, 26 contributing to the regulatory side. 27