Principles relating to the digital instrumentation and control design approach 2017

The design of the instrumentation and control of nuclear facilities uses digital systems that offer increasing computation and interconnection capabilities. They enable advanced functions to be carried out, such as calculation of the critical heat flux ratio, help to detect hardware failures in real time and provide operators with rich, flexible interfaces. However, these evolved functions may be affected by faults that make their logic systematically inadequate in certain cases, which introduces sources of failure other than random hardware failures and raises questions about the informal concept of the increased 'complexity' of instrumentation and control. Appropriate design principles shall therefore be applied so that this logic is as fault-free as possible and can be assessed by an independent body such as IRSN. This document presents the main problems associated with the design of the digital instrumentation and control of a complex facility, as well as the general principles to follow to demonstrate that a satisfactory safety level has been achieved. The doctrine elements presented in this document are the result of the experience acquired during assessments carried out for the French nuclear power plants, enhanced by exchanges with experts from the nuclear sector, and reflect French practice; they apply in other sectors in which a high level of confidence can be attributed to instrumentation and control. The normative texts cited in this document provide detailed requirements that are open to considerable interpretation, as the nature of the problem posed does not enable relevant and measurable criteria to be defined in all cases. This document aims to explain the principles underlying these detailed requirements and to give the means for interpreting them in each situation. (authors)