Hardware reliability assessment of safety related and safety critical systems in nuclear power plants

In nuclear power plants, both new and old, computer based instrumentation and control (I and C) systems are being used increasingly in safety related applications, such as some functions of the process control and monitoring systems, as well as in safety critical applications, such as reactor protection or actuation of engineered safety systems. The dependability of computer based systems important to safety is therefore of prime interest and should be ensured. However computer based systems have some characteristics that make them different from other electronic control (hardwired) systems and hence necessitate a different approach to demonstrate their safety and reliability. Computer based systems are programmable and provide a number of advantages over non-programmable systems. However the features that give advantages also add complexity to software. Unlike in hardwired-based systems, faults in software, which does not wear-out, always result from improper requirements, design or implementation. Also, software implementations are discrete models of the real world and are less tolerant to "small" errors and are more difficult to test. It is also recognized that currently the quantitative assessment of reliability of computer based systems is still an active field of research, primarily due to the software component of these systems. In Indian Nuclear Power Plants computer based systems have been introduced progressively. At NPCIL, a very systematic approach to Internal Verification and Validation of such systems is being practiced to demonstrate their integrity and safety of these systems. The quantitative assessment required for the Hardware used in the computer based systems is fulfilled through Hardware reliability assessment (HRA) using the fault tree methodology coupled with reliability prediction techniques. The assessment of the IA and IB systems has been performed for the TAPS 3&4, RAPS5&6 and KGS 3&4 projects. Methodologies used and summa- y findings of these studies are discussed in this paper.