Differences between analog and digital I&C

Fifty years ago instrumentation and control (I&C) systems at nuclear power plants (NPP) were analog and relied on a mixture of mechanical, pneumatic and electric components. Today analog technology has been replaced with digital technology. Digital I&C has over the years experienced difficulties in the licensing process, which has delayed and escalated costs of both NPP and I&C projects. In the paper it is argued that some of the difficulties are connected to misunderstandings regarding differences between analog and digital I&C. These misunderstandings have led to unrealistic expectations regarding proofs that selected I&C systems can be considered acceptable. To ensure a successful licensing process it would be necessary to agree on evidence for safety that can be considered sufficient. Such evidence should be collected both from the I&C design process and from testing intermediate and final I&C solutions. By a combination of evidence from different sources it should be possible to build a safety case that can be agreed to give sufficient proofs for acceptability. The first component in building the safety case is to make use of safety principles to provide structural evidence that certain classes of design errors have been avoided. The second component is to use simulators and targeted testing to demonstrate functionality of the I&C in different plant situations.