Defence in depth conception in nuclear power plants and requirements for instrumentation and control systems

The aim of this article is to identify and discuss some issues of the safety systems’ design for nuclear power plants equipped with the light water reactors using a defence in depth (D-in-D) conception. Because the functional safety solutions play nowadays an important role for the risk control, the basic requirements for the instrumentation and control systems are specified with regard to relevant international standards. For the design purposes the safety functions are categorized into three categories. The I&C systems implementing these functions are assigned to one of three classes that conform to defined design, manufacturing and qualification requirements. These systems are designed to implement functions of relevant categories. Additional design requirements are discussed, including hardware and software aspects, to achieve and maintain the required reliability commensurate with the importance of the safety functions to be performed to reduce risk.