Novel methodology for functional design chain analysis of a nuclear power plant: A new built Finnish power plant case study

Abstract

Energized by the fact that nuclear power plants (NPP) have a multidisciplinary character as well as a high degree of coupling and interaction between their elements, regulatory bodies worldwide tend to induce a massive number of requirements to ensure the safety of the plants. Mainly, these requirements aim to apply the defence-in-depth (DiD) safety approach. In Fennovoima Hanhikivi 1 (FH1) NPP, the DiD approach is applied through formalizing functional entities that are responsible for defending various DiD levels and preventing them from failure. Each functional entity consists of safety functions that share common non-functional requirements such as safety classification, failure criterion, diversity, and separation requirements.When investigating the safety functions of any NPP, one concludes that they are significantly interconnected and their processing logics exchange a huge number of signals. Therefore, verifying each safety function independently is not effective because a function may seem to perform its function when it is considered alone, but challenges are found when also the interconnected functions are considered. Thus, it becomes essential to verify the whole functional chain. A functional chain can be defined as a set of safety functions that interact together to guarantee the success of plant systems in case of an initiating event. It should be noted that the functional chain term, defined in this study, is different from the safety group term defined in the IAEA safety glossary. The latter is an assembly of equipment, whereas a functional chain is a group of safety functions. In other words, considering the safety engineering process of the plant life cycle, a functional chain is constructed and defined based on plant-level design down to system-level design, while a safety group is constructed based on system-level design.In this study, a methodology for analysing the functional chain is developed. The methodology mainly targets verifying five aspects of the functional chain which are: measurements, actuators, decision-making processing logic, human–machine interface (HMI), and cabling between various components. The methodology utilizes system engineering approach integrated with a relational database that can be utilized to query the plant design data in order to verify the application of non-functional requirements to the plant design components. Additionally, the study also develops a tool that can assist its users to systematically apply the developed methodology and record its results.Finally, a case study for practically applying the methodology to a NPP design is presented. The application of the methodology succeeded in revealing issues that need to be further clarified with the plant supplier. The main usefulness of applying the developed “functional chain analysis” methodology to a NPP is that it provides an additional as-designed verification approach besides the existing verification methods.