Practice and Discussion on Safety Categorisation and Classification of Defence in Depth Level 1/2 Functions Based on IAEA-SSG-30

The safety classification of structures, systems and components (SSC) in nuclear power plants (SSG-30) was issued in 2014 by the International Atomic Energy Agency (IAEA). Both the Hua-long Pressurized Reactor (HPR1000) and UK version of the Hua-long Pressurized Reactor (UK HPR1000) Safety Function Categorisation and System, Structure and Component (SSC) Classification methodology have been developed based on IAEA SSG-30 with consideration of different regulatory requirement. The SSG-30 is a high level guidance to perform the safety categorization and classification and there are still some issues that need to be analyzed in specific application such as failure consequence severity criteria and consequence analysis hypothesis. With the expansion of Defence in Depth (DiD) levels in nuclear power plant design, there are differences about function categorization requirements between different countries. This paper analyzes the differences of definition about DiD levels and the function categorisation requirements in different international standards. The IAEA SSG-30 methodology for application is evaluated to meet different regulatory requirements and the alignment or expansion is needed for application have been identified to determine the function categorisation in DID 1/2 levels. The principles of the HPR1000 and UK HPR1000 on DID 1/2 functions categorisation have been summarized and the subsequent introduce and discussion are about practices to apply these principles on special safety functions and safety features.