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It has been observed that the Reactor Coolant System (RCS) flowrate dissymmetry phenomenon (i.e. imbalance of primary flowrates between loops) appears in several Nuclear Power Plant (NPP) units. As an initial condition of fault studies, the RCS flowrate is very important to nuclear safety. The consequences of RCS flowrate dissymmetry are rarely investigated so far, and thus worthy of studying. This paper aims to analyze the safety margin of Pressurized Water Reactor (PWR) under specific accidents. A plant model is set up and accidents due to malfunction of a component in a single loop are simulated, in order to figure out whether Departure from Nucleate Boiling (DNB) is probable to happen. The results show that the Departure from Nucleate Boiling Ratio (DNBR) criterion is met for Partial Loss of Primary Flow accident with margins. For Main Steam Line Break accident, the flow dissymmetry has little impact on the accident consequences and the minimum DNBR meets the requirements. The Feedwater Line Break accident consequences are sensitive to the primary loop flowrate, while Departure from Nucleate Boiling (DNB) is not probable to happen with dissymmetry larger than 1500 m3/h. The results prove that the plant has enough safety margins and is able to operate under limited flow dissymmetry situation.

As Low As Reasonably Practicable (ALARP) is one of the most important nuclear regulatory concepts in the UK Generic Design Assessment (GDA). ALARP requires that nuclear power plants have the duty to achieve the lowest level of residual risk provided this is reasonably practicable. Unlike As Low As Reasonably Achievable (ALARA), ALARP concept should be considered not only in radioprotection, but also in nuclear safety areas such as fault studies, structural integrity, etc. Different from the traditional licensing requirements in other countries, ALARP concept is not prescriptive but emphasizes self-evaluation, risk-orientation and balance of all aspects. It is therefore difficult to demonstrate that the design has reduced nuclear risks to an ALARP level without a systematic approach during the licensing. In this study, the UK context regulatory requirements of ALARP (including the Tolerability of Risk, Relevant Good Practice, Gross Disproportion, and Numerical Targets) are investigated based on which an ALARP approach for Hua-long Pressurized Reactor (HPR1000) is developed. In this approach, potential improvements can be identified through the review against Relevant Good Practice and risk insights. Risks can be reduced through the implementation of preferred options determined by optioneering process. The ALARP position of the whole plant design can be evaluated through holistic risk review. The ALARP approach has been successfully applied to HPR1000 and recognized by the UK authority during the GDA process. The ALARP approach facilitates the balance of safety, economy and environmental friendliness during nuclear power plant design, and can be guidance to the robust licensing and regulation.

Diversity design is an effective measure to avoid common cause failure, which is of great significance to improve the reliability and safety of nuclear power plants. Especially for those high-frequency occurring accidents mitigation, diversity design plays a significant role. In order to improve its reliability and safety, this paper takes a PWR nuclear power plant as the research object, establishes a diversity design method to mitigate high-frequency occurring accidents, and applies this method to optimize the design of nuclear power plant. In this paper, the safety system of the power station is studied, and a variety of design methods are applied to optimize the design scheme. The results show that this method not only significantly improves the reliability and safety of nuclear power plant, but also plays a great role in the economic operation of nuclear power plant.

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.