Innovating the Safety Review Pathway to Strengthen the Safety Foundation of High-Temperature Gas-Cooled Reactors — National Nuclear Safety Administration Approves Safety Design Criteria for High-Temperature Gas-Cooled Reactors

Recently, the National Nuclear Safety Administration approved the Safety Design Criteria for the High-Temperature Gas-Cooled Reactor of the Jiangsu Xuwei Nuclear Heating and Power Plant (hereinafter referred to as the Safety Design Criteria). This document further refines the specific safety requirements for high-temperature gas-cooled reactors, provides a foundational basis for the safety design of high-temperature gas-cooled reactors, and serves as an important basis for subsequent safety review. This is China's first approved safety design criteria for small nuclear power reactors (hereinafter referred to as small nuclear power reactors), marking that the safety review of small nuclear power reactors has explored a new pathway led by safety design criteria.

I. Relevant Background: The Rapid Development of Small Nuclear Power Reactors Calls for Innovation in Review Methods

In recent years, global advanced nuclear energy technologies have been accelerating their evolution, and the research and development enthusiasm for small nuclear power reactors has continued to rise. China has formed a development pattern of parallel multi-technology routes including high-temperature gas-cooled reactors, small pressurized water reactors, and molten salt reactors. At the same time, the current nuclear safety regulations and standards system is mainly established based on the engineering practice of large light water reactors, and the applicability of some specific safety concepts and design requirements to small nuclear power reactors with vastly different technical characteristics faces challenges. Under such circumstances, how to establish review bases for small nuclear power reactors that both comply with the basic requirements of nuclear safety and fit the characteristics of specific reactor types has become an urgent issue to be resolved.

The National Nuclear Safety Administration, based on China's actual conditions, has actively explored and improved regulatory documents for small nuclear power reactors, and has successively issued the "Safety Review Principles for the High-Temperature Gas-Cooled Reactor Nuclear Power Plant Demonstration Project (Trial)" and the "Safety Review Principles for Small Pressurized Water Reactor Nuclear Power Plants (Trial)". Recently, it has also officially issued the "Technical Opinions on Nuclear Safety Regulation of Small Nuclear Power Reactors", systematically sorting out and clarifying the regulatory technical positions on several important safety issues. Among them, it is explicitly required to develop programmatic design criteria related to safety as early as possible in light of the technical characteristics and safety features of specific reactor types, namely safety design criteria: prepared by the designer in light of reactor type characteristics and reviewed and recognized by the regulator through organization, serving both as the top-level basis for engineering safety design and as the evaluation benchmark for subsequent review. The Safety Design Criteria for high-temperature gas-cooled reactors approved this time are precisely the first implementation practice of this review approach on a specific reactor type, and will lay a good foundation for the safe and high-quality development of small nuclear power reactors.

II. Main Content: Building a Common Safety Benchmark with Criteria as the Guideline

(1) Clarifying Safety Objectives and Condition Acceptance Criteria, with Dual Qualitative and Quantitative Constraints

The Safety Design Criteria establish the basic safety objective: to establish and maintain effective defense against radiological hazards in modular high-temperature gas-cooled reactor nuclear power plants, so as to protect personnel, society, and the environment from harm. At the quantitative level, in response to the configuration characteristics of multiple modules driving one turbine, a probabilistic safety objective for a single module is set, so as to achieve a level of public protection equivalent to or higher than that of large nuclear power plants without relying on off-site intervention measures. The Safety Design Criteria establish condition classification and acceptance criteria, and provide public individual dose limits for normal operation, anticipated operational occurrences, design basis accidents, and selected beyond-design-basis accidents.

(2) Focusing on Basic Safety Functions Based on Risk Analysis, Fully Reflecting Inherent Safety Characteristics

The Safety Design Criteria focus on basic safety functions supported by reactivity control and core residual heat removal, with radioactive containment as the benchmark. In terms of radioactive containment, the tri-structural isotropic (TRISO) spherical fuel element is the first and most important barrier, and the coolant system serves as the second auxiliary barrier; in terms of reactivity control, the core has a full-range negative temperature coefficient, and after the main helium circulator shuts down, reactor shutdown is achieved by relying on negative temperature feedback; in terms of core residual heat removal, natural removal of residual heat is achieved through low core power density, a large heat capacity support structure, and a slender core design, and heat is discharged to the ultimate heat sink by relying on a passive compartment cooling system. With the three basic safety functions as the driver, with the radioactive containment function as the core and ultimate objective, safety-important functions are divided into two major categories: barriers preventing fission product release and measures maintaining barrier effectiveness, and are then further subdivided into 11 subcategories including fuel quality and performance, systematically sorting out the structures, systems, and components required to achieve safety-important functions and determining specific design criteria.

(3) Accurately Dividing Post-Accident States, Adapting to High-Temperature Gas-Cooled Reactor Characteristics

In response to the characteristics of negative temperature feedback and slow accident progression in high-temperature gas-cooled reactors, the Safety Design Criteria define three reactor states: “controllable state,” “safe state,” and “maintainable state”: the negative temperature feedback of the core can ensure that the reactor achieves shutdown and remains in a controllable state after the main helium circulator shuts down; within a relatively long grace period, the reactor can further enter a long-term subcritical safe state through the insertion of the control rod system, etc.; or the reactor can further be placed in a lower-temperature maintainable state. This state division fully reflects the inherent safety characteristics unique to high-temperature gas-cooled reactors and provides a benchmark for the design of safety systems and accident management.

(4) Systematically Formulating 44 Design Criteria, Covering Five Major Levels

Under the basic safety objectives and design safety objectives, a total of 44 specific design criteria have been formulated, putting forward systematic requirements for the safety design of high-temperature gas-cooled reactor units from five levels: general requirements, barrier design, protection systems and reactivity control systems, core heat transfer systems, and fuel and radioactivity control. The formulation of the criteria incorporates a risk-informed approach, further optimizing the allocation of nuclear safety functions and safety classification of systems, and establishing a mapping pathway of “safety classification—quality grouping—code class,” achieving precise transmission from risk assessment results to design requirements.

(5) Accurately Identifying Special Accidents of High-Temperature Gas-Cooled Reactors and Taking Targeted Countermeasures

Unlike light water reactors, high-temperature gas-cooled reactors have special accident conditions such as water ingress and air ingress. The Safety Design Criteria propose targeted countermeasures and mitigation measures and requirements to mitigate the effects of water ingress and air ingress accidents, ensuring that, on the premise of meeting standards, radioactive release is further reduced.

III. Significance: Providing a Demonstration for the Safety Review of Small Nuclear Power Reactors

As the first practice in China for the review of safety design criteria for small nuclear power reactors, this document is of great significance.

First, it has explored and formed a new approach to the safety review of small nuclear power reactors.Against the background of rapid parallel development of small nuclear power reactors, the National Nuclear Safety Administration has actively innovated the review pathway and put forward the requirement to develop safety design criteria, translating the principles established in the "Technical Opinions on Nuclear Safety Regulation of Small Nuclear Power Reactors" into operable design criteria for specific reactor types. The Safety Design Criteria of this project are the first practice of this method, providing a demonstration and reference for the formulation and review of safety design criteria for other reactor types such as molten salt reactors and floating reactors.

Second, it has achieved the systematic consolidation of review experience.The criteria systematically summarize prior review practices such as the high-temperature gas-cooled reactor nuclear power plant demonstration project, condensing key issues of regulatory concern such as safety classification, reliability of high-temperature components, and fuel elements into clear design requirements, achieving an organic combination of experience feedback and forward design, and marking a key step in China's high-temperature gas-cooled reactors moving from demonstration projects to commercial application.

Third, it effectively promotes the high-quality development of small nuclear power reactors.Against the background that regulations and standards still need to be improved and the design characteristics of reactor types differ significantly, the Safety Design Criteria determine in advance the safety principles and requirements recognized by the regulator, using them to guide the establishment of specific safety requirements and safety design, enabling designers and regulators to reach consensus as early as possible on the fundamental question of “what constitutes an acceptable safety design.” This is a pragmatic move to escort the high-quality development of small nuclear power reactors with a high level of nuclear safety.

Looking ahead, the National Nuclear Safety Administration will continue to thoroughly implement the overall national security concept and the nuclear safety concept of “rationality, coordination, and progress together,” take safety design criteria as the guide, continuously improve the safety review methods for new reactors, promote the review of safety design criteria for new reactors from individual reactor types to systematic promotion, and escort the high-quality development of the nuclear energy cause with a high level of nuclear safety.