To support the design optimization of Unit 3 (an HTGR) of Phase I of the Jiangsu Xuwei Nuclear Cogeneration Plant and facilitate its safety review, the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University — the institution responsible for HTGR technology and the designer of the nuclear island — compiled China's first set of safety design criteria for an advanced reactors, which was recently reviewed and endorsed by the National Nuclear Safety Administration (NNSA) and will serve as a reference model for advanced reactor design in China. During the development of the criteria, INET organized the relevant design teams to trace the reactor's safety characteristics to their technical origins and incorporated risk-informed design principles.

The following article is sourced from the official website of the National Nuclear Safety Administration:

The NNSA has recently approved the Safety Design Criteria for the HTGR at the Jiangsu Xuwei Nuclear Cogeneration Plant (hereinafter "the Safety Design Criteria"). The document further specifies the safety requirements applicable to HTGRs, providing a foundational basis for their safety design and an important reference for subsequent safety reviews. It is the first set of safety design criteria for a small nuclear power reactor (hereinafter "SMR") ever approved in China, signifying that a new pathway — one led by safety design criteria — has been explored for the safety review of SMRs.

I. Background: The Rapid Rise of SMRs Calls for Innovative Review Approaches

In recent years, advanced nuclear technologies have advanced rapidly worldwide, and SMRs have become a focal point of international R&D. China has developed multiple SMR technology routes in parallel, including the HTGR, the small pressurized water reactor, and the molten salt reactor. At the same time, China's existing nuclear safety regulations and standards were largely built on engineering experience with large light-water reactors. As a result, the applicability of certain specific safety concepts and design requirements to SMRs — whose technical characteristics differ fundamentally — has come into question. Against this backdrop, establishing review bases for SMRs that both satisfy fundamental nuclear safety requirements and reflect the specific characteristics of each reactor type has become a pressing task.

Grounded in China's national conditions, the NNSA has actively explored ways to improve its regulatory framework for SMRs. It successively issued the Safety Review Principles for the HTGR Nuclear Power Plant Demonstration Project (Trial) and the Safety Review Principles for Small Pressurized Water Reactor Nuclear Power Plants (Trial). More recently, it formally issued the Technical Opinions on Nuclear Safety Regulation of Small Nuclear Power Reactors, which systematically sets out the regulatory and technical positions on a number of key safety issues. Notably, the document calls for the early formulation of overarching, safety-related design criteria — namely, safety design criteria — tailored to the technical characteristics and safety features of each reactor type. Such criteria are to be prepared by the designer in light of the reactor type's characteristics, then reviewed and endorsed by the regulator, serving both as the top-level basis for engineering safety design and as the evaluation benchmark for subsequent safety reviews. The HTGR Safety Design Criteria approved on this occasion is the first concrete application of this review approach to a specific reactor type, and will help lay a solid foundation for the safe, high-quality development of SMRs.

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

(1) Defining safety objectives and acceptance criteria for plant states with both qualitative and quantitative constraints

The Safety Design Criteria establish the fundamental safety objective: to establish and maintain effective defense against radiological hazards at the modular HTGR nuclear power plant, thereby protecting people, society, and the environment from harm. On the quantitative side, reflecting the plant's multi-module, single-turbine configuration, probabilistic safety objectives are set for each individual module, ensuring a level of public protection comparable to — or higher than — that of large nuclear power plants, without reliance on off-site intervention. The Criteria also establish a classification scheme for plant states together with corresponding acceptance criteria, specifying public dose limits for individual members of the public under normal operation, anticipated operational occurrences, design basis accidents, and selected beyond-design basis accidents.

(2) Anchoring Fundamental Safety Functions in Risk Analysis to Bring Inherent Safety Features into Full Play

The Safety Design Criteria center on fundamental safety functions anchored in radiological containment and supported by reactivity control and core residual heat removal. For radiological containment, the tri-structural isotropic (TRISO) spherical fuel elements constitute the first and most important barrier, with the coolant system serving as a second, auxiliary barrier. For reactivity control, the core features an all-range negative temperature coefficient: once the main helium circulator shuts down, negative temperature feedback alone brings the reactor to a shutdown state. Forcoreresidual heat removal, the combination of low core power density, a support structure with large heat capacity, and a slender core profile allows residual heat to be removed naturally, with the passive cavity cooling system ultimately discharging it to the final heat sink. Guided by these three fundamental safety functions — with radiological containment as both the core and the ultimate goal — the Criteria classify safety-significant functions into two broad categories, namely barriers preventing the release of fission products and measures preserving the effectiveness of those barriers, and further subdivide them into 11 subcategories such as fuel quality and performance. On this basis, the structures, systems, and components needed to fulfill these functions are systematically identified, and specific design criteria are established for each.

(3) Accurately Defining Post-accident States to Match HTGR Characteristics

Given the HTGR's negative temperature feedback and its slow accident progression, the Safety Design Criteria define three reactor states: the "controlled state," the "safe state," and the "maintainable state." The core's negative temperature feedback guarantees that, once the main helium circulator shuts down, the reactor achieves shutdown and settles into the controlled state. Within the extended grace period, actuation of the control rod system and other means can further bring the reactor into the long-term subcritical safe state, or into the maintainable state at an even lower temperature. This state delineation fully captures the inherent safety features unique to the HTGR and provides a benchmark for the design of safety systems and for accident management.

(4) Formulating 44 Design Criteria in a Systematic Manner across Five Dimensions

Beneath the fundamental safety objective and the design safety objective, 44 specific design criteria have been formulated, setting out systematic requirements for the safety design of HTGR units in five dimensions: general requirements; barrier design; protection and reactivity control systems; core heat transfer systems; and fuel and radioactivity control. A risk-informed approach runs through the development of the Criteria, further optimizing the allocation of nuclear safety functions among systems and their safety classification, and establishing a mapping pathway from safety classification through quality grouping to code class — ensuring that risk assessment results are translated precisely into design requirements.

(5) Identifying HTGR-Specific Accidents and Adopting Targeted Mitigation Measures

Unlike light-water reactors, the HTGR is subject to distinctive accident conditions such as water ingress and air ingress. The Safety Design Criteria set out targeted response and mitigation measures for these accidents, so that radioactive releases are further reduced while the applicable standards are met.

III. Significance: Setting a Model for the Safety Review of SMRs

As China's first review practice involving safety design criteria for SMRs, this document carries considerable significance.

First, it has pioneered a new approach to the safety review of SMRs. With SMRs advancing rapidly along multiple tracks, the NNSA has innovated its review pathway by requiring the formulation of safety design criteria — turning 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 for this project is the first application of this approach, and will serve as a model and reference for developing and reviewing safety design criteria for other reactor types, such as molten salt reactors and floating reactors.

Second, it has captured and codified review experience. The Criteria distill the lessons of earlier review practice — notably the HTGR Nuclear Power Plant Demonstration Project — and crystallize key regulatory concerns, such as safety classification, the reliability of high-temperature components, and fuel elements, into explicit design requirements. In doing so, they organically integrate experience feedback with forward design, marking a pivotal step in China's HTGR journey from demonstration toward commercial deployment.

Third, it effectively underpins the high-quality development of SMRs. With regulations and standards still taking shape and reactor designs varying widely in their characteristics, the Safety Design Criteria put regulatory-endorsed safety principles and requirements in place up front, guiding both the development of specific safety requirements and the safety design itself. This allows the designer and the regulator to reach early consensus on a fundamental question: what constitutes an acceptable safety design. It is a pragmatic move to safeguard the high-quality development of SMRs with a high level of nuclear safety.

Looking ahead, the NNSA will continue to fully implement the holistic approach to national security and the nuclear safety concept of "rational, coordinated and balanced" development. Guided by the safety design criteria, it will continue to refine its safety review methodologies for new reactor types, expanding the review of safety design criteria from individual reactor types to systematic application across the board — thereby safeguarding the high-quality development of nuclear energy with a high level of nuclear safety.

Original article link: https://nnsa.mee.gov.cn/ywdt/gzdt/202609/t20260929_1175484.html