Safety Classification of Modular HTGRs Based on the IAEA SSG-30 Method

Safety classification is one of the most important engineering aspects of a nuclear power plant (NPP). In 2014, the International Atomic Energy Agency released Safety Guide SSG-30 for the safety classification of NPPs. This method, which is technology neutral and compatible with various reactor designs, can provide a new approach to the safety classification of high-temperature gas-cooled reactors (HTGRs). The applicability of the SSG-30 method to modular HTGRs is discussed in this paper.

This study establishes different dose limits for determining the severity of the consequences caused by the failure of safety functions or design provisions. In addition, the definitions of the controlled state and the safe state are proposed for modular HTGRs. After that, the SSG-30 method is applied to the safety classification of reactivity control systems of the HTR-PM, including the control rod system and the small absorber ball system. The safety classes of the systems derived from this method are verified through an iteration process that involves quantitative accident analyses and engineering judgment.

The classification is further extended to individual components of the two systems. The results obtained by the SSG-30 method are compared with those obtained using the traditional deterministic method. The comparison shows the possibility of optimizing the safety classification for future HTGR projects in China by applying the SSG-30 method.