ANSI/ASME/ANS RA-S-1.4-2021, “Probabilistic Risk Assessment Standard for Advanced Non-Light Water Reactor Nuclear Power Plants,” has just been issued. Approved by the American National Standards Institute (ANSI) on January 28, 2021, this joint American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) standard sets forth requirements for probabilistic risk assessments (PRAs) used to support risk-informed decisions for commercial nuclear power plants and prescribes a method for applying these requirements for specific applications.
ANSI/ANS-RA-S-1.4-2021 and its preview are available in the ANS Standards Store.
Scope: This reactor-technology-neutral standard supports a diverse mixture of reactor concepts, including high-temperature gas-cooled reactors (HTGRs), liquid metal–cooled fast reactors, molten salt reactors, microreactors, and small modular reactors, through the use of established technology-neutral risk metrics common to existing light water reactor (LWR) Level 3 PRAs. Such risk metrics include frequency of radiological consequences (e.g., dose, health effects, and property damage impacts).
To support a wide range of applications defined by the non-LWR stakeholders, the scope of this standard is very broad and is comparable to a full-scope Level 3 PRA for an LWR with a full range of plant operating states and hazards. Because some of the advanced non-LWR designs supported by this standard include modular reactor concepts, requirements are included that support an integrated risk of multi-reactor facilities, including event sequences involving two or more reactors or radionuclide sources concurrently.
Improvements from trial use: ANSI/ASME/ANS RA-S-1.4-2021 supersedes a 2013 version of the standard issued for trial-use and pilot application. This trial-use version was extensively piloted in the development of a number of advanced non-LWR PRAs that were under development and being built around the world. These advanced non-LWR PRA pilots included one to support the licensing of the High Temperature Reactor–Pebble Module plant in the Republic of China and a modernization of GE Hitachi Nuclear Energy’s Power Reactor Innovative Small Module PRA, which piloted a major fraction of this standard’s technical requirements in 2018.
The experience with pilot applications of this standard has been extended to support the development of the Traveling Wave Reactor design and molten chloride fast reactor at TerraPower, the pebble bed HTGR under development at X-Energy, the Versatile Test Reactor being developed for the U.S. Department of Energy, the fluoride-cooled high-temperature reactor at Kairos Power, the eVinci Micro Reactor at Westinghouse, and advanced HTGRs under development in Japan. The 2021 version of this standard captures the lessons learned from these pilot applications and incorporates improvements that have been made in other PRA standards that are applicable to advanced non-LWRs.
In preparing the technical requirements in this standard, the Non-LWR Working Group members made use of applicable source material from PRA standards that have been developed for LWRs including ANSI/ASME/ANS RA-Sb-2013, “Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications,” as well as trial-use PRA standards developed by ASME and ANS for Low-Power-and-Shutdown PRA, Level 2 PRA, and Level 3 PRA.