Passive safety systems are engineered to maintain reactor safety through natural physical forces, gravity, natural convection, thermal conduction, and inherent material properties, without requiring electrical power, mechanical actuation, or human operator intervention. This design philosophy represents a fundamental departure from the active safety systems of older reactor generations, which relied on pumps, diesel generators, and operator actions that could fail under extreme conditions, as demonstrated at Fukushima Daiichi in 2011. Passive safety is a defining characteristic of virtually every SMR and advanced reactor design under development and is central to their licensing cases with the NRC and international regulators.
The implementation of passive safety varies across reactor technologies. NuScale's VOYGR modules operate with natural circulation cooling, eliminating reactor coolant pumps entirely; the modules are submerged in a large below-grade pool that serves as the ultimate heat sink, capable of indefinite cooling without external water or power. Westinghouse's AP1000 (the basis for the AP300 SMR) pioneered large-scale passive safety with gravity-driven core cooling and passive containment cooling using natural air circulation. TerraPower's Natrium leverages the thermal properties of liquid sodium, which has a boiling point far above operating temperatures and enables passive decay heat removal through Reactor Vessel Air Cooling Systems (RVACS). The BWRX-300 from GE-Hitachi uses an isolation condenser system that passively removes decay heat through natural circulation.
For TRISO-fueled reactors, passive safety extends to the fuel itself. X-energy's Xe-100 and Kairos Power's KP-FHR use TRISO particles whose ceramic coatings retain fission products at temperatures exceeding 1,600 degrees Celsius, far above any credible accident scenario. South Korea's SMART100 received Standard Design Approval from the NSSC in September 2024 specifically based on its fully passive safety systems. The NRC's new Part 53 licensing framework, expected to be finalized by end of 2027, is explicitly designed to accommodate the risk-informed safety cases of passively safe advanced reactors, potentially saving applicants $53-68 million compared to traditional Part 50/52 licensing pathways. Passive safety is not merely a technical feature but a commercial differentiator, reducing emergency planning zone requirements and enabling siting closer to population centers and industrial facilities.