Sodium-Cooled Fast Reactor (SFR) System Safety Assessment - 2017
A report by the GIF Risk and Safety Working Group (RSWG).
The Sodium-cooled Fast Reactor (SFR) utilises low-pressure liquid sodium as the reactor coolant, enabling high power density with low coolant volume fraction. The oxygen-free environment prevents corrosion, but sodium's reactivity with air and water necessitates a sealed coolant system. The SFR can be designed for various plant sizes, ranging from 50 to 1,500 MWe, with an outlet temperature of 500-550°C. The SFR closed fuel cycle facilitates the regeneration of fissile fuel and the management of minor actinides, although this requires the development and qualification of recycle fuels.
Key safety features of the SFR include a long thermal response time, a significant margin to coolant boiling, operation near atmospheric pressure, and an intermediate coolant loop between the radioactive sodium in the primary system and the power conversion system. Different working fluids, such as water/steam or supercritical carbon-dioxide, are considered for the power conversion system to enhance safety and reliability. The SFR aims to be economically competitive and extend uranium resources through a fast neutron spectrum, making it a near-term deployable GEN IV system, especially for actinide management.
The SFR system has evolved from extensive global experience, with more than 20 reactors built and nearly 400 reactor years of operation. Historical feedback from construction and operation has been integrated into ongoing safety-related research and development, enhancing fuel, safety, and material behaviour knowledge over the past seventy years.
This safety assessment focuses on reactivity control, decay heat removal, and confinement of radioactive materials. Reactivity control is achieved through diverse and independent active shutdown systems, passive shutdown systems, and inherent reactivity feedback mechanisms. Decay heat removal systems incorporate redundancy, diversity, and passive systems to ensure reliability, while confinement of radioactive materials is managed through multiple safety barriers and design measures to prevent containment bypass.
Design extension conditions (severe accidents) are addressed to improve safety beyond design basis accidents. Strategies for preventing and mitigating severe accident sequences, such as unprotected transients and loss of heat removal systems, are implemented. Measures to practically eliminate situations that could lead to core damage or containment failure are emphasised, ensuring enhanced core cooling capabilities and robust containment integrity.
The safety of the SFR fuel cycle involves managing different types of fuels and implementing advanced processing methods like pyroprocessing to recover transuranic elements and manage waste. Chemical and radiation risks are addressed through containment integrity and radiation protection measures.
In summary, the SFR system requires ongoing research and development to address reactivity control, decay heat removal, and confinement challenges, ensuring safety and reliability for future deployment.