Core Physics Characteristics and Issues for the Advanced High-Temperature Reactor (AHTR)

Abstract

Graphite-coated nuclear fuel has traditionally been used for high-temperature reactors that are cooled with high-pressure helium gas. A next-generation, high-temperature reactor concept is being developed that utilizes the same graphite-coated fuel and graphite moderator as helium-cooled reactors, but uses low-pressure liquid fluoride salt as the primary coolant. The Advanced High-Temperature Reactor (AHTR) combines attractive features of gas-cooled reactor fuel, liquid-salt reactor coolant, and liquid- metal-cooled facility design to yield a reactor concept with exceptional safety and economic features. A physics viability study for the AHTR was completed recently, which concluded that there are no fundamental physics barriers with the concept. The study also identified a number of engineering challenges and future research and development needs. As part of the viability study, an initial analysis of the reactor core physics performance was performed, including coolant reactivity effects, fuel burnup cycle length, and transient behaviors. Considerable attention was given to the coolant void reactivity feedback effects, which are highly dependent on salt composition and core geometry. The coolant void coefficient ranges from negative for lighter element salts such as lithium fluoride and beryllium fluoride, to substantially positive for heavier element salts such as sodium fluoride and zirconium fluoride. Due to the large thermal inertial of the graphite-moderated core and coolant pool, and the excellent passive decay heat removal characteristics of the system, temperature transients appear to be very slow and relatively benign. Detailed results of the physics analyses and future development needs will be presented.