Fuel requirements for the advanced high-temperature reactor: Graphite coated-particle fuel and molten fluoride salt coolant

The technological base for high-temperature reactors is the graphite-matrix coated-particle fuel that can operate at temperatures approaching 1250 deg. C with allowable accident temperatures approaching 1600 deg. C. Historically, the reactor coolant has been helium. However, another reactor coolant is also compatible with graphite-based fuels: molten fluoride salts. Oak Ridge National Laboratory, Sandia National Laboratories, and the University of California at Berkeley are developing a new reactor concept, the advanced high-temperature reactor (AHTR), which uses graphite-matrix coated-particle fuel with a clean high-temperature, low-pressure molten-fluoride-salt reactor coolant. The molten salt has a boiling point near 1400 deg. C. Recent studies have developed a preconceptual design for 2400-MW(t) AHTR. Two outlet coolant temperatures were evaluated: 800 deg. C and 1000 deg. C. The low pressure and high-temperature output matches the need for heat to produce hydrogen using thermochemical production techniques or electricity at high efficiency. While the AHTR uses the same coated-particle fuels as those used in helium-cooled reactors, the difference in coolant characteristics and reactor design will likely change some of the fuel requirements. The superior heat transfer characteristics of liquid molten salts compared with those of gaseous helium reduces peak fuel operating temperatures. The decay-heat-cooling system reduces peak accident temperatures by several hundred degrees Celsius. The ability of the molten salt to absorb fission products reduces those fuel quality requirements necessary to minimize off-site radiation exposures under accident conditions. Because more fuel blocks must be moved during a refueling outage, the larger power output of the AHTR implies longer refueling times if the fuel has the same geometry and power densities as modular gas-cooled reactor fuel. Consequently, there are strong economic incentives to increase the power density, increase fuel burnup, and modify the fuel geometry to reduce refueling times. Neutronic requirements may require other modifications as well. (author)