Fuel burnup quantifies the total thermal energy extracted from nuclear fuel over its residence time in a reactor core, most commonly expressed in megawatt-days per metric ton of initial heavy metal (MWd/tHM or MWd/tU). Higher burnup means more energy is extracted from each fuel assembly before it must be replaced, directly improving fuel economics, reducing the volume of spent fuel generated per unit of energy produced, and extending the interval between refueling outages. Typical burnup for conventional light-water reactors using LEU fuel ranges from 40,000 to 60,000 MWd/tU, though the industry is pursuing NRC approval for burnup levels up to 75,000 MWd/tU and beyond under extended burnup programs.
Advanced reactor designs using HALEU fuel can achieve substantially higher burnup levels than conventional reactors. The higher initial fissile content (5-20% U-235 compared to 3-5% for LEU) provides more fissile atoms available for fission, enabling longer fuel campaigns and higher energy extraction before fuel replacement. TRISO fuel particles used in HTGRs and FHRs are designed for burnup levels exceeding 150,000 MWd/tU, far above conventional fuel limits, enabled by the robust silicon carbide and pyrolytic carbon coatings that maintain structural integrity and fission product retention at extreme burnup levels. Fast-spectrum reactors like TerraPower's Natrium and Oklo's Aurora use metallic HALEU fuel designed for high burnup in a fast neutron environment, where the harder spectrum improves fissile utilization efficiency.
The economics of fuel burnup are intertwined with the entire nuclear fuel cycle. Higher burnup reduces the number of fresh fuel assemblies required per operating cycle (lowering front-end fuel costs), reduces the volume of spent fuel requiring storage and eventual disposal (lowering back-end costs), and extends the refueling interval (increasing capacity factor and revenue). However, higher burnup also increases the radioactivity and decay heat of individual spent fuel assemblies, requiring longer cooling times before dry cask storage and potentially affecting repository capacity calculations. Lightbridge Corporation (NASDAQ: LTBR) is developing advanced metallic uranium-zirconium alloy fuel rods that operate approximately 1,000 degrees Celsius cooler than standard fuel, targeting enhanced burnup performance for both existing reactors and new SMR designs, with irradiation testing underway at Idaho National Laboratory's Advanced Test Reactor since November 2025.