Developing a U.S. Fuel Source for Advanced Reactors When uranium is extracted from the earth, more than 99 percent of atoms are U238, while the fissionable isotope U235 represents less than one percent.  To fuel one of today’s commercial nuclear reactors, the uranium must be enriched so that the U235 concentration, or “assay,” is raised … Continue reading High-Assay Low-Enriched Uranium