In February 2010, to refuel the Tehran Research Reactor which produces medical isotopes, Iran began using a single cascade to enrich uranium "up to 19.8%", to match the previously foreign supplied fuel. 20% is the upper threshold for low enriched uranium (LEU). Though HEU enriched to levels exceeding 20% is considered technically usable in a nuclear explosive device, this route is much less desirable because far more material is required to achieve a sustained nuclear chain reaction. HEU enriched to 90% and above is most typically used in a weapons development program.