Low-enriched uranium (LEU)[edit]
Low-enriched uranium (LEU) has a lower than 20% concentration of 235U. For use in commercial
light water reactors (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5% 235U. Fresh LEU used in
research reactors is usually enriched
12% to 19.75% U-235, the latter concentration being used to replace HEU fuels when converting to LEU.
[3]
Highly enriched uranium (HEU)[edit]
Highly enriched uranium (HEU) has a 20% or higher concentration of 235U. The fissile uranium in
nuclear weapon primaries usually contains 85% or more of 235U known as
weapons-grade, though theoretically for an
implosion design, a minimum of 20% could be sufficient (called weapon(s)-usable) although it would require hundreds of kilograms of material and "would not be practical to design";
[4][5]