Low-enriched uranium (LEU)
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 percent 235U. It is not a direct bomb risk. LEU used in research reactors is usually enriched 12% to 19.75% U-235, the later concentration being used to replace HEU fuels when converting to LEU.
Slightly enriched uranium (SEU)
Slightly enriched uranium (SEU) has a concentration of 235U between 0.9% and 2%.
This new grade is being used to replace Natural uranium (NU) fuel in some heavy water reactors like the CANDU. Costs are lowered because less uranium and fewer bundles are needed to fuel the reactor. This in turn reduces the quantity of used fuel and its subsequent waste management costs.