After the enrichment, the enriched ‘product’ containing a higher concentration of 235U, which will be used to make nuclear fuel (e.g.,, PWRs and BWRs require 3% – 5% of 235U), and the ‘tails’ containing a lower concentration of 235U (e.g.,, 0.2-.0.3 of 235U), and known as depleted uranium (DU). Depleted uranium is also produced during fuel burnup and is typically found in spent fuel elements.
Naturally-occurring uranium contains 0.72% of the 235U isotope. The remaining 99.28% is mostly the 238U isotope which is a fissionable isotope, but is not a fissile isotope. Most reactors require uranium to be enriched from 0.7% to higher concentrations. The process of increasing the concentration of one isotope relative to another is called “enrichment.”
Definition by IAEA: Depleted uranium – uranium in which the abundance of the isotope 235U is less than that occurring in natural uranium, e.g., uranium in spent fuel from natural uranium fuelled reactors and tails from uranium enrichment processesIAEA/NVS/3