1.NounEnriched uranium with 0.9-2% uranium-235, requiring a special moderator with very low neutron absorption (such as heavy water or high-purity graphite) in order to sustain a nuclear fission chain reaction; used to increase power output and efficiency in a few types of nuclear reactors capable of running on natural uranium and for downblending of highly-enriched uranium from decommissioned nuclear weapons into low-enriched uranium suitable for use in commercial power reactors.