- the radioactive uranium isotope having a mass number 238, comprising 99.28 percent of natural uranium: used chiefly in nuclear reactors as a source of the fissionable isotope plutonium 239.
Origin of uranium 238
First recorded in 1940–45
Also called U-238, U 238.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018