uranium-238n. The most common isotope of uranium, having mass number 238 and half-life 4.47 × 109 years, nonfissionable but may be irradiated with neutrons to produce fissionable plutonium-239.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.