One of two or more atoms of the same element that have the same
number of protons in their nucleus but different numbers of
neutrons. Isotopes have the same atomic number but different mass
numbers. Isotopes of an element are denoted by indicating their
mass numbers as superscripts to the element symbol, e.g.,
233U or 239Pu, or as numbers following the
name or symbol of the element, e.g., uranium-233 or Pu-239. Some
isotopes are unstable to the extent that their decay needs to be
considered for nuclear material accountancy purposes (e.g., 241Pu
has a half-life of 14.35 years).