PLUTONIUM

Plutonium, element number 94, was discovered in 1940 by Glenn T. Seaborg, Arthur C. Wahl and Joseph W. Kennedy at the University of California. There are a total of 15 isotopes of plutonium whose half lives range from 20 minutes to 76 million years and whose atomic weights range from 232 to 246. All plutonium isotopes except the isotope of atomic weight 241, areoemitters; some of the longerlived isotopes also em it some (3-radlation and plutonium 241 isa nearly pure g-emitter. These isotopes are formed by a variety of mechanisms, most of which Involve the bombardment of certain uranium isotopes with a-partlcles, neutrons or deuterons. Plutonium does occur in nature, although in such small amounts that it does not constitute a practical source of the element. Some pitchblende ores (once a principal source of uranium and radium) contain as much as 1 part plutonium in 10". Plutonium is formed in nuclear power reactors principally through the neutron bombardment of U-238. Since most of the commercial nuclear power reactors are fueled with uranium which has a low level of enrichment in uranium-235, the bulk of the uranium contained in the fuel is U-238. The principal plutonium isotope produced is Pu-239. U-238 + n —> U-239 -^ Np-239 ^—>• Pu-239 However, depending upon the operating conditions of the reactorandthedegreeof exposure to neutrons, a number of the other plutonium isotopes are produced. For example. Table 1 shows the principal isotopes of plutonium in the spent fuel of a typical nuclear power reactor, at current design exposure levels.