Plutonium-241

Plutonium-241 (²⁴¹Pu or Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes (especially ²³⁹Pu), ²⁴¹Pu is fissile, with a neutron absorption cross section about one-third greater than that of ²³⁹Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Decay to americium

²⁴¹Pu has a half-life of 14 years, corresponding to a decay of about 5% of ²⁴¹Pu nuclei over a one-year period. The longer spent nuclear fuel waits before reprocessing, the more ²⁴¹Pu decays to americium-241, which is nonfissile (although fissionable by fast neutrons) and an alpha emitter with a half-life of 432 years; ²⁴¹Am is a major contributor to the radioactivity of nuclear waste on a scale of hundreds or thousands of years.

Americium has lower valence and lower electronegativity than plutonium, neptunium or uranium, so in most nuclear reprocessing, Am tends to fractionate not with U, Np, or Pu, but with the alkaline fission products: lanthanides, strontium, caesium, barium, yttrium, and is therefore not recycled into nuclear fuel unless special efforts are made.

In a thermal reactor, ²⁴¹Am captures a neutron to become americium-242, which quickly becomes curium-242 (or, 17.3% of the time, ²⁴²Pu) via beta decay. Both ²⁴²Cm and ²⁴²Pu are much less likely to absorb a neutron, and even less likely to fission; however, ²⁴²Cm is short-lived (half-life 160 days) and almost always undergoes alpha decay to ²³⁸Pu rather than capturing another neutron. In short, ²⁴¹Am needs to absorb two neutrons before again becoming a fissile isotope.