Plutonium-241

Plutonium-241 (Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Unlike ²⁴⁰Pu, ²⁴¹Pu is fissionable, with a neutron absorption cross section about 1/3 greater than ²³⁹Pu, and a similar chance of fissioning on neutron absorption, around 73%. Otherwise, neutron capture produces plutonium-242. In general, isotopes of odd mass number are both more likely to absorb a neutron, and more likely to fission on neutron absorption, than isotopes of even mass number.

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Effects of Pu-241 decay

²⁴¹Pu has a half-life of 14 years, corresponding to a decay of about 5% of Pu-241 nuclei over a one-year period. The longer spent nuclear fuel waits before reprocessing, the more ²⁴¹Pu decays to americium-241, which is nonfissionable (except in a fast reactor) and dangerous as a nuclear waste (halflife 432 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, 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 Cm-242 and Pu-242 are much less likely to absorb a neutron, and even less likely to fission; however, ²⁴²Cm is short-lived (halflife 160 days) and almost always undergoes alpha decay to Pu-238 rather than capturing another neutron. In short, Am-241 needs to absorb three neutrons before again becoming a fissile isotope.