241Pu is a fissile isotope, which means 241Pu is capable of undergoing fission reaction after absorbing thermal neutron. Moreover 241Pu meets also alternative requirement that the amount of neutrons produced by fission of 241Pu (~2.94 per one fission by thermal neutron) is sufficient to sustain a nuclear fission chain reaction. Its fission cross-section for thermal neutrons is about 1012 barns (for 0.025 eV neutron). For fast neutrons its fission cross-section is on the order of barns.
Most of absorption reactions result in fission reaction, but a part of reactions result in radiative capture forming 242Pu. The cross-section for radiative capture for thermal neutrons is about 363 barns (for 0.025 eV neutron). Therefore about 74% of all absorption reactions result in radiative capture of neutron. About 26% of all absorption reactions result in fission.
It is a man-made isotope and can be found in an irradiated uranium fuel or in a spent uranium fuel. Isotope 241Pu is formed in a nuclear reactor from fertile isotope 240Pu. Absorption of a resonance or thermal neutron by the 240Pu nucleus yields 241Pu.241Pu decays via beta decay into 241Am with half-life of only 14.3 years. 241Am has relatively high cross-section for radiative capture for thermal neutrons (~680 barns – 0.025eV). This two phenomena (decrease in fissile isotope and increase in neutron absorber) cause slight decrease in reactivity of irradiated fuel when stored in a spent fuel pool.