Making plutonium in a nuclear reactor.
A neutron released by the fissioning of a chainreacting U-235 nucleus is absorbed by the nucleus
of a U-238 atom. The resulting U-239 nucleus decays
with a half-life of 24 minutes into neptunium, which
in turn decays into Pu-239. Each decay is accompanied by the emission of an electron to balance the
increase in charge of the nucleus and a neutrino.
From IPFM, Global Fissile Material Report 2011
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