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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