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According to this page on Wikipedia:

Reactor-grade plutonium (RGPu) is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up.

Since plutonium is not found naturally, how does it end up at nuclear waste from nuclear fission reactors?

NOT_A_ROBOT
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    Because that is the way how 239Pu is created. ( 238U + n -> 239U -> 239Np -> 239Pu ). What is the desired intermediate product in 239Pu production, is a waste in 235U fission based power plants. – Poutnik Mar 01 '21 at 02:33
  • @z1273 Thanks! Also, the tour and help center links are for the biology stackexchange :) – NOT_A_ROBOT Mar 01 '21 at 03:31

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Wikipedia:

The fissioning of an atom of uranium-235 in the reactor of a nuclear power plant produces two to three neutrons, and these neutrons can be absorbed by uranium-238 to produce plutonium-239 and other isotopes. Plutonium-239 can also absorb neutrons and fission along with the uranium-235 in a reactor.

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As noted by @IanBush, Pu-239 has a half-life of 24110 years and Pu-240 has a half-life of 6560 years, so they will stay as it is for years before fissioning to any other isotopes.

Nilay Ghosh
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    It's worth nothing Pu-239 has a half life of ~24,000 years, and Pu-240 ~6500 years, so once they are produced by the above mechanism they are going to hang around for a while, and not just disappear in minutes – Ian Bush Mar 01 '21 at 09:04