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In case of Uranium, things are lot easier, because $UF_6$ is gaseous from around 60 $\,^{\circ}\mathrm{C}$.

I know, that in most cases, plutonium isotope separation is not needed, because there is no need to separate its fissile isotopes. But there is an exception: radioisotope thermoelectric generators (used to power remote devices such as spacecraft and unmanned polar equipment) need ${^{238}}Pu$.

I think, Plutonium can be separated from the depleted fuel cells with chemical methods, but how to separate ${^{238}}Pu$ from the result?

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    Laser isotope separation (AVLIS/MLIS/SILEX/CRISLA/CHEMLIS). – Deer Hunter Mar 17 '15 at 10:11
  • @DeerHunter Thank you! I checked $PuF_6$, and it is also gaseous from 60 C. But I don't know if it is really used. Probably a big disadvantage that it is much more radioactive, there are many isotopes (between 238 and 242), and in the most "interesting" case there is only a single difference in the nucleon number (238 vs. 239). Afaik, in most cases they try to avoid the Pu isotope separation if they can (for example, the ratio of the $Pu_{240}$ weapon-grade Pu is limited by the removing of the fuel cells early, instead of create much more Pu, but with more $Pu_{240}$). – peterh Mar 17 '15 at 12:49

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Pu238 is not usually isotopically separated from spent nuclear fuel for exactly the reason you pointed out, it would be very difficult. Instead most of what we have comes from one of two different processes.

The first is bombardment of Np237 (also made in a nuclear reactor) with neutrons. It will become Np238 and then undergo a beta emission (fancy way of saying it spits out an electron from one of the neutrons in the nucleus) forming your Pu238.

The other option is to bombard Am241 with neutrons to produce Am242, (excluding the metastable nuclei) this in turn will do another beta decay (like our Np238) and turn into Curium-242 ( Cm242 ) . Cm242 is unstable and will decay by alpha emission (fancy way of saying it coughs out a helium nucleus) thus forming our Pu238.

And that is about it, there are other paths to Pu238 but these are the easiest to perform so far. Eventually we will run a thorium fuel cycle, when that happens there will be an abundance of this radioisotope.

Hope this helps!

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