Does humankind know technique to transform uranium 238 to uranium 235 ? Is it using it for obtaining uranium 235 for nuclear plants ? Or is it selecting uranium 235 from uranium ? Why not transform uranium 238 to 235 ? This would be more efficient, isn' it ?

  • $\begingroup$ sorry, I mean nuclear plant. ok, I fix it $\endgroup$ Commented Nov 7, 2019 at 11:47
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    $\begingroup$ A third option is to "use" U-235 while still in the original natural uranium isotope mix. See en.wikipedia.org/wiki/CANDU_reactor $\endgroup$
    – DJohnM
    Commented Nov 7, 2019 at 21:49

1 Answer 1


Yes, we can convert uranium-238 to uranium-235, but the last step of the conversion is a very slow process, so it's not practical. The standard way to obtain U-235 is to separate it out from natural uranium, typically using uranium hexafluoride; see Enriched uranium for descriptions of the various techniques.

Here are the steps to convert U-238 to U-235.

  1. Irradiate U-238 with neutrons to produce U-239, which has a half-life of about 23.45 minutes.
  2. U-239 beta decays to neptunium-239, which has a half-life of about 2.356 days.
  3. Np-239 beta decays to plutonium-239, which has a half-life of 24,110 years.
  4. Pu-239 alpha decays to U-235.

Obviously, waiting 24,110 years is not practical. ;) But that's ok, because plutonium-239 is fissile, so it can be used as reactor fuel and in the production of fission weapons.

The above half-life data comes from https://en.wikipedia.org/wiki/Isotopes_of_uranium#Uranium-239 and https://en.wikipedia.org/wiki/Plutonium-239

  • $\begingroup$ Thanks a lot PM 2Ring. $\endgroup$ Commented Nov 7, 2019 at 11:48
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    $\begingroup$ Substituting plutonium for uranium does lead to completely different design and engineering problems in both reactors and weapons... $\endgroup$
    – DJohnM
    Commented Nov 7, 2019 at 21:45
  • $\begingroup$ @DJohnM That's certainly true, I didn't intend to imply that all fissile isotopes are equal. But that's a topic for another question, and questions about reactor design are possibly better suited to the engineering site. $\endgroup$
    – PM 2Ring
    Commented Nov 8, 2019 at 3:28

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