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 ?
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.
- Irradiate U-238 with neutrons to produce U-239, which has a half-life of about 23.45 minutes.
- U-239 beta decays to neptunium-239, which has a half-life of about 2.356 days.
- Np-239 beta decays to plutonium-239, which has a half-life of 24,110 years.
- 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