The GT version of Uranium 238, called "Uranium 238 Ingot", has oredict ingotUranium, and only comes from ores.
The NC version of Uranium 238, called "Uranium-238", has oredict ingotUranium238Base and ingotUranium238, and only comes from re-processing spent fuel.

As a result, the two aren't interchangeable. For some recopies, only one can be used, and for others, only the other.

For example, LEU-235 Fuel (and all the other lowly enriched uranium fuels) can only be crafted with the GT version, whereas HEU-235 Fuel (and all the other highly enriched uranium fuels) or MOX fuel can only be crafted from the NC version.

For what it's worth, it is possible to convert the NC version to the GT version by Fluid Extractor-ing it, and then re-solidifying it in a ingot mold. Converting the other way is only kind of possible by running the reactor.

If this is intentional behavior, then please close this, although I see no reason that should be the case.

fixed in a crude way.

fixed in a better way