I dont know if it's a bug or it's not implemented yet.
But if i put an depleted isotope cell in a rector it becomes instant an re-enriched uranium cell. But it should take a while. And it should be enriched faster on high temperature like in the original mod.

I used the 1.2.1.5.2 version
(https://wiki.industrial-craft.net/index.php/Depleted_Isotope_Cell)

Did you get it from Creative?

yes i cheated it for testing

Now i tried to craft it and put it in the reactor, it works. Idk why i didnt come to this on my own, but thanks for the help!

Btw great mod! IC2 is now for me playable again, with that old ic2 Feeling. Thanks <3