The Depleted Isotope Cell is obtained by combining a Near-Depleted Uranium Cell with Coal Dust. After this, it can be enriched in a Nuclear Reactor to create a Re-Enriched Uranium Cell. Enriching is performed by surrounding the Depleted Isotope Cell with one or more Uranium Cells. This will cause the uranium cells to pulse an additional time. This pulse will recharge the isotope, producing heat but no EU.
When placed in a Nuclear Reactor, the Depleted Isotope Cell will recharge with different speeds depending on the reactor temperature. This means that if you want to make more Re-Enriched Uranium Cells you must heat up your reactor. Just don't let it get too hot (unless you like fire and lava and a nuclear explosion!)