Note this is the easy way - not the most efficient !
First, here's a suitable reactor design.
The extra chamber isn't a mistake - it's to ensure that the reactor doesn't get to 85% heat externally, and start spewing lava everywhere.
The way to use it - you simply load it up, and start it. It's a positive breeder, so it will get hot all by itself, but it's designed to run out of Uranium before it actually overheats. Once finished, leave it to cool until absolutely cold. Then put in another uranium cell (don't start it off early, or you'll just have a nice hole in return for your efforts!) This time, when it finishes, you'll get 4 re-enriched uranium.
No pre-heating. No monitoring. No timing. And a relatively cheap reactor too.
For your safety, please note that this reactor requires external water cooling, and does get hot enough to evaporate water blocks. You are advised to build it with source water blocks above the reactor, with at least two clear blocks between them and the central reactor block to ensure they aren't evaporated. It also gets hot enough to give radiation damage - stand well clear....
The overall maths - for each initial chunk of Uranium you get 8 isotope cells. These get converted into 8 uranium cells, but with this method you'll need to put 4 of them back into the breeders in order to run them. Overall, you get a surplus of 4 uranium cells per initial piece of Uranium, and additionally 8 million Eu (generated at efficiency 1) from the breeder reactors. If you burn the other four cells at efficiency 3 in some other reactor, that nets you 32 Million EU per uranium chunk, all told.