Equal Breeder 1 to 4
(Uses 1 Uranium to process 4 Depleted Cells)
This reactor is very safe but difficult to get going (Requires lots of start up heat) but can safely run at 60% heat.
The Math:
1x Uranium Cell
4x Isotope Cells
13x HD
35x Coolant Cells
Hull Cooling: 1 + 6(Chambers) x 2 + 20(Air) x 0.25 = 18 per tick
Internal Cooling: 35 (Coolant cells) per tick
Cooling Rate: Hull Cooling + Internal Cooling = 53 per tick
Heat Buildup: 5(U Pulses per tick) *10 + 4(I Cells) = 54 per tick
Excess Heat per tick: 54 Heat - 53 Cooling = 1 per tick
Heat buildup per cycle with all air cooling = 1
The 1 heat can be offset by having 2 blocks of water near the reactor. Because you will run breeders very hot, the water should be streaming by within 2 blocks of the 3x3x3 cube of the reactor so that when the water near it evaporates, it is replenished by the stream.
By doing this, the Hull cooling rate becomes: 1 + 6(Chambers) x 2 + 18(Air) x 0.25 + 2 (streaming water blocks) = 19.5 -> 19 per tick (No decimals I'm assuming)
This brings the cooling up 1 making it a Perfect-Breeder.
Max Heat Capacity: 16k(Hull) + 48(Cooling Components) x 10k = 496k
It needs to be heated up to at least 60% Max heat to be effective (4 completed cells in 1 cycle)
This is the tricky part. Using lava it takes 149 buckets of lava to reach 60%. Each lava bucket is 2k heat and 60% Max heat is 297.6k. It can dissipate heat across internal cooling at a rate of 13HDs x 25 = 325 per tick; that means (2000/325) 6.15 seconds per bucket to avoid loud bang. If you do this it takes 916 seconds (about 15 minutes). This is the fasted method since it is consistently absorbing heat at 325 per tick but not practical at all (works for testing).
A more practical method is to replace the 4 Isotope cells with active uranium cells and even place 1 extra uranium cell in the bucket slot to produce excess heat on purpose. By doing this, you can produce 180 heat per tick. The only thing you have to watch by doing this is the adjacent coolant cells that are now next to uranium, they will heat up first. Once they heat up to the percentage you want, you can store them in you inventory and put them back in later after the rest of the reactor heats up. This method at 180 heat per sec takes 28 minutes (1654 seconds).
You can easily maintain the temperature once it is heated up by controlling the external cooling blocks surrounding it. It is safe to run at +60% max heat and I have been successful at running it at 90% max heat for a full cycle too.
Edit: Turns out this design was already made by Root Infinity -> here. I just happened to recreate it flipped vertically.