I was looking at the optimum way to breed Uranium with as little Uranium as possible for input. This was the result. Using only one (1!) Uranium, it breeds four Uranium from it. It can be tweaked to cool one more per cycle (negative breeder - add an extra cooler in the blank spot) or down as far as you'd like (positive breeder - just randomly remove coolers). It must be heated to at least 3000 heat to break even, 6000 heat is optimum however as it cycles through all four in a cycle - if you want to micromanage run it at 9000+ and change them halfway through. Unfortunately it has the worst possible efficiency rating, so it is not really useful in the real world.
Quote from Design
T = Integrated Heat Dispenser
C = Cooling Cell
D = Depleted Isotope Cell
U = Uranium Cell
To start, put an additional uranium call in the blank space, and/or remove one or more of the dispensers. It must be submerged fully in water to operate however!
Quote from Calculations
All heat gain goes directly to the reactor hull, so that's all good - no overloading components here! Every heat dispenser is connected to at max four cooling cells, and every cell is connected to at least one dispenser, so it can distribute heat properly. This means that it is a simple heat gain/heat loss equation.
heat loss = Reactor Loss + Chamber Loss * # Chambers + # Water Blocks + # Cooling Cells = 1 + 2*6 + 20 + 21 = 54
heat gain = Uranium Gain * # Pulses + Depleted Gain = 10*5 + 4 = 54