It's been a while since I last dug out the reactor planner. I was looking to find the highest efficiency possible (for fluid reactors) that is easy to automate, and I found this:
EU/Tick: 260
Efficiency: 4.33
HU/s: 1199.99
Efficiency: 11.67
Overall Efficiency: ???
Cost: 372 Copper, 64 Gold, 460 Iron, 10 Redstone, 30 Rubber, 125 Tin, 12 Uranium Fuel
Running Costs: 9 Iron Plates, 6 Copper Plates
Reactor Code: 03030C0D11090C0000030C0D0C0D0C0D0C000C0D0C0D0C0D0C09000D0C0D0C0D0C0D1100110D0C0D0C0D0C0000090C0D1109110D0C00
I think it's one of the cheapest full size reactors I have come across, and provides a high efficiency (and so works well in a fluid reactor). I think the best automatable uranium -> HU conversion is using reflectors, but I think that this is likely a close second. In a non-Fluid setup, it can even be pushed into a five chamber design (as shown above), should you ever need to save on resources, or fit a reactor into a 3x3x2 space.
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In the quest for the "best" Fluid Reactor, I started exploring Reflector-based designs, and dug up a few older ones (with a few tweaks to make them cheaper), including this:
EU/Tick: 260
Efficiency: 4.33
HU/s: 1343.86
Efficiency: 21
Overall Efficiency: ???
Cost: 512 Coal, 728 Copper, 92 Gold, 4 Iridium Reinforced Plate, 485 Iron, 14 Redstone, 42 Rubber, 646 Tin, 8 Uranium Fuel
Running Costs: 6 Iron Plates, 4Copper Plates
Reactor Code: 2303230C09110D0C0903230C0D0C0D0C0D11230C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D0C0D110D140D140D110D11
With ~1,350 HU/s, it appears to be near the peak of HU/s, and also simultaneously requires somewhere around the least fuel input, despite outputting the most power. I understand that makes it a slow MOX Breeder, but does anybody have a superior design for a liquid cooled reactor?