NOTE: It wasn't until after posting this (Murphy's law) that I noticed some glaring issues with the design. Those will be fixed shortly. I have fixed the flaws
After playing around with reactors for a bit, I've come up with a Mark-I reactor that I'm reasonably happy with. This core design requires all six reactor chambers to be attached, and needs a full water sheath around the central core to avoid an overheat. Other than that, this unit has a decent output at 60EU/t, though at an unfortunately low efficiency rating of 1.5. In the event that you do not need 60 EU/t or wish to achieve a higher efficiency rating you could easily remove some of the single rods and their surrounding coolant cells, this will also reduce construction and operation costs.
This reactor can be configured to support low-efficiency negative breeding by swapping the central uranium rod for the isotope to enrich. Replacement of cooling cells with water buckets and/or removal of some cells is recommended for faster breeding.
Required Construction Materials:
1 Nuclear Reactor
6 Reactor Chambers
35 Coolant Cells
11 Integrated Heat Dispersers
Required Operation Materials, per cycle:
8 Uranium Cells