Ive made a number of configurations of a 12 fuel rod core big reactor, from a 5x14x15 (a single, 12 high fuel rod), to a 9x3x11 configuration (12 1 high fuel rods), with the fuel rods arranged in a grid, with one block of coolant between each other and the reactor walls.
Flatter configurations seems to be more efficient than taller configurations, but they all seem to put out (with no moderation) ~>6kRF/t, consume about 0.08mb/t - when cooled with ender. For a nett yield of 60-70,000,000RF per ingot of uranium. Cooled with water halved the power output, for a nett yield of ~30,000,000MRF per ingot. Cryotheum yielded about 80,000,000RF per ingot - I thought ender was the best coolant but it is out performed by cryotheum.
I though there would be more variance in the designs as the reactivity was 100% for the single 12 high fuel rod design to 400% for the flatter designs, but the increase in temperature that the multiple core designs got seemed to mostly compensate for higher reactivity resulting in only a relatively small improvement in effiency.
I don't know if there is a more efficient way to pack in the fuel rods, rather than in a grid, all separated with a block of coolant.
