Here's my entry. It's very expensive, but hey, what's expense when you're filling 5+ MFSUs a cycle?
http://www.talonfiremage.pwp.b…ut690xhxoa00tz21792qm5vcw
6 chambers
4.33 efficiency
MK1, generates 52m EU per cycle @ 260 EU/t (well, when quad-cells last an entire cycle anyways)
no neutron reflectors used
Materials:
536 copper
112 tin
384 iron
28 gold
31 rubber
16 redstone dust
2 glowstone + lapis
burns 12 uranium per cycle.
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Also, I've tried to make a Mk2, but it's very difficult! The amount of heat and the power of cooling is so much, that there's a very fine line between stable and explosion.