EDIT: Meh, I gave up, this is the better you'll get and it's everywhere around the forum design
Woot! My design. Yay.
Anyway, check my old post for why you can't do better than a 2.33 efficiency Mark I. The math shows it won't work. The next practical step up is a 2.5, and with all the heat it generates you can't cram enough cooling cells AND the HDs to distribute heat to them. Math says that a six-chamber, 2.5 leaves room enough for only 11 HDs in the entire design. Eleven HDs can distribute to at most 44 CCs, you need 39 CCs minimum, BUT you must sacrifice roughly 8 CCs worth to cool off the huge heat pulses from the uranium. So it's a no go.