I decided to make a max efficiency MkI reactor. With multi-uranium cells and reflectors I came up with the following design:
MkI Reactor ; 180 eu/t ; 36m eu / cycle ; 22 eu / t / uranium.
If the single uranium cells are replaced with reflectors, it becomes:
MkI Reactor ; 140 eu/t ; 28m eu /cycle ; 35eu / t / uranium.
Not gonna bother to tabulate resource costs: They're high. But, if you're playing the efficiency game, I feel the second one sets the bar pretty high for a Mk1 reactor.