I believe that the tringular/hexagonal lattice is better from a neutronic standpoint. The neutron leakage is more before the mean free path of a fast neutron is much greater in a fast reactor than in a thermal reactor. In the latter, fast neutrons slow down relatively quickly.
The thermalhydraulics in a fast reactor is a bit different since it is liquid metal, Na or NaK, or if one wants to get really exotic, Li. One has to be concerned with the positive void coefficient.
I believe that the control elements were B4
C based. It's been a while since if reviewed the technology.
I'll see if I can find a good reference on fast reactor T/H. Probably Waltar and Reynolds is the best text on fast reactors.