4 buckets/s in 4 slots on the reactor is enough to cool the remaining slots full of uranium cells. Heat generated by a full 5 chamber minus the 4 slots for buckets is 1,920 and 4 buckets is 2,000 cooling at > 4,000 heat in hull. That gives you a max of 6,000 heat. But it IS VERY important that you do not miss a single beat in the reactor at those figures or it will go pop in 6 seconds...
EDIT: Oh and i hit a snag with my electric engines design... They feed off the power the reactor makes and goes pop due to overvoltage. Lucky me a had a security system set up to stop the reactor but i did press the PANIC button several times as well.
So now it's back to the drawing board and try to make something happen with that other addon instead or use the unreliable redstone engines or bulky stone engines... Since combustion engines cannot be stopped and started as easily (in the future release of BC)
So 4 it is. My outlet in reactor is perfect, it's hard to notice a bucket in 'empty state' there. Now there are problems with inlet system though (see below).
Bucket leaks fixed. Imagine that they can actually get ejected upwards. How isn't that weird. It took 5 obsidian pipe branches to contain it.
And now I can run 32-cell reaction non-stop and this requires literaly nothing - no external power supply, fuel, ice, buckets. Nothing. About 40+ cell reaction... I kept up one based on 40 for circa 120 seconds in 5k-6k heat range and then it decided to shot over 10k, so I terminated it It looks like if there was some problem with pumps. I have 4 of them powered by the reactor itself. They seemed to be unable to provide enough coolant for 8 bucket-fillers. I could increase power supply to them, but would it be worth it...? What if power gain from additional cells is less than increase in pumps' power consumption...
EDIT: And since they are buffing reactors (which is good ) now I have to bring down all piping around HV-transformer for it to be able to accomodate 4 fibre cables to transmit voltages around 1800 EU/t. Eh...