stable mark 1, its the high output low efficiency reactor from the regular reactor thread.
I would use the maximum efficiency thought as it is a lot more efficient (more than double)
I might have said this several times before but all the designs found in this thread works for 5x5 reactors and they are all stable mark 1 reactors
[Official] New Reactors design thread.
To calculate the 5x5 output just double the heat generated in the old reactor planer.
Due to how the heat generation scales with efficiency for 5x5 generators it is a lot better to use efficient designs rather than high output low efficiency generators.
The best full scale stable mark 1 reactor from that thread is this one
Stable, cheap and produces 1280 Hu/s with only 12 uranium rods, half of which isnt even quad core. doesn't require any diamonds. Also since it produces 1280 Hu/s you can also easily automate it with ic2 only. You use 6 steam cycles (just watch memenths video) with 12 liquid heat exchangers fully loaded for 1200 Hu and you then dump the rest into a liquid heat exchanger with only 8 heat exchangers and a stirling which uses exactly 80 hu.
IMO the best reactor by far for 5x5 as it stands right now both with respect to price, ease of setup and efficiency.
Ofc it still gets crushed cost for cost by windpower but as far as 5x5 reactors go, this is the one you should go with. it also works well with a stirling setup or just as a regular reactor so you can easily adjust how complicated you wanna go.
Also about windpower: The carbon wind turbines last FOREVER. we are talking hundreds of in game hours. i had one since the start of my current survival server and its down to like 80%. The amount of power you can produce before you break one of those wind turbines is absolutely insane