little question

Each water block in a 3x3x3 cube around the reactor provides 1 cooling. Water source blocks and flowing water both counts as water.

Important addition, if you add additional reactor chambers to your reactor, they eat into that space where water could be.

So a single core reactor will have 25 water surrounding it ike this:
Below:



Middle:



Above:

But a fully expanded reactor will have a max of 20 like this:

:Tesla Coil::Tesla Coil:
:Tesla Coil::Nuke TNT::Tesla Coil:
:Tesla Coil::Tesla Coil::Tesla Coil:

The water surrounding the add-on reactor chambers doesn't count.

And if you want to run in redstone wire to turn it on/off, you may lose another if you're not careful with placement.