I got Direwolf's reactor code and modified it a bit, it works for 1-8 turbines (all in sync, you can't control them separately) I also fixed a small bug where if the reactor barely generates the required steam it will end up setting the control rods to -1, -2, -3 etc forever, if it tries -1 it goes to 0 and assumes that 0 is the right value.
Here is my modified code
Original Reactor Code: Direwolf20 (I'll remove this once he puts it on his front page)
Button Code: Direwolf20 (Same Deal)
Also I recommend changing the first line of the button API to read
local mon = peripheral.find("monitor")
that will avoid having to change the API every time you attach your monitor