If you know how to use combinators, it is easy. You need to cool it down if core temperature is too high and pump water in the reactor it if water level is to low.
Here is how I wire them:
1) Install reactor & two towers (let's say, one on the left and one on the right) with one square of space between them (for pumps).
2) Connect 4 express pumps (you need Flow Control mod for these) between towers and the reactor (one should pump water reactor->tower, another should pump tower->reactor). Make sure towers are rotated properly, so one pipe directed to the reactor and one directed from the reactor.
3) Connect 2 express pumps to pump into reactor. Let's say, they are on the top.
4) Connect 2 express pumps to pump water from the reactor. Let's say, they are on the bottom.
5) Connect bottom pumps to 2 turbines (these are new things, come with this mod).
6) Connect top pumps to 5-6 offshore pumps (they should be as close as possible or you need express pumps to pump water).
7) Wire all 8 pumps, reactor control panel and ~4 combinators (both to output and to input). You will get a single circuit network.
8) One combinator should be set to "Core temperature > 150" and emit "C" signal (cool).
9) One combinator should be set to "Water < 250" and emit "W" signal (water).
10) One combinator should be set to "Water > 100" and emit "T" signal (turbines).
11) One combinator should be set to "Coolant temperature > 80" and emit "T" signal (turbines). I don't think it is really needed, though. It actually might make things worse as it will shut down turbines if reactor cools too much.
12) Make top pumps pump water on "W > 0", coolant tower pumps (4 of them) to pump on "C > 0" signal and turbine pumps to pump on "T > 1" (so they pump when reactor has enough water and it is hot enough).
13) Fuel it!
Additionally, you can wire circuit network to emit all three signals ("C=1", "T=2" and "W=1") when reactor emergency shutdown is > 0. Otherwise, once it overheats, it will take a lot of time for it to cool down on its own.
They are good as base load power plants, but they don't work well when demand varies a lot. If load is too low, you will end up burning fuel for nothing. If load is so high that your pumps don't have enough electricity, reactors might will starve of water and overheat.
