The way I handled it was to use an IC2 Detector cable on the power feed from the reactor. For me, I just had a red rp2 lamp to show that the reactor was active. When the cycle ended, the light would go out. You could of course use a NOT gate or inverter such that when there is no power coming from the reactor, then the light goes on.
Mk1 reactors don't really need rp2 stuff (other than a lamp) since they're completely stable.
I use this design. I have 4 reactors and use an rp2 sequencer that has the 4 sides paired such that the reactors run on a 50% duty cycle. I had trouble with RP2 timing circuits on SMP so this design is perfect. It can run a full cycle (in cast something goes wrong with the rp2 stuff) without exploding and running a 50% cycle means I can run them continuously. Expensive and tedious to set up but once in place, run forever very efficiently.