You may know of two methods for doing this already.
1. A wooden block carrying a redstone signal within the 5x5x5 radius of the reactor. Once the reactor hits 40% heat, it starts setting fire to such blocks, breaking the redstone signal. This can be used to shut off the reactor.
Upsides - pretty simple.
Downsides - destroys the wooden block, which will need to be replaced.
2. The chicken kill switch. Imprison a chicken on top of a (stone, I think) pressure plate. When the reactor hits 65/70% heat, the radiation level increases, and the chicken dies, releasing the pressure plate.
Upsides - simple.
Downsides - You need to replace the chicken.
The third method is to detect the water evaporation which happens at 50% heat, and you can do that by placing a wall like the one pictured around part of your reactor. In this demo, standing on the pressure plate arms the detector by first powering the pistons at the bottom, then via a redstone wire along the top. (Note the extra delay for the line along the top.) When power is released, the piston's don't retract, and this is the state you see in the picture. At least, they don't until a block update occurs next to one of the pistons. This can detect evaporating water, but in this mockup you can emulate that by placing or removing a block next to one of the pistons. When you do that, the whole wall will retract.
To turn this into a redstone signal, I usually mount the last piston in the row sideways, and make it sticky. You can then use standard redstone techniques to turn this into a signal to turn off your reactor when the piston retracts.
Once set up, it doesn't consume anything, and it'll turn off your reactor within about 20 seconds of it hitting 50% heat.