I just used a squirrel because it has 1 hp, and I had a few poke... erm... Safari Balls. The second the reactor shoots one tick of radiation, it dies and shuts the whole thing down. You could use any creature, and pre-damage them if you wanted it to shut down quickly. Also, the piston is powered by the pressure plate the animal is standing on, so pushing/pokeballing the creature into the cage seals it until the squirrel has a tragic accident.
I considered using a yellow bird from Twilight Forest (canary!), but I wasn't sure if it would stay on the pressure plate constantly.
Also, no squirrels harmed in the making of this picture. Yet.