By making a water sensor (for the ones not knowing how to make one here is a example: http://www.youtube.com/watch?v=M9aBcRbZ2fk) and then making a hole in the botom of your reactor containment chamber and putting the water source block on a spot where it can evaporate its possible to shutdown your reactor once it passes 50% max heat. Since this is so simple and the water sensor part is already explained by the video i dont feel the need to post screenshots. Diferent designs of watersensors are possible but the way they work remains the same.
Advantages:
-Reactor shutsdown @ 50% max heat
-No need to replace anything except the water source block
-Simple to make
Disadvantages:
-Through its activated earlier its pretty slow to react and will not work on for instance mk5 reactors which overheat in seconds even through they react earlier
-May shutdown your reactor earlier than you want (not suited for 9000 heat breeders for instance)
Just adding another option to the emergency shutdown arsenal 