The modern ones use water as the moderator, if it goes next to nothing reacts because things are wazzing around to fast and the reaction dies.
The bench mark i think is under 5 seconds from SCRAM to less than 90% reaction rate.
And once you design a truely civilian energy reactor and they don't have to fanny around with the flux things become a whole heap easier and safer apparently.
The ESBWR has alot going for it, the major one being it doesn't breed.
It can also be run in a self cooling convective mode so you don't have to worry about keeping any pumps running, you just have to top up the coolant tanks which its pretty easy to be automatic without requiring power on site
Its not the most effecient though. And the power generation turbines become contaminated.