[cyberax]

Yes. Reactors now are designed to never have positive feedback loops that can result in uncontrollable power spikes. They do the worst case simulations to prove that.

Russian atomic regulator downright shut down the project to design a light water breeder reactor, saying that it's not going to be licensed. Light water breeder reactors are barely theoretically possible, but they require trade-offs to limit the amount of water within the reactor core. So the trade-offs result in a positive void coefficient. It's supposed to be offset by other safety features, but better be safe than sorry.