content: There are a couple reasons and less waste is certainly one of them.  These use liquid metal at ambient pressure as the coolant instead of pressurized water, which reduces the possibility of a loss of coolant incident. Also, the coolant loop is designed to operate under 'natural convection' so even if the pumps shut off (Fukishima) the coolant will still flow As the temperature increases more neutrons (the drivers behind fission) will escape the core making it self-moderating and reducing the chance of a run away reaction. Light-water reactors have this too, but they need external controls to make this happen while (at least the Integral Fast Reactor) breeder reactors do not Most of the 'meltdown' heat comes not from the actual fuel, but the decay heat from the fission products that are created in the reactor core during operation. I'm not positive on this but I think that by burning most of the produced fissile materials during operation there is less present to contribute to decay heat once the reaction is stopped