What happened at Three Mile Island, Chernobyl and Fukushima? What role did water coolant play in each one? Why do we use water as a coolant, given it has a liquid range of only 100 degrees Celsius?

Today's reactors, which are cooled by pressurized water, were viewed as a stop-gap technology until "breeder reactors" could be commercially deployed. Breeder reactors convert fertile material (such as Thorium) into fissile material (such as Uranium-233). It is not practical to operate a breeder reactor with water coolant, so a new coolant must be chosen. We examine two coolant options (from a field of many): liquid salt, and liquid sodium. What advantages do each have over water? Is one safer than the other?