Current pressurized water reactors and boiling water reactors are systems that have been designed with one kind of passive safety feature . In the event of an excessive-power condition , as the water in the nuclear reactor core boils , pockets of steam are formed . These steam voids moderate fewer neutrons , causing the power level inside the reactor to lower . The BORAX experiments and the SL-1 meltdown accident proved this principle
SL-1
SL-1
Passive nuclear safety