most fission nuclear reactors use uranium-235 or plutonium-239 nuclear fuel submerged in water to generate thermal energy by means of boiling the water, circulating the water and turning it into steam to power turbines, which convert elctro-mechanical energy to electricity. because of intense radioactivity, these fuel rods have to be fully submerged to prevent fumes from getting into open air (when they are exposed or not fully immersed). prolonged exposure outside of water will cause the heat thru' constant fissile reaction to melt the core, which when it becomes molten will be extremely difficult to control and/or contain. standing water alone will not be able to keep rods cooler as they will boil away the water (just like you boil water during ns for your instant noodle with a thermal coil, and when your water is boiled away, the coil will start melting your plastic cup). the water has to be circulated or replenished with colder water. when a rod is spent, it is still decaying and generating heat. same thing happens when a reactor is shut down, rods are still going thru' neutron absorption and producing heat thru' radioactive decay. spent rods are placed in a cooling tank, fully submerged, with the water coolant being constantly circulated. reactor that is shut down for maintenance or during an emergency has rods taken out and placed in cooling tanks. in cases where rods are left in the reactor due to acute emergencies, the water in the reactor also needs to be constantly replenished or cooled.
