Example of Natural Circulation Cooling

Natural circulation is often the primary means of cooling for pool-type reactors and for irradiated fuel assemblies store up in pools of water after elimination from the reactor. The heat source is a fuel assembly. The heat sink is the volume of the water in the pool.

Water at the underneath of a fuel assembly absorbs energy produced by the assembly. The water rises in temperature and reduces in density. The gravity pulls cooler (i.e., more dense) water into the bottom of the assembly displacing the warmer water. The warmer (or lighter) water is forced to give up its location to the cooler (or heavier) water. The warmer (or lighter) water increases higher in the assembly. Since water travels up the length of the assembly, it soaks up more energy. The water becomes lighter and lighter being constantly forced upward by more dense water moving below it. In return, the cooler water absorbs energy from the assembly and is also forced to increase as natural circulation flow carries on. Water exiting the top of the fuel assembly provides up its energy as it mixes with the volume of the water in the pool. The volume of the water in the pool is generally cooled by circulation during heat exchangers in a separate procedure.