The two areas must be in contact so that flow between the areas is possible. If the flow path is
obstructed or blocked, then natural circulation cannot occur.
Example of Natural Circulation Cooling
Natural circulation is frequently the primary means of cooling for pool-type reactors and for
irradiated fuel assemblies stored in pools of water after removal from the reactor. The heat
source is the fuel assembly. The heat sink is the bulk of the water in the pool.
Water at the bottom of a fuel assembly absorbs energy generated by the assembly. The water
increases in temperature and decreases in density. Gravity pulls cooler (more dense) water into
the bottom of the assembly displacing the warmer water. The warmer (lighter) water is forced
to give up its position to the cooler (heavier) water. The warmer (lighter) water rises higher in
the assembly. As water travels up the length of the assembly, it absorbs more energy. The water
becomes lighter and lighter being continuously forced upward by more dense water moving in
below it. In turn, the cooler water absorbs energy from the assembly and is also forced to rise
as natural circulation flow continues. Water exiting the top of the fuel assembly gives up its
energy as it mixes with the bulk of the water in the pool. The bulk of the water in the pool is
commonly cooled by circulation through heat exchangers in a separate process.
Flow Rate and Temperature Difference
The thermal driving head that causes natural circulation is due to the density change caused by
a temperature difference. In general, the greater the temperature difference between the hot and
cold areas of fluid, the greater the thermal driving head and the resulting flow rate. However,
it is good practice to keep the hot fluid subcooled to prevent a change of phase from occurring.
It is possible to have natural circulation take place in two-phase flow, but it is usually more
difficult to maintain flow.
Various parameters can be used to indicate or verify natural circulation is occurring. This is
dependent on plant type. For instance for a pressurized water reactor (PWR) selected Reactor
Coolant System (RCS) parameters that would be used are as follows.
RCS DT (THot - TCold) should be 25-80% of the full power value and either steady or
slowly decreasing. This indicates that the decay heat is being removed from the system
at an adequate rate to maintain or reduce core temperatures.
RCS Hot and Cold leg temperatures should be steady or slowly decreasing. Again, this
indicates that heat is being removed and the decay heat load is decreasing as expected.
Steam generator steam pressure (secondary side pressure) should be following RCS
temperature. This verifies that the steam generator is removing heat from the RCS
If natural circulation for a PWR is in progress or is imminent, several actions can be performed
to ensure or enhance core cooling capabilities. First, pressurizer level can be maintained greater
than 50%. Secondly, maintain the RCS subcooled by 15oF or greater.