Flow Rate and Temperature Difference
The thermal driving head which causes natural circulation is due to the density change caused by a temperature difference. In common, the greater the temperature differences among the hot and cold regions of fluid, the greater the thermal driving head and the resultant flow rate. Though, it is an excellent practice to maintain the hot fluid sub-cooled to prevent a change of phase from taking place. It is possible to contain natural circulation occur in two-phase flow, though it is generally more hard to sustain flow.
Different parameters can be employed to point out or confirm natural circulation is taking place. This is reliant on plant type. For illustration for pressurized water reactor (PWR) chosen Reactor Coolant System (RCS) parameters which would be employed are as follows.
1. RCS ΔT (THot - TCold) must be 25-80% of the full power value and either stable or gradually reducing. This points out that the decay heat is being eliminated from the system at a sufficient rate to sustain or decrease core temperatures.
2. RCS Hot and Cold leg temperatures must be stable or slowly reducing. Again, this point out that heat is being eliminated and the decay heat load is reducing as predicted.
3. The steam generator steam pressure (i.e., secondary side pressure) must be following RCS temperature. This confirms that the steam generator is eliminating heat from the RCS coolant.
When natural circulation for a PWR is in evolution or is imminent, numerous actions can be performed to make sure or enhance core cooling capabilities. At first, pressurize level can be maintain greater than 50%. Secondly, maintain the RCS sub-cooled by 15oF or greater.
Both of these actions will aid make sure steam or vapor pockets are not produced in the RCS where they would restrict RCS flow. Thirdly, sustain steam generator water level ≥ normal range. This gives a sufficient heat sink to make sure heat elimination is adequate to prevent boiling of the RCS.