Both of these actions will help ensure steam/vapor pockets are not formed in the RCS where they
would restrict RCS flow. Thirdly, maintain steam generator water level normal range. This
provides an adequate heat sink to ensure heat removal is sufficient to prevent boiling of the RCS.
The main points of this chapter are listed below.
Natural Circulation Flow Summary
Natural circulation flow is circulation of a fluid without the use of
Forced circulation flow is circulation of a fluid through a system by pumps.
Thermal driving head is the driving force for natural circulation caused
by the difference in density between two areas of fluid.
Three items are necessary to support natural circulation:
There must be a heat sink and a heat source.
The heat source must be located below the heat sink.
Flowpaths must exist between the warm fluid and the cold fluid.
Generally, the greater the temperature difference, the higher the natural
circulation flow rate.
Natural circulation in a PWR can be verified by monitoring:
RCS DT -
25%-80% full power value
THot / TCold -
steady or slowly decreasing
S/G steam pressure - tracking RCS temperature
Natural circulation in a PWR can be enhanced by:
maintain pressurizer level >50%
maintain RCS 15oF subcooling
maintain adequate heat sink, S/G level normal range