Fluid Flow Rate and Temperature Difference Review
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.
1. 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.
2. 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.
3. Steam generator steam pressure (secondary
side pressure) should be following RCS temperature.
This verifies that the steam generator is removing heat from
the RCS coolant.
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
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.