Temperature gradients in typical PWR steam generator.
As was written, after synchronization of generator, the reactor control system is usually switched to automatic control and the additional power increase is in this mode. The power plant is then controlled by plant control system, that coordinates the NSSS and the turbine control system. The interfacing variable is in this case the core inlet temperature, which is fully determined by steam pressure inside steam generators. Note that, the core inlet temperature and the steam pressure are interconnected, since heat (or power) transferred across a steam generator is:
q = h . ΔT
- q is amount of heat transferred (heat flux), W/m2 i.e., thermal power per unit area
- h is heat transfer coefficient, W/(m2.K)
- ΔT is the difference in temperature across the steam generator (in this case, the difference between the average temperature of the reactor coolant – Tavg and the saturation temperature determined by system pressure.
For all practical purposes, the heat transfer coefficient (h) is constant, since the heat transfer coefficient is a function of the materials used in the construction of the steam generator and the U-tubes are completely covered with water.
See also: Steam Turbine
When a reactor is in the automatic control, it follows the core inlet temperature – Tin (or the core average temperature – Tavg). Note that Tavg = (Tout + Tin) / 2. When there is a difference between actual Tin, actual and the temperature Tin, set, which is set in the system, the reactor control system initiate control rods movement. For example, when Tin, actual > Tin, set, the reactor control system automatically inserts control rods in order to decrease Tin, actual. The reactor thermal power remains constant, because the rods movement only offsets the reactivity from the difference (Tin, actual – Tin, set) x MTC = moderator defect.
On the other hand, if the energy demand in the external system increases, more energy is removed from reactor system causing the temperature of the reactor coolant (Tin) to decrease. As the reactor coolant temperature decreases, positive reactivity is added and a corresponding increase in reactor power level results. This reactor power increase occurs without any change in control rods position and without any change in boron concentration. The same inherent stability can be observed as the energy demand on the system is decreased.
Schema of a steam turbine of a typical 3000MWth PWR.
It must be added, in case of disconnected automatic control, the turbine controls the steam pressure and the reactor control system is in manual regime. In this case a control rods insertion causes a decrease in reactor thermal power since the steam pressure remains constant. The turbine load decreases as the reactor thermal power decreases.