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

 where:

• 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 – T_in (or the core average temperature – T_avg). Note that T_avg = (T_out + T_in) / 2. When there is a difference between actual T_{in, actual} and the temperature T_{in, set}, which is set in the system, the reactor control system initiate control rods movement. For example, when T_{in, actual} > T_{in, set}, the reactor control system automatically inserts control rods in order to decrease T_{in, actual}. The reactor thermal power remains constant, because the rods movement only offsets the reactivity from the difference (T_{in, actual} – T_{in, 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 (T_in) 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.

Back to the automatic control, in which reactor control system controls the floating T_{in, actual} and is actuated when T_{in, actual}  – T_{in, actual} is greater than the preset deadband. From this point of view (T_in or T_avg control), usually three regimes are defined:

• Constant Pressure Regime (sliding T_avg). Constant pressure (p_steam) regime means that the reactor control system maintains the steam pressure inside steam generators (or at the turbine inlet) constant as turbine load changes. Also in this case, the automatic system follows  T_{avg, set}, but T_{avg, set} is a function of turbine load. For example, increasing turbine load causes steam pressure to decrease. The reactor control system would sense this decrease in steam pressure and withdraw control rods to increase the reactor coolant temperature. This type of reactor control produces ideal steam conditions at the main turbine inlet for all loads from initial load (e.g. 30%) to 100% load. The temperature difference (ΔT) between primary side and secondary side is increased by raising T_avg or proportionally by raising T_in.  The disadvantage of this type of control scheme is that just minimal changes in turbine load requires (negative MTC) an intervention of the reactor control system. Moreover there are problems with in a high reactor outlet temperature (T_out), which approaches saturation values. Therefore it is not widely used in PWRs especially for U-tube steam generators.
• Constant T_avg Regime (sliding p_steam). Constant T_avg regime means that the reactor control system maintains the average core temperature of the reactor coolant constant as turbine load changes. The automatic system follows  T_{avg, set}, which is constant. For example, increasing turbine load causes steam pressure to decrease. This, in turn, causes T_avg to decrease, because the turbine uses more energy than that produced by the reactor. The reactor control system would sense this decrease in T_avg and withdraw control rods to increase the reactor coolant temperature. This regime causes minimal changes in reactivity of the core, due to minimal changes in the average coolant temperature. Since the coolant temperature remains constant the coolant volume does not change and therefore the pressurizer level is remains constant for all load conditions. The disadvantage of this type of control scheme is that steam conditions (steam pressure) significantly changes from hot zero power to full load, which is unacceptable.
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  Sliding Tavg and Sliding psteam Regime. The automatic system follows Tavg, set, but Tavg, set is a function of turbine load and it is programmed for best performance of the entire system.

  Sliding T_avg and Sliding p_steam Regime. This regime combines the previous regimes. The automatic system follows  T_{avg, set}, but T_{avg, set} is a function of turbine load and it is programmed for best performance of the entire system. Sometimes, there are many programmed functions of T_{avg, set}, which are for different purposes (e.g. for initial power increase, for load-follow, for coast down). The control scheme most often selected to control the reactor is the sliding T_avg and sliding p_steam regime. This regime is a compromise between the other two regimes of control and contains some of the advantages and disadvantages of each.