Constant Pressure – Constant Temperature Regime
Back to the automatic control, in which reactor control system controls the floating Tin, actual and is actuated when Tin, actual – Tin, actual is greater than the preset deadband. From this point of view (Tin or Tavg control), usually three regimes are defined:
- Constant Pressure Regime (sliding Tavg). Constant pressure (psteam) 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 Tavg, set, but Tavg, 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 Tavg or proportionally by raising Tin. 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 (Tout), which approaches saturation values. Therefore it is not widely used in PWRs especially for U-tube steam generators.
- Constant Tavg Regime (sliding psteam). Constant Tavg 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 Tavg, set, which is constant. For example, increasing turbine load causes steam pressure to decrease. This, in turn, causes Tavg to decrease, because the turbine uses more energy than that produced by the reactor. The reactor control system would sense this decrease in Tavg 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.
Sliding Tavg and Sliding psteam Regime. This regime combines the previous regimes. 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. Sometimes, there are many programmed functions of Tavg, 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 Tavg and sliding psteam regime. This regime is a compromise between the other two regimes of control and contains some of the advantages and disadvantages of each.