Basic Components of PWR ( part 4 )

External Components

Pressurizer

According to the criteria the pressure of the system must be higher than the saturation pressure of liquid to avoid bulk boiling of the coolant.

The main function of pressurizer :

1-   Maintains the coolant pressure during steady state operation.

2-   Limits pressure change caused by thermal expansion and contraction during normal load transients.

3-   Prevents coolant pressure from exceeding limits.

Pressurizer Level

Under steady state operation, pressurizer level represents a balance of water flow into the reactor coolant system and water letdown flow into the chemical and volume control system. Therefore, when water level begins to change we have an imbalance of in and out water flow conditions. This imbalance represents a change in water inventory in the reactor coolant system. Transient load condition will also produce a change in pressurizer water level. This is brought about by the fact that in the pressurized system, water volume changes as a function of reactor coolant average temperature, when coming down in load the decrease in reactor coolant average temperature causes the total water volume is not change in water inventory. However, the level does change with temperature. By programming the pressurizer level set point as a function of reactor coolant average temperature, the level control system will not respond from external load disturbances. Under steady state conditions, the level controller will only take control action to maintain level at the programmed set point. This will serve to correct for changes in water inventory in the reactor coolant system then when the level does deviate from the set point, the level control system will re-establish the in and out water flow conditions, and maintain the required water inventory by restoring the level back to the set point.  

Pressurizer Pressure Control System   

The pressurizer pressure control system limits pressure excursions, which might induce reactor trip, changes in reactivity, and actuation of the relief valves. The pressurizer pressure is maintained at a fixed set point the pressurizer pressure controller is provided with proportional, rate, and reset adjustments. Deviation of pressurizer pressure from its set point is the pressure compensation signal sent to the average temperature control system for rod speed control. The out put of the pressurizer pressure controller is compared with fixed set points of the proportional spray controllers and the proportional heaters controls. The control system will perform as follows some of the heater is energized to provide heat input to compensate for pressurizer heat losses and achieve equilibrium between water and steam.

 For small pressurizer pressure variations, the proportional heaters will increase or decrease heat to restore system pressure back to set point.

For large pressurizer pressure variation caused by larger in surges, steam is compressed and pressure is raised above the set point.