The pressuriser control system is designed to:

* Maintain the mass inventory of the reactor coolant system by controlling the pressuriser level accord­ing to the reactor coolant temperature, so that the coolant in-surges and out-surges which occur during load changes may be accommodated without undue loss from or make-up to the reactor coolant system, or a reactor trip.

• Control the primary coolant pressure so as to avoid undue discharge through the pressuriser relief or

safety salves.

Pressuriser level

The pressuriser water level is compared with its de­manded value to form the water level error signal, which is then compensated by a proportional/integral — derivative (PID) controller to provide the demanded charging flow. The letdow-n flow is isolated if the pressuriser water level falls below a low level setpoint to avoid uncovering the heaters and to maintain the »’Vater inventory of the primary circuit. Reactor coolant ls discharged to the letdown line from a crossover leg of the RCS.

The charging valve position and valve position de­mand are checked for correct response to control system demands.

The demanded value of the pressuriser water level is programmed as a function of the estimated average coolant temperature in the primary circuit, so that at part-load the water level is reduced from its full load value in order to match as closely as possible the contraction of the water in the primary circuit as its temperature is reduced.

Pressuriser pressure

The pressuriser pressure is compared with its demanded value to form the pressure error signal which is then compensated by a PID controller to form the actuation signals for the pressuriser heater and spray controls. Increasing the heater power increases the pressuri­ser pressure, and increasing the spray rate decreases the pressuriser pressure. The pressuriser proportional heater controller demand is checked for correct re­sponse to control system requests.

The 78 electrical heaters are located near to the bottom of the pressuriser. Eighteen of the heaters are proportionally controlled to correct pressure devia­tions arising from small disturbances in the reactor and primary circuit. The remaining (back-up) heaters are switched on when the pressuriser pressure control requires more heat than can be supplied by the pro­portional heaters. Operation of all the heaters is inhibited when the pressuriser water level is low and likely to uncover the heaters. Heat losses from the pressuriser, including heat losses due to the small, continuous spray, require the proportional heaters to be at approximately half power at full load steady — state conditions.

Pressuriser spray from the nozzle located in the top of the pressuriser is initiated when the pressure con­troller spray demand exceeds a given value. The spray rate then increases proportionally with increasing spray demand, until the maximum spray flow is reached. Steam condensed by the spray reduces the pressuri­ser pressure back towards its demanded value. A small, continuous spray is normally maintained to reduce thermal stresses and to help maintain uniform water chemistry and temperature in the pressuriser. The spray valve positions are checked for correct re­sponse to control system demands.

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