Civil Engineering Reference
In-Depth Information
Fig. 3.14
Overview of US-APWR design [
13
]
3.2.7 Control Systems
At nominal power all PWRs have negative feedback coefficients of coolant tem-
perature and power (see Chap.
2
). Any reduction in core coolant temperature
therefore will result in an increase in reactivity and power. If higher loads are
demanded by the generator and the turbine, more heat must be extracted from the
primary cooling system through the steam generators. This is done by opening the
turbine governor valve which causes the primary coolant temperature in the reactor
core to drop and subsequently the reactor power to rise; the power automatically
balances out at a slightly higher level. However, in order to prevent the steam
temperature and the steam pressure from dropping too far, the control elements are
also moved up at the same time.
Reactivity changes in the core are balanced by axial movements of the control
elements: slow changes of the kind brought about by fuel burnup and fission
product build up are also controlled by changing the boric acid concentration of
the primary coolant. Also gadolinium is mixed with the UO
2
fuel as a burnable
poison if very high burnup of the fuel shall be attained.
The coolant pressure is kept at the systems pressure by the pressurizer (Fig.
3.6
).
The water level in the pressurizer is controlled by the volume control system. The
addition of feed water to the steam generator must be matched by the feed water
control system as a function of the amount of steam extracted for the turbine.
