Civil Engineering Reference
In-Depth Information
- temperatures, pressures, pump speeds, water levels in the reactor pressure vessel,
in the pressurizer, and in the steam generators.
The thermal reactor power is determined by measurement of the inlet and outlet
temperatures and the coolant flow in the four cooling circuits.
Disturbances and off-normal conditions, respectively, initiate countermeasures
by the control systems. The control system is no safety system. Its actions coun-
teract the course of disturbances. In case of power changes, the control system
supports self-regulation of the reactor by control rod movement (PWR) or by
changing the speed of the main coolant pumps and initiate control element move-
ments (BWR). The pressure and the level of water in the pressurizer are regulated
by heating the water or spraying water for condensation of the steam in the
pressurizer. On the whole, the control system keeps a number of important
safety-related measured parameters stable within preset
limits,
in this way
preventing unnecessary actuation of the reactor protection system.
When limits of normal operating ranges are exceeded, e.g. 112 % of nominal
power, the reactor protection system automatically intervenes to support the control
system (Fig.
5.7
). It can shut down the reactor by dropping the absorber rods
(scram) or reduce reactor power specifically by feeding boric acid. In the case of
a reactor scram, the reactor protection system at the same time automatically
initiates emergency core cooling, emergency power supply, and isolation of the
reactor building (containment).
The reactor protection system captures the data necessary for accident detection,
e.g., reactor power too high, water levels too low, pump speed too low, etc.. It has
triple redundancy and operates in a 2-out-of-3 logic, i.e. when the initiation criteria
are exceeded in two out of three redundant lines, the reactor is shut down.
5.6.4 Mechanical Design of a PWR Primary Cooling System
The primary cooling system of a PWR consists of the reactor pressure vessel, steam
generator, pumps, a pressurizer, and the piping connected to the reactor pressure
vessel (Fig.
5.3
). In normal operation, this system is under a coolant pressure of
15.5 MPa. The reactor pressure vessel e.g. is made of high-strength 16 MND5 steel
in the EPR (Chap.
3
) or 22NiMo37 in a German BWR, with a stainless steel liner on
the inside. Also the pipes, pump casings, pressurizer, and parts of the steam
generators have stainless steel liners.
5.6.4.1 Reactor Pressure Vessel Design
German reactor pressure vessels made by KWU for the PWR-1300 design—as an
example—are made up of forged rings joined by circumferential welds (Fig.
5.8
).
The pressure vessel is designed to a pressure of 17.6 MPa, a temperature of 230
C,
