Civil Engineering Reference
In-Depth Information
The saturated steam flows from the reactor pressure vessel directly to the turbine
and is pumped back from the condenser and feedwater tank to the pressure vessel.
The condenser is cooled by water from a cooling tower or from a river (Fig.
3.17
).
3.4.2 The ABWR Safety and Depressurization Systems
The reactor shutdown systems and the backup shutdown systems are similar to
those described in Sect.
3.3.2
and Fig.
3.21
. Instead of boric acid also sodium
pentaborate is used [
18
].
The depressurization chamber (wet well), the dry well part of the inner contain-
ment and the outer secondary containment of the ABWR are shown in Fig.
3.30
.
The inner containment is a prestressed concrete building clad by steel plates and
filled with nitrogen.
Ruptures of any pipe in the primary coolant system cause the main steam pipes
and the main feedwater pipes to be blocked by two series-connected main steam
isolation valves (MSIV) and by series-connected non-return valves. In this way the
reactor pressure vessel within the inner containment is isolated from the turbine-
condenser system. Under accidental conditions with overpressure in the reactor
pressure vessel, the safety relief valves (SRV) are actuated automatically (Auto-
matic depressurization system) (Fig.
3.31
). Steam is discharged into the large water
pool in the pressure suppression chamber (wet well) (Figs.
3.30
and
3.31
). If in case
of a core melt also fission products and hydrogen are streaming into the pressure
suppression pool, the gaseous fission products and hydrogen will collect in the
upper dry well part of the inner containment. The inner containment is inertized by
nitrogen. This avoids hydrogen detonations. In addition hydrogen recombiners will
decrease the content of hydrogen there.
3.4.3 Emergency Cooling and Afterheat Removal System
of the ABWR
In case the water level in the reactor pressure vessel drops during a loss of coolant
accident, water is automatically fed into the reactor pressure vessel by the following
systems:
- two automatic high pressure core water flooding systems. Their pumps are
motor-driven. The water is taken from the large suppression pool or from the
condensate storage tank (Fig.
3.31
).
- one automatic reactor core isolation cooling system. The pump of this system is
driven by a steam turbine, the steam is coming from the reactor pressure vessel.
The flooding water is taken from the suppression pool or from the condensate
storage tank.
