The Design of Light Water Reactors - The Risks of Nuclear Energy Technology - page 53

tanks , e.g. in case of the KWU-PWRs. If the flooding tanks or accumulators have
run dry, the low pressure injection system feeds water from the reactor building
sump into the primary systems in case of KWU-PWR. The building sump can
collect leakage water from the primary system (sump recirculation operation). In
case of AP1000, US-APWR or EPR, water discharged inside the containment is
collected in the in-containment refueling water storage tank (IRWST) located at
the lower part of the containment. The low pressure safety injection and residual
heat removal systems can take water from the IRWST (AP1000, US-APWR,
EPR).
Smaller leaks cause the pressure to drop only gradually, so that initially only the
high pressure feed systems will start to function. However, the pressure and
temperature drop in the main coolant system is supported by a temperature drop
of 100 C/h on the secondary side.
In case of AP1000, US-APWR and EPR the primary coolant system under high
pressure can also be depressurized automatically via several depressurization valve
lines on top of the pressurizer into the IRWST. After the coolant pressure and
temperature have dropped sufficiently below 1 MPa, the low pressure feed systems
will be started up.
The afterheat is discharged into river, lake or sea water, a cooling pond or special
cooling towers by way of intermediate heat exchangers (secondary parts of the
emergency cooling and afterheat removal system), which are also fourfold redun-
dant (KWU-PWRs, US-APWR, EPR) and driven by emergency power.
3.3 Boiling Water Reactors
The Boiling Water Reactors (BWRs) built today by a number of manufacturers in
the USA, Europe and Japan are characterized by almost identical technical designs.
This section will deal with the standard BWR-1,300 of Kraftwerk Union [ 14 ] and
the SWR-1,000 (KERENA) as designed by AREVA [ 15 ]. In addition the advanced
boiling water reactor (ABWR) built by General Electric (USA), TOSHIBA and
HITACHI (Japan) will be presented. Figure 3.17 shows the functional design
diagram of a BWR plant. In a BWR steam is directly produced in the reactor core
and dried in upper steam dryer structures of the reactor pressure vessel. The steam
then flows directly to the turbine generator system. The steam condenses in the
condenser of the turbine system. The condenser is cooled by water from a cooling
tower. Condensate water is then pumped back through a feedwater tank to the
reactor core.