Civil Engineering Reference
In-Depth Information
serves as a shield against radiation towards the outside in case the inner contain-
ment was radioactively contaminated in an accident.
The space between the containment and the prestressed outer concrete shell is
held at a slightly negative pressure relative to the internal pressure and the atmo-
spheric pressure of the external environment by means of a blower. This makes
uncontrolled leakages to the outside impossible in normal operation. The air taken
in is discharged from the stack through filters.
5.6.5.2 Safety Systems in the Containment
The safety systems in the containment are summarized schematically in the illus-
tration of Fig.
5.10
for the EPR containment.
- The containment spray system, which cools the atmosphere of the containment
after a loss-of-coolant accident, condenses the steam released and thus acceler-
ates pressure reduction. The water for spraying is taken from the in-containment
refueling water storage tank (IRWST) (see Fig.
5.10
or Sect.
3.1
for PWRs and
Sect.
3.2
for BWRs).
- During the recirculation phase the low pressure emergency core cooling system
takes in the water from the containment sump for cooling (Kraftwerk Union
PWR) or from the IWRST (EPR and AP1000 or US-APWR in Sect.
3.1
) or from
the water pool or the pressure/suppression chamber of BWRs (Sect.
3.2
).
- A containment heat removal system decreases the temperature and pressure in
the containment over the medium term (Fig.
5.10
).
- Two redundant valves are used for containment isolation, one of which is
installed inside, the other one outside the containment. Building isolation is
initiated especially after a loss-of-coolant accident or when higher radioactivity
levels are detected in the containment.
5.6.6 Analyses of Operating Transients (Safety Level
3, Design Basis Accidents)
The course of various operating transients must be studied for the following
accidents with and without failure of the scram system. Transients with failure of
the scram system are also referred to as ATWS (Anticipated Transients without
Scram). The operating transients listed below—as an example—must be studied as
a design basis:
- Failure of the main heat sink, e.g., as a result of closing of the main steam valve
with the off-site (auxiliary) power supply functioning.
- Failure of the main heat sink with the off-site (auxiliary) power supply
unavailable.
