Civil Engineering Reference
In-Depth Information
Passive containment
cooling system (PCCS)
gravity drain water tank
Natural convection
air discharge
Water film evaporation
Outside cooling
air intake
Steel containment
vessel
Air baffle
Internal condensation and
natural recirculation
Steam generator
Reactor pressure
vessel
In-containment
Refueling Water
Storage Tank
(IRWST)
Accumulators
Fig. 3.12
AP1000 passive containment cooling system [
2
,
9
]
3.2.5.1 Emergency Core Cooling System (ECCS)
The passive core cooling system (PXS) protects the AP1000 plant against core
cooling accidents (leaks and ruptures at different locations of the reactor cooling
system). The PXS provides safety injection of coolant water, depressurization of the
primary system and residual heat (afterheat) removal.
In case of e.g. a double-ended rupture of a main reactor cooling line the safety
injection and depressurization systems start to act (Fig.
3.13
). The PXS uses three
sources of cooling water to maintain core cooling through safety water injection.
The safety water injection systems including two core water makeup tanks (CMTs)
at high coolant pressure (15.5 MPa borated water), the two accumulators with
borated water at medium coolant pressure (4.8 MPa) and the IRWST with borated
water which starts to act at pressures below 0.18 MPa (after full depressurization).
These three water injection systems are connected to separate injection nozzles at
the reactor pressure vessel [
2
,
9
]. In this way no safety injection water can be lost in
case of breaks in the main cooling lines (Fig.
3.13
).
In case of an accident sequence with a threat of core melting, the primary coolant
system can be depressurized after reactor shutdown by the automatic depressuriza-
tion system (ADS) (Fig.
3.13
) into the IRWST via two depressurization stages with
threefold valve lines at the pressurizer and a third and a fourth depressurization
stage connected to the main coolant lines [
2
].
