Civil Engineering Reference
In-Depth Information
3.2.8.3 Emergency Feedwater System
The emergency feed water system (Figs.
3.9
and
3.16
) supplies feed water to the
steam generators, if the main feed water pumps can no longer do so. They are
supplied by emergency power in case the main power supply was to fail. The
emergency feed water system is equipped with fourfold redundancy (KWU-PWR,
US-APWR, EPR), having borated water reserves (feed water tank and
demineralized water tank) which allow the removal of afterheat to be maintained
for many hours. Removal of the afterheat from the reactor core is ensured first by
natural convection of the primary cooling water flowing through the core and steam
generators. Natural convection is enforced by installing the steam generators at a
higher level than the reactor core. After the coolant pressure in the reactor cooling
system has dropped to a sufficiently low level the afterheat removal system takes
over this function.
3.2.8.4 Emergency Cooling (Safety Injection) and Afterheat Removal
Systems
The emergency cooling (safety injection) system of PWRs consists of the high
pressure safety feed system, the flooding tank or accumulators and the low pressure
safety injection system. The low pressure safety injection system is combined with
the residual heat or afterheat system which serves for both operational and safety
related purposes (see Figs.
3.8
,
3.9
,
3.13
, and
3.16
):
- When the power of the PWR is shut down by the protection system, the
emergency cooling (safety injection) and afterheat removal system are started
up automatically, after the pressure and the temperature in the primary systems
have dropped to sufficiently low levels. These systems then remove the afterheat
and continue to cool the reactor core and coolant circuits.
- In a loss-of-coolant accident the emergency cooling (safety injection) and
afterheat removal system have to maintain the coolant level in the reactor
pressure vessel and ensure cooling of the reactor core. The emergency cooling
(safety injection) and afterheat removal system for AP1000 was described in
Sect. 3.2.5.1. It has fourfold redundancy for the KWU-PWR, US-APWR and
EPR and is supplied by emergency power. It can feed water both into the cold
(inlet) and the hot (outlet) main coolant lines by means of a high pressure feed
system (e.g. 15.5 MPa in case of AP1000, 11 MPa in case of KWU-PWRs,
9.2 MPa in case of EPR). In addition there is the medium pressure injection by
accumulators (e.g. 4.5 MPA for AP1000 and 2.5 MPa for KWU-PWRs) and a
low pressure feed system
1 MPa. Major leaks would cause the pressure in the
reactor cooling system to drop so quickly that the high pressure feed system
would not be started up. In that case, borated water will be injected into the
primary main cooling system directly from the flooding tanks or accumulators
and through the low pressure injection systems at
<
1 MPa from the flooding
<
