Civil Engineering Reference
In-Depth Information
In case of core melting the IRWST can be drained into the reactor cavity
surrounding the reactor pressure vessel. The reactor pressure vessel is then
surrounded by water. This prevents failure of the wall of the reactor pressure vessel
The design pressure of the inner containment is 0.41 MPa. The thickness of the
steel wall of the inner containment is 44 mm. The thickness of the reinforced
concrete wall of the outer shield building is 91.4 cm [
11
,
12
]. The design basis
for seismic events is 0.3 g ground acceleration.
The core coolant inventory in the containment for recirculation cooling and
boration of the core is sufficient to last at least 30 days. Hydrogen igniters and
autocatalytic recombiners shall prevent hydrogen explosions or detonations.
3.2.6 The US-APWR Containment Design
The US-APWR design with 1,700 MW(e) power output (Table
3.2
) is based on the
Mitsubishi (Japan) APWR design modified to meet the US-licensing requirements.
It is a four loop design (Fig.
3.14
). Its core has 257 fuel elements of 17
17 fuel
rods (Fig.
3.14
). The core is reloaded every 18 months [
3
,
5
,
13
] (Fig.
3.15
).
The US-APWR has a single wall concrete containment and an in-containment
refueling water storage tank (IRWST) with large water volume. In case of threat of
core melting the steam/water mixture in the reactor pressure vessel can be released
into the IRWST via drain lines after opening of depressurization valves above the
pressurizer (Fig.
3.16
). Water of the IRWST can be pumped into the reactor cavity.
In this way the lower part of the outside pressure vessel is flooded with water to
prevent core melt through to the bottom. The bottom of the reactor cavity is clad by
a steel liner where the core debris shall be spread into a thin melt layer for long term
cooling. In case of boiling water in the IRWST the steam would rise into the upper
part of the containment. It shall be condensed there by water of a containment spray
system. The containment atmosphere can additionally be cooled by an alternative
containment cooling system. Hydrogen formed during a core melt accident by
chemical reaction between steam and the hot zirconium cladding will be monitored
in the upper part of the containment. The hot hydrogen shall be burnt by a number
of igniters.
Water for the containment spray system can be fed from the IRWST by a pump
through a heat exchanger (CS/RHR HX) or by a fire-water pump from an outside
water tank. Similarly a pump can take water from the IRWST and feed it back to the
cylindrical cavity around the reactor pressure vessel. If needed, water can be
pumped to the IRWST from the outside water tank via the fire-water pump.
Emergency feed water for the steam generators can be provided by emergency
feed water pumps from outside water sources.
