A Robust
Reactor
By Tom Geer, Westinghouse Electric
Company.
Response to questions by Newal
Agnihotri, Editor of Nuclear Plant
Journal.
1.
What’s the difference, functionally
and structurally, between the outer
containment for the AP1000
®
plant
as compared to the conventional
Westinghouse PWR plant containment?
The AP1000 plant design has a free-
standing steel containment vessel sur-
rounded by a concrete shield building. As
with conventional pressurized water reac-
tors (PWRs), the AP1000 reactor’s con-
tainment vessel acts as a leaktight barrier
to contain the release of airborne radio-
activity following postulated design-basis
accidents and to provide shielding for the
reactor core and the reactor coolant sys-
tem during normal operations. Unlike
conventional PWRs,
theAP1000 reactor’s
containment vessel
is also an integral
part of the passive
containment cooling
system, whose func-
tion is to provide
the safety-related
ultimate heat sink.
Heat removal from
the exterior of the
containment shell
is enhanced by a di-
rected-flow natural
convection design
and a passive, ex-
ternal cooling water
distribution system.
The shield building is the structure
and annulus area that surrounds the
containment vessel and provides the
natural convection flow path and the
external cooling water for passively
cooling the containment vessel. The
shield building structure is a combination
of reinforced concrete (typical of
conventional PWRs) and concrete-filled
steel plate construction (SC) similar
to that being employed for structural
modules throughout the AP1000 plant.
2.
How is indefinite cooling of the plant
and the fuel pool achieved after 72 hours,
when the onsite and offsite power may
still be unavailable?
For the first 72 hours, the passive
containment cooling heat transfer
process is aided by the use of water that
is gravity-drained from a tank on top of
the shield building onto the containment
vessel. Beyond 72 hours, the passive core
cooling system will continue to remove
heat from the core to the containment,
and the passive containment cooling
system will continue to remove heat from
the containment vessel to the atmosphere.
Because the water tank located on top of
containment will be emptied in 72 hours,
water must be re-supplied to aid in the
heat transfer to the atmosphere. This is
done via on-site, permanently installed
equipment and water tanks, or a safety-
related connection point conveniently
located at ground level for use with a small
transportable pump. This same pump
could also be used to provide makeup
water to the Spent Fuel Pool (SFP).
To support the continued operation
of the passive safety systems (core and
containment cooling) and SFP makeup,
only about 135 gallons per minute is
required.
For more information regarding the
AP1000 plant design’s coping ability
during station blackouts, please refer to
the following website:
.
westinghousenuclear.com/station_
blackout_home/
3.
How is the reliability of the passive
safety system achieved to alleviate
concerns of a stuck valve or anundesirable
leakage of the passive cooling system?
The AP1000 plant is designed to
mitigate events, including loss of coolant
accident (LOCA) pipe breaks up to
a double-ended rupture of the largest
reactor coolant system (RCS) pipe.
The passive systems are also designed
to handle a single failure occurring, in
addition to the initiating event. So, the
plant can mitigate a LOCA with any
remotely operated valve failing to open
(sticking closed).
Portions of the passive safety
injection systems are connected to the
RCS such that any potential leakage from
these areas would be RCS leakage. RCS
leakage is monitored very closely by
redundant and diverse leakage monitoring
instruments. If the leakage became more
than an insignificant amount, the plant
would be shut down to repair the leak.
Note that the AP1000 plant does
not recirculate radioactive water outside
containment, as is done in active safety
plants where core cooling following a
Tom Geer
As vice president, Engineering &
Licensing, in the
Nuclear Power
Plants organization,
Tom Geer is
responsible for
the licensing of
Westinghouse
new-plant designs
globally. He also
is accountable
for ensuring
that engineering
deliverables are
coordinated with
construction
activities at the U.S.
AP1000
®
new-plant
sites.
Mr. Geer has more
than 30 years of management and
engineering experience.
He chairs the Texas A&M Nuclear
Engineering Advisory Council and
serves on the North Carolina State
Nuclear Engineering Department
Advisory Board.
Mr. Geer holds both a bachelor’s
degree and master’s degree in nuclear
engineering from Texas A&M University.
He also received technical nuclear
certification at McGuire Nuclear Station
and completed senior nuclear plant
manager training through the Institute of
Nuclear Power Operations (INPO). He
is a registered Professional Engineer in
North and South Carolina.
40
NuclearPlantJournal.com Nuclear Plant Journal, July-August 2013
