POWER May 2012 - 44

NUCLEAR
(from, for example, a lake, river, or ocean),
or other offsite supplies. The diesel fuel
supply for the ancillary diesel engines will
also need to be replenished.
The design basis for containment cooling
utilizes continuous water distribution on the
vessel steel shell. In the highly unlikely case
of an operator not being able to supply water
to the top of the containment after seven
days of cooling, the vessel shell would become
dry and the flow of air through the
annulus region would provide heat removal.
Pressure within the containment vessel
would slowly increase but would not reach
the normal design pressure for over two
days. Even in the case of this very unlikely
event, the steel containment vessels have a
very large design margin and will not exceed
the ASME Service Level C pressure
limit. Therefore, even " air-only " cooling
decay heat removal for the containment vessel
steel shell following seven days of water
cooling will prevent damage to the fuel in
the reactor core.
Design-Basis Flood. Because the Vogtle
site is about 130 miles from the coast and
220 feet above sea level, its location is not
vulnerable to floods, tsunamis, dam breaks,
or other events on the Savannah River, including
the failure of all upstream dams.
Nuclear power plants are designed to
effectively manage flooding levels up to
the design-basis flood with some degree
of margin beyond the design level as part
of the plant's standard design. The maximum
flood level assumed for the AP1000
is the plant design grade elevation. Flooding
of intake structures, cooling canals, or
reservoirs or channel diversions does not
prevent safe operation of the plant. In the
instance of a design-basis external flood,
the AP1000 standard plant response is to
stop all unnecessary plant evolutions (such
as maintenance or testing) and close exterConstruction
Is a Cooperative, Global Effort
Large safety-related and non-safety-related components are being
shipped in from suppliers located all over the world, including
Japan, South Korea, and Italy.
To be clear, the term " safety-related " is a classification applied
to items that must function during or following a design-basis
event, such as an earthquake. Specifically, a safety-related function
protects the integrity of the reactor coolant pressure boundary,
the capability of the reactor to shut down and stay in a safe
shutdown condition, and the capability to prevent or mitigate offsite
exposures based on NRC guidelines. Safety-related also applies
to documentation and quality assurance requirements during
manufacturing, in accordance with 10 CFR 50, Appendix B quality
assurance requirements.
In 2010, the first critical components for the new plants started
arriving from Japan at the Port of Savannah. That shipment
brought massive plates for the bottom head of the Unit 3 contain5.
Taking the long way. Condenser components arrived at
Plant Vogtle in late January via railcars after the long trip from South
Korea. Courtesy: Southern Company Inc.
nal portals. In the unlikely scenario that an
AP1000 suffers a severe flood that exceeds
the design-basis elevation, the reactor core
and spent fuel pool remain protected.
The two most important features of the
AP1000 that provide defense against flooding
and other external hazards are that safe
shutdown and core cooling are provided
by systems located inside the containment
vessel that are designed to " fail safe " upon
loss of power, loss of instrumentation and
control, and loss of instrument air. The
containment vessel is a 1.75-inch-thick
steel pressure vessel that is not affected by
flooding. The spent fuel cooling is from
water stored in pits that are at least 35 feet
above the grade elevation.
High Winds and Tornados. Severe
winds, such as those generated by hurricanes
or tornados, pose a threat of wind
loading on a structure that may cause damage
or collapse; damage may also result
ment vessel; each bottom head consists of 62 plates. There will be
close to half a million inches of weld on the bottom head alone.
The Chicago Bridge and Iron Co. (CB&I) was awarded the contract
to fabricate and assemble both the Unit 3 and 4 containment vessels.
CB&I supported Westinghouse for over 10 years in the design
of the containment. Here are some interesting statistics pertaining
to the pressure vessels:
■ Each containment vessel weighs ~4,000 tons.
■ Each 1.75-inch-thick containment vessel is approximately 130
feet in diameter by 215 feet tall.
■ The vessels have approximately 70 penetrations ranging from ½
inch to 16 feet in diameter.
■ The containment vessels will be enclosed inside the shield
building with a 4.5-foot annulus area between them.
■ Each containment vessel is subassembled into five major sections
and transported to the nuclear island for placement and
welding in-place.
Toshiba, located in Japan, and South Korean engineering
company BHI Co. Ltd., for example, designed and manufactured
the steam condenser for Unit 3. The assembly, weighing
about 3,600 tons, left BHI's plant in Sacheon on November
21, 2011, to begin its journey by sea from the port of Masan,
South Korea, through the Panama Canal, to the Port of Savannah
in Georgia, where it arrived in early December. Fortunately,
none of the major component fabrication facilities in Japan or
South Korea was damaged by the March 2011 earthquake and
tsunami (Figure 5).
Components manufactured in the U.S. are shipped either by
truck or rail. For instance, moisture separator reheaters are being
shipped from Oklahoma, and reactor control instrumentation is
coming from Westinghouse facilities in Pennsylvania.
44
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