POWER May 2012 - 42

NUCLEAR
ground acceleration (PGA), about equal
to an earthquake of magnitude 6.6 on the
Richter Scale at the epicenter (Figure 3).
The AP1000 design is also evaluated for
a seismic margin analysis, which is 67%
above the SSE with a 0.5g PGA. This condition
is called the Review Level Earthquake
(RLE). Seismic margin analysis assumes a
95% probability that the SSCs will retain
structural integrity after an RLE.
Furthermore, an independent seismic
peer review submitted to the NRC confirmed
that Plant Vogtle is capable of
sustaining an earthquake ground motion
representing an earthquake magnitude of
about 7.0. As a comparison, the magnitude
of the Fukushima Daiichi seismic event
was 9.0 on the Richter Scale at the epicenter
with an observed maximum 0.52g PGA
at 109 miles. The Vogtle site has recorded
no earthquake ground motion in the past
20 years.
AP1000 Response to a 0.3g SSE.
Should an SSE occur, the analysis conservatively
assumes that the seismic event causes
general infrastructure damage leading to a
loss of offsite power (LOOP) event concurrent
with a reactor and turbine trip. All Seismic
Category 1 structures remain intact and
functional, and damage done to other structures
on site will not prevent the functional
performance of Category I SSCs.
For the first 72 hours, reactor core decay
heat is removed via natural circulation
from the passive heat exchanger and the
in-containment water tank to the containment
vessel and the passive containment
cooling system water tank on top of the
shield building (Figure 4). The passive
core cooling methods occur automatically
without operator action and without the
use of AC power. Boiling off existing water
inventory cools the spent fuel pool. Minor
operator action is eventually required
for a one-time valve alignment to provide
makeup water to the spent fuel pool from
the cask washdown pit.
From 72 hours to seven days, makeup
water for decay heat removal from the
spent fuel pool and containment (the reactor
core) is provided by the ancillary water
tank located at grade level. Ancillary diesel
generators are small (80 kW) and rugged,
and they support a few specific plant power
needs, such as the makeup pumps used to
move water from the ancillary water tank
to the spent fuel pool and to the top of the
containment vessel.
The ancillary diesel generators also provide
power to support main control room
displays and lighting as well as selected
ventilation systems. In addition, offsite
portable diesel generators and diesel-pow42
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ered
pumps from prearranged sources may
be brought to the site to provide backup for
the ancillary diesel generators and water
transfer pumps.
After seven days, the plant continues to
function in the same manner, except that
additional water supplies are required, either
from plant storage tanks, raw water
3. Targeted seismic design. The AP1000 is composed of systems, structures, and components
(SSCs) that are designated as Seismic Category I, II, or Non-Seismic. Seismic Category
I SSCs are designed to withstand the Safe Shutdown Earthquake (SSE) and continue to perform
their safety-related function. Seismic Category II SSCs are designed to withstand the SSE
without damaging a safety-related SSC. Seismic Category II SSCs are not required to remain
functional after the earthquake. Non-Seismic SSCs are designed to the typical industry building
codes. Source: Westinghouse
Seismic Category I
Shield
building
Auxiliary
building
Radwaste
building
Diesel
building
Annex
building
4. Efficient heat rejection. Transfer of reactor decay heat to the atmosphere is via natural
circulation through a passive heat exchanger located on top of the shield building. Courtesy:
Westinghouse Electric Co.
PCS gravity drain water tank
Water film evaporation
Inside containment refueling
water storage tank
Outside cooling air intake
Natural convection air discharge
Seismic Category II
Non-seismic
Turbine building
first bay
Turbine building
Shield building
Internal condensation
and natural recirculation
Containment condensate
Steel containment vessel
Air baffle
Reactor core
POWER | May 2012

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POWER May 2012

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