POWER October 2013 - 79

NUCLEAR POWER
stand the environment of the reactor pool as
well as the high pressure and temperature of
any design basis accident.
Containment vessel pressure is maintained
at a vacuum under normal operating conditions,
providing for reduced moisture that could contribute
to component corrosion and impact the
reliability of instrumentation and other systems
within the containment vessel. The vacuum essentially
eliminates convection heat transfer,
removing the need for " direct-contact " reactor
pressure vessel insulation. The vacuum also enhances
steam condensation rates that would occur
during an accident with ECCS actuation and
limits the available oxygen, which is beneficial
for severe accident combustible gas control.
Following an actuation of the ECCS, heat
removal through the containment vessel reduces
the containment pressure and temperature
and maintains them at less than design
conditions for extended periods of time.
Steam is condensed on the inside surface of
the containment vessel, which is passively
cooled by conduction and convection of heat
to the reactor pool water.
Decay Heat Removal System. The decay
heat removal system (DHRS) provides
secondary side reactor cooling for non-ECCS
events when normal feedwater is not available.
The system is a closed-loop, two-phase
natural circulation cooling system. Redundant
trains of decay heat removal equipment are
provided, one attached to each steam generator
loop. Each train is capable of removing 100%
of the decay heat load to cool the reactor coolant
system. Each train has a passive condenser
submerged in the reactor pool. The condensers
are maintained with sufficient water inventory
for stable operation.
Emergency Core Cooling System. The
ECCS consists of two independent reactor
vent valves and two independent reactor
recirculation valves. The ECCS provides a
means of decay heat removal in the event of a
loss of coolant accident or a loss of the main
feedwater flow in conjunction with the loss
of both trains of the DHRS.
Following a small-break loss of cooling accident
or other condition resulting in an actuation
of the ECCS, heat removal through the containment
vessel rapidly reduces the containment
pressure and temperature and maintains them
at acceptably low levels for extended periods of
time. Steam is condensed on the inside surface
of the containment vessel, which is passively
cooled by conduction and convection of heat to
the reactor pool water. Since the containment
vessel is evacuated to a low absolute pressure
during normal operation, only a small amount
of noncondensable gas will be present inside
the containment vessel.
Chemical and Volume Control System.
October 2013 | POWER
The primary functions of the chemical and
volume control system (CVCS) are to purify
reactor coolant, adjust the boron concentration
in the reactor coolant, and supply spray
flow to the pressurizer. Equipment within
the CVCS also allows for chemical addition
to the reactor coolant and heats the reactor
coolant during startup. The CVCS contains
safety-related valves credited with stopping
inadvertent boron dilution and inadvertent
addition of reactor coolant inventory.
Will the Market Respond?
Whether or not a commercial market exists
for SMRs remains to be seen. But in the quest
for zero-carbon baseload power generation capacity
that is also scalable and capable of being
sited close to the load center, designs like
NuScale's may well be a game changer. ■
-David Wagman is content director for
the ELECTRIC POWER conference, which
will be held April 1-3, 2014, at the Ernest N.
Morial Convention Center in New Orleans.
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