POWER August 2012 - 65

NUCLEAR
that of conventional PWRs, it incorporates
existing light water reactor technology; for
example, its fuel assemblies are just smaller
versions of the standard commercial 17 x 17
fuel assembly (Table 1). Several modules can
be combined into a larger-sized power station,
based on demand. Other unique features
include the plant's safety design, its underground
installation, and spent fuel handling.
Integral Design Approach
Mowry said the B&W mPower reactor's integral
design means that the entire reactor
and nuclear steam supply system (NSSS) are
incorporated into one reactor vessel. It is approximately
the size of a conventional PWR
steam generator, is rail shippable, and does
not require on-site NSSS construction. From
bottom to top the nuclear core/fuel assemblies,
control rod drive mechanisms, steam generator,
reactor coolant pump impellers, and pressurizer
all are inside the single vessel instead
of multiple vessels connected by large hot leg/
cold leg piping (Figures 1 and 2).
Mowry said removing the reactor vessel's
primary cooling circuit penetrations below
the core eliminates the possibility of a worstcase
design-basis accident in which a large
loss of reactor cooling water is caused by a
break in the piping. In contrast to the larger
PWRs, this reactor's small core combined
with low power density reduces fuel and clad
temperatures during accidents.
Furthermore, the low power density combined
with a large coolant water inventory results
in operating and safety margins that are
significantly more robust than those required
by the NRC-two to three orders of magnitude
safer or about 10-8
to 10-6). The CDF expresses
pressure boundary integrity. In a conventional
PWR, this requires that the core remains
subcritical and covered with water. Current
PWRs perform these functions through
design features such as multiple high- and
low-pressure injection pumps; low-pressure,
closed-loop decay heat removal systems; and
active cooling systems to maintain containment
pressure within design-basis limits.
Mowry said that because the B&W
mPower reactor employs conventional balance-of-plant
systems, including the NSSS
arrangement and system components within
a single pressure vessel, most transients and
accidents described in NUREG-0800, Standard
Review Plan for the Review of Safety
Analysis Reports for Nuclear Power Plants:
LWR Edition are either identical or very similar
to operating PWRs or the advanced PWRs
employing passive safety systems. He said no
unique thermal-hydraulic or neutronics phenomena
have been identified for this design.
The B&W mPower reactor's inherent safety
compared to the
current NRC or EPRI Utility Requirements
Document core damage frequency (CDF)
benchmarks (10-5
the likelihood that, given the way a reactor
is designed and operated, an accident could
cause the fuel in the reactor to be damaged.
The reactor module is located inside its
own underground steel containment and nuclear
island, effectively isolating the reactor
along with dedicated safety systems (there is
no sharing of safety systems) from external
man-made threats such as aircraft and projectiles,
and from Fukushima-type natural
disasters. Small penetrations reduce the magnitude
of a design-basis loss of coolant accident
and, therefore, the rate of energy release
to the containment. The watertight underground
nuclear island contains all emergency
cooling water sources needed to protect the
reactor core for an extended period of time,
including the refueling water storage tank.
The primary function of reactor safety
systems is to prevent core damage due to
overheating and to maintain reactor coolant
66
features use gravity-driven or natural convection
systems rather than engineered pump-driven
systems and thus do not require AC power
(either onsite or offsite) to power any safety
systems. For example, pumps are not required
to inject cooling water to the core. Instead, the
decay heat removal system serves as an emergency
core cooling system, is powered by gravity,
and maintains a minimum volume of water
on top of the core after a transient. Natural
circulation removes decay heat, and a gravitydrained
storage tank supplies makeup water to
cool the reactor core. This ultimate heat sink
provides at least 14 days of cooling without
the need for external intervention or AC power
to maintain reactor core cooling and safe shutdown.
This is attributable to the SMR having
a combination of a much lower level of decay
heat than larger plants and the unique ultimate
heat sink design. This allows operators to focus
on long-term event mitigation rather than immediate
emergency actions.
Unlike at Fukushima, no diesel generators
are required to provide power for any of these
safety systems to perform their intended functions.
However, Mowry said that in keeping
with mPower's defense-in-depth philosophy,
two back-up diesel generators are provided
in seismically qualified structures for added
protection. A three-day battery supports all
plant monitoring and control without reliance
on AC power. Finally, passive hydrogen
recombiners prevent the buildup of hydrogen
either from the reactor core or the spent fuel
pool. All of the inherent safety systems, including
the ultimate reactor cooling water
source (the ultimate heat sink), batteries,
battery recharging system, and hydrogen
recombiners are housed inside the protected
underground nuclear island.
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The B&W mPower design includes a fully
protected spent fuel pool located within the underground
nuclear island. As observed at Fukushima,
protection of spent fuel is most critical in
the first few years after it is removed from the
reactor core. Therefore, the spent fuel pool is designed
with a large heat sink to ensure that more
than 30 days of fuel cooling is available without
the need for external intervention and before
sufficient pool water is lost through boiling to
uncover the spent fuel. At Fukushima, sufficient
water loss may have been experienced within
one week of the March 2011 accident.
Instead of rotating the fuel for up to three
refueling cycles, as in conventional PWRs,
the mPower reactor fuel has a single four-year
run, and then the entire core is replaced in one
load. A gantry crane lifts off the top half of the
vessel, exposing the reactor core for replacement.
The spent fuel pool would store enough
spent fuel for a 20-year lifetime. It has been
suggested that potential customers consider
buying an extra steam generator that could be
swapped in during a refueling outage. The previous
steam generator then could be inspected
once the reactor is back online and off the
critical path, saving time and money.
1. Familiar steam circuit. This drawing
shows the integral reactor arrangement and
primary loop (reactor coolant) flow through the
pressure vessel. The once-through steam generator
is a vertical shell counterflow straighttube
heat exchanger design, which directly
generates superheated steam as the feedwater
flows through the steam generator in a single
pass. Source: Babcock & Wilcox mPower Inc.
Pressurizer
Reactor coolant pumps
Steam generator
Riser
Mid flange
Control rod drive
mechanism
Upper reactor
vessel internals
Control rod
guide frame
Core barrel
Core
Reactor vessel
POWER | August 2012
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POWER August 2012

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