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NuclearPlantJournal.com Nuclear Plant Journal, March-April 2015
Research &
Development
Shape-Changing Seal
EPRI is developing a seal for water-
circulating pumps in nuclear power plants
that could control leakage by adjusting its
shape while in use, potentially reducing
plant shutdowns and maintenance costs.
In pressurized water reactors, reactor
coolant pumps circulate water that
transfers heat from the fuel in the reactor
core to a steam generator. Mechanical
pump seals control water leakage. To
ensure seal reliability and durability,
it’s critical to avoid excessive leakage,
which can happen as a result of seal wear,
chemical deposition, or changes in water
properties. If leakage rates are too high,
plant operators must shut down the plant
to repair the seal. Such events happen a
few times per year across the U.S. fleet,
which can take a week to repair and incur
costs of $500,000 for the seal repair and
$1 million for replacement power for a
lost day’s output.
EPRI researchers are investigating
coolant pump seal designs in which
leakage could be controlled while in
service. The idea is to adjust the seal’s
shape, changing the thickness of the
lubricating film between seal faces to
reduce leakage to an acceptable level.
In
2014,
EPRI
collaborated
with Georgia Technology Research
Corporation, Duke Energy, and Southern
Nuclear to complete a feasibility study.
EPRI developed three conceptual designs
for such a seal (3002002344). In two
of the designs, the seal, made of either
stainless steel or carbon graphite, has
several internal chambers containing
hydraulic fluid—an incompressible
material that can change the seal’s shape
when subjected to certain pressures. In
the third design, a carbon graphite seal is
bonded to a piezoelectric crystal, which
can change the seal’s shape when a
controlled voltage is applied.
Researchers simulated the behavior
of these three designs through modeling.
Results showed that while all three
options are feasible, the carbon graphite
seal with hydraulic fluid-filled cavities
offered the best ability to control leakage
over the largest range.
EPRI also is considering the
feasibility of hardware- and software-
based control systems for the seals.
Researchers envision control using a
fuzzy logic system—one that would
control the seal based on a set of “if-then”
rules involving human decision making.
They modeled such a system for use with
the seal and it demonstrated excellent
control.
EPRI is reviewing, manufacturing,
and testing material samples for the
controllable seal. The current focus
is on composite materials, since the
seal cannot be made from traditional
materials. When viable materials are
identified, EPRI will build and test a
prototype in 2015. Collaboration with
component manufacturers and utilities
will be essential to ensure that the seal
is compatible with coolant pumps and
other equipment, complies with rigorous
industry standards, and is commercially
viable.
While the controllable seal concept
was originally intended to address
leakage problems with reactor coolant
pumps in nuclear plants, it can potentially
be applied in other industries, such as
petroleum and chemical processing,
where high-pressure fluids are pumped
and seals are used to control leakage.
Contact: Gary Boles, telephone:
Radiation Reduction
EPRI researchers are exploring how
“big data,” enhanced decision logic, and
new technologies could reduce radiation
fields at nuclear power plants to further
safeguard worker health.
The radiation fields created at
nuclear power plants are of significant
concern to worker health. While working
in radiation fields are tightly controlled
by regulatory bodies and by industry
ALARA principles (as low as reasonably
achievable), achieving and maintaining
exposure goals is an ongoing challenge.
Safety is the primary concern, of course,
but there are economic implications
as well, such as costs for additional
shielding, engineering assessments, and
personal protection equipment.
The nuclear industry has established
ambitious targets to reduce radiation
exposure. In the United States, nuclear
plant owners have committed to
collective radiation exposure goals
(CRE) of 55 annual person-REM for
individual PWR units and 110 annual
person-REM for individual BWR units
by 2015. While about two-thirds of the
PWR fleet has achieved this goal, the
BWR fleet has been challenged because
of its larger inventory of cobalt sources
and the presence of more components
in the radiation-affected primary circuit.
Outage CREs can run as high as 200
person-REM for BWR units.
To enable more effective, sustainable
radiation exposure reductions, EPRI has
developed a new roadmap for radiation
field source term control. Only if the
radiation field’s source term is managed
more effectively in a long-term and well-
integrated approach, will nuclear plants
be able to achieve and maintain the
ambitious CRE goals. EPRI research aims
to answer several challenging questions:
What if we could use all the data
we have? Recent computing technology
developments offer advanced pattern
recognition that can expand our
analytical perspective beyond two or
three parameters. Combining this tool
with more efficient data and text mining
enables us to look at data collected
over decades through different lenses.
This “big data” approach supports
the discovery of source term control
strategies not currently envisioned. The
current approach involves exploring one-
to-one relationships such as that between
zinc injection and radiation fields. But
plants often don’t implement changes
one at a time. If, say, zinc implementation
coincides with the removal of cobalt-
based Stellite™ components, but the
plant is experiencing degrading efficiency
in the coolant cleanup system paired with
many reactor trips, the resulting radiation
Radiation Reduction Roadmap.
