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NuclearPlantJournal.com Nuclear Plant Journal, January-February 2016
Research &
Development...
through coolant flow streams into the fuel
core where it is irradiated and converted to
radioactive cobalt-60. This circulates back
to other parts of the plant, resulting in a
major source of worker radiation exposure.
Stainless steel–based hardfacing
alloys have the potential to reduce cobalt-
related radiation in nuclear plants by
15–20%. In the 1980s, EPRI developed
such a material, called NOREM. But this
and similar alloys are difficult to apply
through welding and are susceptible to
significant galling at temperatures above
200°C. When galling develops on the
surface of a valve seat, for example, the
valve may seize—potentially leading to
plant safety risks. Since the 1980s, the
nuclear industry has evaluated more than
two dozen cobalt-free hardfacing alloys,
but none has demonstrated adequate wear
and galling resistance—until EPRI’s
stainless steel–based NitroMaxx.
NitroMaxx grew out of four years of
research and development to characterize
the structural properties and degradation
mechanisms of existing cobalt- and
stainless steel–based alloys. In particular,
EPRI researchers gained a better
understanding of how galling develops.
Through this work, the team figured out
how to create a durable alloy that could
effectively resist galling and wear.
To design NitroMaxx, researchers
super-saturated the matrix of a stainless
steel alloy with nitrogen—an approach that
has long been known to increase hardness.
One key to NitroMaxx’s galling resistance
is its high strain-hardening rate—a property
that allows the alloy to become harder at
the surface when subjected to strain.
The manufacture of NitroMaxx is made
possible through the use of powder metallur-
gy and hot isostatic pressing, which involve
heating and consolidating metal powders.
With powder metallurgy, manufacturers can
optimize an alloy’s composition and struc-
ture with great precision, allowing the ap-
plication of hardfacing alloys to components
without welding.
NitroMaxx has potential application
on many nuclear plant components, in-
cluding valves, gates, and certain reactor
pressure vessel internals.
From the Laboratory to the Field
In laboratory tests, EPRI researchers
subjected samples of NitroMaxx, Stellite,
NOREM, and other alloys to various
sliding wear and galling tests at a typical
nuclear plant operating temperature
(343°C). Using a laser microscope to
examine the resulting degradation, they
determined that NitroMaxx’s resistance
to galling and wear was much greater
than NOREM’s and similar to Stellite’s
(see images above).
In 2015, EPRI performed additional
tests in simulated nuclear plant
environments to gauge NitroMaxx’s
durability, corrosion resistance, and
performance during temperature and
pressure cycles. The next step is to work
with utilities and manufacturers to field-
test components in noncritical plant
applications.
Reducing Workers
Exposure
By Robert Ito
EPRI is developing a prototype
system using tablet computers equipped
with camera or video connected to
databases to verify the open-or-closed
status of a valve or switch in a nuclear
power plant. Testing began in June, 2015
at the Tennessee Valley Authority’s
Bellefonte Nuclear Generating Station,
and a second testing phase is in progress.
If successful, the device will perform
vital verification tasks now done by
humans—saving time, reducing human
error and radiation dose, and improving
plant reliability.
Independent Verification in the
Hands of Machines
Since the dawn of the nuclear power
industry, plant workers have been tasked
with double-checking the work of their
colleagues. In a typical scenario, when
a worker opens or closes a valve, an
independent verifier follows and rechecks
everything, ensuring that the first worker
didn’t make any mistakes. With roots in
the U.S. nuclear navy, such independent
verification has long been a core tenet of
the industry’s safety culture.
There are potential downsides to
human verification. Every check pulls
a worker away from another job, with
possible radiation exposure. People are
prone to attention lapses, particularly
during repetitive verification tasks, and
may be reluctant to question a trusted
colleague’s work.
But what if a handheld tablet
computer could do the work of a human
verifier? To investigate this question,
EPRI developed a prototype. Here’s how
it works. The user performs a procedure—
such as closing a valve—and at each
step photographs the component with
the tablet’s digital camera. The tablet’s
software compares each photograph with
a laser-scanned three-dimensional model
of the component, recording and detecting
whether the component is open or closed.
As the software determines that a given
Laser micrographs of NOREM (left), Stellite (center), and NitroMaxx (right)
samples subjected to the same stresses at plant operating temperature
reveal almost no galling (indicated by the thick streaks) for NitroMaxx and
significant galling on NOREM.
