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NuclearPlantJournal.com Nuclear Plant Journal, September-October 2015
Cavitation
Peening
By Gary Poling and Curtis Van Cleve,
AREVA Inc.
Gary Poling
Gary Poling is responsible for Strategic
Projects and Research and Development
(R&D) in Component Replacement &
Repair (CR&R), as well as the regional
R&D coordinator for the Installed Based
Americas business unit at AREVA Inc.
Gary is a graduate
of The Ohio State
University earning
Bachelor of Science
and Master of
Science degrees
in mechanical
engineering.
Gary began his
career with AREVA
in 1999 as a tooling
engineer supporting
CR&R internals
segmentation
activities for Maine Yankee
decommissioning. He has continued to
serve in various leadership roles for
CR&R bringing new technologies and
tool designs to the market. Gary is an
active member of the ASME Code Task
Group for High Strength Nickel Alloy
Issues.
Extending the Life of Nuclear Reactor Operations
The nuclear reactor fleet in the United
States is an aging one. Despite being
the best maintained and safest operating
fleet in the world, the fact remains that
all of our nuclear energy facilities began
construction in the 1970s or even earlier.
One of the biggest concerns related to aging
stems from primary water stress corrosion
cracking, which can lead to increased costs
for operation, maintenance, assessment,
repair and replacement of boiling water
reactor (BWR) and pressurized water
reactor (PWR) components.
Stress corrosion cracking has three
main contributing factors:
• Tensile surface
stresses at the ex-
posed, wetted surface
• Material conditions
such as microstruc-
ture, roughness, cold
working and chemi-
cal composition
• Corrosive environ-
ment within a pri-
mary reactor vessel
(borated water at a
high temperature and
pressure)
In particular, al-
loy 600 and 82/182 materials, widely used
in PWR systems, are susceptible to this
type of deterioration. In the United States,
primary water stress corrosion cracking
has been reported on reactor pressure
vessel top head penetration nozzles, reac-
tor vessel bottom mounted nozzles and
dissimilar metal welds of primary system
piping. Cracking has also been observed
on other pressure boundary components,
such as steam generator tubes and plugs,
pressurizer heater sleeves, pressurizer in-
strumentation nozzles and reactor hot leg
piping instrumentation nozzles.
Fortunately, there is a solution:
cavitation peening. This technique is
designed to reduce surface stresses,
effectively eliminating one of the three
contributing factors to primary water
stress corrosion cracking.
Cavitation: A Physical
Phenomenon
Cavitation is the formation of vapor
bubbles in a liquid, resulting from rapid
pressure changes. After a vapor bubble
is formed, additional pressure forces it
to implode while still submerged. This
collapse or implosion creates a significant
shockwave, including a brief area of high
temperature and light.
Cavitation
peening
harnesses
this physical phenomenon to “pre-
stress” metal components and makes
them resistant to corrosion and other
degradation. This process, as well as how
to eliminate unwanted application of it, is
a major area of study in the field of fluid
dynamics.
Uncontrolled, sustained cavitation
can cause pitting and erosion on metal
surfaces. However, when controlled,
cavitation can be beneficial. For example,
a form of cavitation is the basis for
ultrasonic cleaning baths; in this case,
using cavitation caused by acoustic forces,
vapor bubbles are made to oscillate in
shape or size before imploding, rubbing
along the surface to remove dirt.
Harnessing Cavitation
Cavitation peening is a way to
control the initial degradation of a metal
surface so as to lessen the effects of aging
over time. Rather than allowing stresses
to create multiple, random fractures
in an area, cavitation peening creates
compressive stresses on the surface of the
material in a controlled manner. It uses
submerged, ultra-high-pressure water
jets to work the surfaces of reactor vessel
components in order to improve material
properties and enhance resistance to
corrosion. High-pressure water is forced
through a very small orifice, exiting at
a very high velocity. This high-pressure
flow creates a low vapor pressure region
above the material’s surface, which
generates cavitation bubbles. As these
vapor bubbles collapse on the material
surface, shock waves travel into the
surface of the material and create
compressive residual stresses.
