POWER November 2016 - 46

NUCLEAR O&M
Ultra-High-Pressure Cavitation
Peening Offers New Way
to Maintain Components in
Operational Nuclear Reactors
To power generation professionals, the word " cavitation " typically has negative
connotations, but a new process offers a way to control this normally
destructive effect and deploy it as a cost-effective life-extension technique.
Mike Epling and Benjamin Youman
A
s operators of commercial nuclear
energy facilities consider subsequent
license renewals and life extensions,
finding new ways to safely maintain reactor
components remains of particular importance.
When faced with the required inspections
and maintenance, as well as the potential
for emergent repairs, operators' options
to preserve these reactor components have
been limited to either supporting continued,
extensive and costly repairs, or to considering
complete component replacement. However,
given the ever-changing economics in
the power industry, as well as the necessity to
reliably and cost-effectively generate baseload
power, operators are demanding new and
innovative techniques and technologies to
maintain their reactor components.
One such technique, known as ultra-highpressure
cavitation peening, was recently
introduced for the first time on an in-service
reactor vessel closure head. This work was
completed at Exelon's Byron Generating Station
Unit 2 in spring 2016.
With ultra-high-pressure cavitation peening,
power generators are no longer faced
with only two options-repair or replace-
when planning for long-term use of nuclear
energy facility components that contain materials
susceptible to primary water stress
corrosion cracking (PWSCC). Today, this
surface stress remediation technique provides
a third option for efficiently and costeffectively
maintaining components, offering
an alternative to the unpredictability and impacts
of performing localized repairs or the
extensive costs of component replacement.
Ultra-high-pressure cavitation peening targets
PWSCC-prone materials-such as alloy
600, 82, and 182-and mitigates these
materials' susceptibility to stress corrosion
degradation.
46
What Is Ultra-High-Pressure
Cavitation Peening?
Ultra-high-pressure cavitation peening is a
technique that harnesses the physical phenomenon
of cavitation-the formation and
subsequent implosion of vapor bubbles in a
liquid, resulting from rapid pressure changes-to
significantly reduce the probability of
PWSCC on reactor components.
With ultra-high-pressure cavitation peening,
the cavitation cloud is controlled so that
the vapor bubbles implode on a targeted component's
surface, alleviating the initial material
property fabrication and operating tensile
stresses so as to lessen the effects of tensile
stress fatigue over time (Figure 1). The newly
imparted compressive stresses from the cavitation
peening process significantly curtail
known fabrication and operation stresses
within the material's lattice structure, thereby
minimizing the creation of multiple, random
fracture sites within the material's surface
that are then susceptible to PWSCC attack
and subsequent crack growth (Figure 2).
Other industries, such as aerospace and
defense, have used surface stress mitigation
techniques similar to cavitation peening to
provide various safety-critical applications.
In fact, lower-pressure cavitation peening
has been implemented in Japan on pressurized
and boiling water reactor vessel closure
heads, but only prior to the vessel heads being
placed in service. Prior to its implementation
at Byron Generating Station Unit 2,
this technique had not been considered for
nuclear energy facilities, nor had it ever been
performed on an in-service reactor head.
AREVA's Ultra-High-Pressure
Technique
AREVA benchmarked the ultra-high-pressure
cavitation peening technique with a
focus on the safety-critical aerospace industry.
After thoroughly evaluating the science
behind the technique, AREVA's engineers
determined that a refined and targeted process
could benefit the nuclear energy industry.
They then set to work adapting the
process to meet the needs of power generators.
AREVA's U.S. operations developed
the technology and therefore were capable
of deploying it in the U.S. market first. Most
impressively, by simply using water, the effects
of cavitation peening are expected to
extend the economic life of a component by
as many as 20 years beyond its originally
1. Under pressure. High-pressure water exits at a high velocity and creates a cavitation
cloud. Courtesy: AREVA
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POWER | November 2016
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POWER November 2016

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