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NuclearPlantJournal.com Nuclear Plant Journal, September-October 2013
Research &
Development
Welding Capabilities
Through its Welding & Repair
Technology Center, EPRI is developing
new machine welding capabilities
for large-scale weld overlay repair of
thinning walls in pressure vessels and
heat exchangers. Compared to today’s
manual repair practices, automating the
wall reinforcement process is expected to
expand applicability, reduce downtime,
and improve quality control for weld
overlay techniques.
Pressure-retaining
components
are subject to a number of degradation
mechanisms contributing to wall thinning.
In some circumstances, such degradation
could compromise the structural integrity
of the wall. Weld overlays can provide
the structural reinforcement necessary
to restore wall thickness, maintain
component integrity, and eliminate the
need for either an internal weld repair or
total replacement. Avoiding component
replacement can offset millions of dollars
in capital expenses and replacement
power costs.
Across the nuclear power industry,
a number of large weld overlay repairs
have been successfully implemented
using labor-intensive manual operations
by highly skilled craftsmen. To create
an automated repair capability, Welding
& Repair Technology Center (WRTC)
is applying an advanced gas metal arc
welding (GMAW) process based on
wave-form technology.
Initial tests have been performed
on a heat exchanger mockup with
diameter of approximately 7 feet and
wall thickness varying from 1/16th to
5/8th inches. Results demonstrate that
the precision afforded by the wave-form
GMAW process allows large-scale weld
buildup over thin areas while avoiding
the negative effects of burn-through.
Ongoing developmental work, conducted
in collaboration with welding equipment
suppliers, focuses on optimizing height
controls for the welding arc.
During fall 2013, this innovation will
be transferred to vendors and utilities for
weld procedure qualification using EPRI-
developed parameters. The ability to
implement automated large-scale overlay
repairs may enable continued service
of pressure vessels or heat exchangers
without the extensive downtime that
might otherwise be required for manual
repair.
Contact: Jon Tatman, telephone:
Placement Guide
Quality control during concrete
placement is essential for long-lived
infrastructure assets such as containment
buildings and cooling towers because
these structures likely will operate for
40 years or more. Entities pursuing
new plant builds should recognize the
potential consequences from the lack of
quality control so mitigative strategies
can be deployed.
With nearly 70 nuclear plants
under construction around the world,
EPRI’s Advanced Nuclear Technology
Program is addressing this issue with
projects focused on the quality control
of concrete during construction and
operation. EPRI recently published a
field guide for site engineers responsible
for concrete inspection:
General Outline
for Conducting Quality Inspections and
Tests of Concrete Placement at Nuclear
Facilities
(EPRI Report 3002000520)
.
This field guide provides a
field-deployable resource for utility
professionals responsible for inspecting
and testing concrete placements at nuclear
facilities. It addresses development and
submittal of concrete mixes, qualification
and inspection of batching facilities,
qualification of testing agencies and
personnel, review of construction
procedures, and development of
inspection and testing plans.
It also provides checklists for
pre-placement
inspection;
concrete
conveyance;
placement
inspection;
plastic and hardened concrete testing;
finish, cure, and protection inspection;
and minor and structural defect repair.
Finally, it describes problems that have
been encountered during construction
of nuclear concrete containments and
provides a list of relevant standards and
guidelines.
The guide is available in an
optimized version for iPad and tablets, in
a traditional pdf file for printing, and in a
small hard copy field guide.
Contact: Ken Barry, telephone: (704)
595-2540, email:
.
Peening Technologies
Primary water stress corrosion
cracking (PWSCC) of Alloy 600/82/182
components is a well-known materials
degradation mechanism in pressurized
water reactors. Several technologies
have been developed to address PWSCC,
including weld overlays, mechanical
stress improvement, and component
replacements using PWSCC-resistant
materials. Some Alloy 600/82/182
components, however, cannot be
mitigated or economically replaced, such
as the reactor vessel top head nozzles,
reactor vessel bottom mounted nozzles,
and some piping dissimilar metal butt
welds. For these components, peening is a
viable solution that has been widely used
in PWRs and BWRs in Japan, as well
as in other industries to address stress
corrosion cracking and other degradation
mechanisms. EPRI’sMaterials Reliability
Program is nearly finished with technical
documentation characterizing the use of
peening to mitigate PWSCC, avoid costly
replacement, and potentially reduce
inspection requirements.
Peening is a surface treatment that
generates long-lived compressive residual
stresses at the surface, thus preventing the
future initiation of stress corrosion cracks
and tending to arrest stress corrosion crack
and fatigue crack growth of any existing
cracks located in the compressive stress
zone. Testing and operational experience
have shown that the compressive stresses
resulting from peening improve material
