Nuclear Plant Journal, September-October 2013 NuclearPlantJournal.com
47
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performance with respect to stress
corrosion cracking, fatigue failures, and
foreign object damage. Over the last
decade in Japan, both laser and cavitation
peening techniques have been applied
in dozens of PWR and BWR units,
extending inspection intervals in some
cases to 10 years.
Peening will likely be implemented
for asset management both with and
without inspection relief. Inspection
requirements for reactor vessel top heads,
piping butt welds, and bottom mounted
nozzles, for example, are specified in
ASME Code Cases N-729-1, N-770-1,
and N-722-1, respectively. Mitigation/
replacement options are component and
design specific and therefore need to be
assessed on a case-by-case basis. Peening
is a principal mitigation option for bottom
mounted nozzles and reactor vessel top
heads, as well as for Alloy 82/182 piping
butt welds at reactor vessel outlet/inlet
nozzles and safety injection nozzles,
especially when access to these locations
from the outside is limited. Peening
with inspection relief and optimized
inspection intervals is anticipated for
the reactor vessel top head penetrations
and piping dissimilar metal welds given
the requirement for periodic volumetric
examination of these components. In the
United States, nuclear plants may seek
inspection relief on a case-by-case basis
following the 10 CFR 50.55a process.
The Materials Reliability Program
(MRP) has published a technical basis
document for peening (MRP-267,
Revision 1, 1025839), a topical report
defining optimized inspection intervals
(MRP-335, Revision 1, 3002000073),
and guidelines for specifying and
implementing peening in a PWR plant
(MRP-336, 1025841). In 2013, both the
topical report and the technical basis
document were submitted and accepted
by the Nuclear Regulatory Commission
for a formal safety evaluation review
(SE). The NRC is currently reviewing the
documents and preparing the schedule
for the SE process. If approved, the SE
review will provide a formal “NRC
agency position” on the requirements
for mitigating PWSCC by peening
with optimized post-peening inspection
intervals. The peening documents also
are being reviewed by the ASME Code
for incorporation into Code Cases N-729-
1 and N-770-1. Both the NRC SE and
ASME Code Case reviews and approvals
are expected to be completed by 2014.
In June 2013, based on technology
readiness and the Materials Reliability
Program’s technical documentation
efforts, two U.S. PWRs have contracted
with a vendor to implement peening
for PWSCC mitigation. These will be
the first implementations of peening
technology in the U.S. fleet. Both plants
plan to conduct the peening in refueling
outages in 2016.
Contact: Paul Crooker, telephone:
Source: Electric Power Research
Institute’s (EPRI) Nuclear Executive
Update, July, 2013.
Laser and Cavitation Peening
Technologies.
