22
NuclearPlantJournal.com Nuclear Plant Journal, March-April 2015
to the nuclear industry to support long-
term plant operations.
Evaluation
Reactor internal components are
complex and, due to their consistent
exposure to the harsh environmental
conditions under which they operate,
particularly susceptible to aging
mechanisms during long-term operation.
It is imperative to safe plant operations
that the reactor vessel internals maintain
structural integrity throughout long-
term plant operation. One component
that is both critical to achieving this yet
has also been shown operationally to be
susceptible to aging mechanisms is the
baffle-to-former bolt (or baffle bolt).
Baffle bolts are essential to maintaining
the structural integrity of the internals.
These stainless steel bolts attach the
vertical baffle plates to the horizontal
former plates within the internals. The
original plant design basis is that all
baffle bolts remain intact throughout the
original plant licensing period. The aging
mechanism of concern for baffle bolts
when considering long-term operation
is irradiation-assisted stress corrosion
cracking (SCC).
Westinghouse has developed an
engineering analysis technique to address
concern about SCC affecting baffle bolts
to the point of the bolts not serving their
design purpose. The technique, called
Acceptable Baffle-former Bolt Pattern
Analysis, is based on an NRC-approved
methodology and demonstrates that even
with many non-functional baffle bolts, a
plant can operate safely. The Acceptable
Baffle-former Bolt Pattern Analysis
consists of detailed modeling of the
internals and then evaluating the thermal-
hydraulic loads on the baffle bolts during
normal, upset and faulted conditions
to define maximum loads on bolts with
varying numbers and distributions of
intact bolts. The method provides a new
basis for the number and location of
baffle bolts necessary to maintain the
structural integrity of the reactor internals
baffle region. This analysis can also be
done in real time for added versatility in
analyzing a plant’s as-found condition.
The results allow plants to assess
the impact of bolts found to be non-
functional during plant inspections and
then to continue operation while planning
their replacement, if necessary or desired,
to avoid outage delays. Unless major
modifications are made to the reactor
internals, the results are in effect for the
duration of licensed plant operation.
Mitigation
Developing mitigation techniques
for SCC in reactor internals is another
area of importance for reactor internal
components. For reactor dissimilar metal
weld configurations that require new
solutions (for example, control rod drive
mechanism nozzles and bottom-mounted
instrumentation nozzles), Westinghouse
has brought laser peening to the U.S. for
use in PWRs by leveraging the Toshiba-
developed technique previously used in
Japan. Laser peening eliminates tensile
stress – one of three factors required to
produce SCC and known to contribute
to SCC initiation. Eliminating the risk of
new cracks allows plants to use standard
crack growth rates to predict future
degradation accurately, and it eliminates
plant costs and risks associated with
emergency repairs due to new cracking.
The laser peening technology does
not require any surface preparation. It
uses a short-pulse laser to convert tensile
stresses into comprehensive stresses.
For 10 nanoseconds the laser irradiates
the metal surface in water and a high-
pressure (5-gigapascal) plasma forms
on the metal surface. This high-pressure
plasma impinges the metal surface; its
pressure exceeds the yield strength of
the surface material (regardless of the
pre-peening residual stress condition),
converting the tensile stresses in the
surface layer to comprehensive stresses.
The comprehensive stresses are impinged
approximately 1 millimeter into the
surface, although deeper compressions
can be achieved depending on the
application and method. Laser peening
can also stop the growth of existing
SCC-induced cracks within the zone of
compressive stress.
Comprehensive
stresses
are
permanent;
therefore,
additional
mitigation efforts against SCC are not
needed for the duration of plant operation
wherever they are created. Because a
laser performs the peening, there is no
tool-reaction force. The laser peening
tool is lightweight and unobtrusive to
refueling work and the results of this
peening process can be confirmed by
visual examination. Fiber laser peening
has been successfully applied to reactor
vessel main and penetration nozzles at
several plants.
Repair
In terms of repair technology,
laser welding has been added as it has
significant advantages over traditional
welding. It is a remote process that
uses a laser beam, rather than tungsten,
and a filler wire to provide accurate and
repeatable underwater welding with first-
time use success in many applications.
The filler metal used for underwater
laser welding is the same as that used
for the more traditional gas tungsten arc
welding. But unlike traditional welding,
it is automatic and remote, making it
ideal for high-radiation areas. It does not
require an operator to make adjustments
throughout the welding process and the
laser unit can be placed up to 1,000 feet
away from the work area.
Since the laser light is delivered
optically, the entire weld system is
simpler. Optical delivery also allows
weld heads to be developed for easy fit
into various challenging geometries.
Additionally, the heat input to the base
material is very low, at 2.5-7.6 kJ/in
(1-3 kJ/cm), which is approximately
10 percent of the heat input of standard
tungsten inert gas welding, minimizing
distortion as well as the impact on the
adjacent material. Using the laser welding
process can eliminate small cracks and
possibly negate the need for expensive
Reactor Vessel...
Underwater Laser Welding Machine.
