POWER August 2014 - 50

NUCLEAR
Welding and Fabrication
Innovations Mitigate Reactor
Pressure Vessel Embrittlement in
Nuclear Plant Construction
New weld filler materials and fabrication processes are capable of eliminating toughness
losses associated with fast-neutron irradiation of reactor pressure vessel (RPV)
weldments in new nuclear power plants. This article examines these and other
techniques that could revolutionize the manufacturing of RPV components.
David Sandusky and David Gandy
R
eactor pressure vessel (RPV) shells in
the existing U.S. fleet of nuclear power
plants were typically constructed by
forging ring segments from ingots of lowalloy
steel offering sufficient fracture toughness
to withstand radiation-induced embrittlement
across long-term operations. Thick,
roll-formed segments were joined, typically
by the submerged arc welding (SAW) process,
via horizontal and vertical seam welds
incorporating low-alloy steel filler metals fed
in wire form from a spool into the weld pool.
Often, a thin coating of copper had been applied
to the spooled wire to prevent rapid
oxidation of exposed surfaces, a practice that
continues today in the petrochemical and
other industries.
During the U.S. boom in nuclear plant construction
in the late 1960s and early 1970s,
weld metal specifications targeted low-alloy
steel fillers that contained copper, along with
other residual elements and impurities. The
amount of additional copper incorporated in
individual RPV seam welds attributable to
the use of copper-coated wire was relatively
small, with no apparent detrimental effect on
their basic strength, ductility, and toughness.
However, any copper addition is ultimately
detrimental to toughness properties of irradiated
low-alloy steels. Only after construction
had been completed on a significant number
of U.S. boiling water reactor (BWR) and
pressurized water reactor (PWR) vessels did
the potential for accelerated neutron-induced
embrittlement of welds in the reactor beltline
due to elevated copper content and other
tramp elements become apparent.
The factors underlying this in-service
degradation mechanism have been well understood
since the mid to late 1970s. Under
50
neutron fluence, copper is the most active element
in the low-alloy steel matrix, highlighting
the need to ban the use of copper coatings
on filler wire. Controlling copper content
during filler metal manufacturing is essential.
Initial reference temperature, which defines a
filler metal's transition from high-toughness
ductile behavior to low-toughness brittle behavior,
represents another key parameter.
The reference temperature for the nil ductility
transition (RTNDT
) of low-alloy steels, as
defined by the American Society of Mechanical
Engineers (ASME) Section III, Subsection
NB, increases under prolonged exposure
in high neutron flux environments, especially
for fillers with higher levels of copper. As this
parameter increases, RPV integrity analyses
must employ more conservative toughness
curves to retain the same margin against nonductile
failure as existed in the weld filler's
unirradiated state. Low-alloy steel filler metals
with lower initial RTNDT
values thus offer
more favorable toughness characteristics
after long-term neutron exposure.
According to a study recently completed
by the Electric Power Research Institute
(EPRI), lessons learned regarding irradiation
embrittlement of low-alloy weld metals have
been successfully applied as the construction
of advanced light water reactors (ALWRs)
has continued in Asia and Europe across the
past two decades. Metal manufacturing techniques
in common worldwide use today are
capable of meeting tight specifications for
low-alloy steel fillers, and advanced welding
technologies have proven ability to join RPV
shell forgings.
Accordingly, seam welds in new plant construction
are expected to be resistant to radiation-induced
embrittlement over the lifetime
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of new RPVs, even when located in the highfluence
beltline region. Alternatives to conventional
fabrication methods are emerging
that could provide significant benefits. Powder
metallurgy, for example, offers the potential to
revolutionize the U.S. and international manufacturing
and fabrication of RPV shells and,
eventually, small modular reactors.
Current Practice
Today, RPV manufacturing and fabrication
methods employ single, large, forged ring
sections that eliminate the use of vertical
seam welds within the beltline region. In all
but one of today's ALWR designs, this single-piece
ring forging also is tall enough to
span the height of the active fuel. As a result,
even the circumferential welds joining ring
sections are located above and below the active
core, limiting their exposure to neutron
fluence.
The welding technologies applied to make
these full-thickness structural joints are significantly
advanced relative to RPV fabrication
practices from decades ago. Typically, a
narrow-gap or reduced-volume welding process,
as opposed to a conventional V-groove
weld preparation, is involved. Most service
providers employ SAW with the narrow
groove reducing the amount of weld wire,
the number of welding passes, and the overall
weld volume.
Gas metal arc welding (GMAW) also is
applied by at least one RPV manufacturer;
it requires just a single weld bead per layer,
with mechanical oscillation applied to
weave the weld wire across the narrow gap to
achieve good sidewall fusion.
Generally, both SAW and GMAW narrow-gap
processes reduce the time required
POWER | August 2014
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POWER August 2014

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