POWER August 2014 - 51

NUCLEAR
to complete the weld, the total heat input,
the width of the heat-affected zone, and the
amount of weld shrinkage and distortion. Use
of automation, along with tight process controls
and improved filler metals, enables the
implementation of high-quality RPV shell
seam welds offering exceptional resistance
to in-service degradation mechanisms.
Filler metal manufacturing processes have
dramatically improved, enabling suppliers to
produce low-alloy steel weld metals with extremely
low concentrations of copper. These
weld materials also have reduced levels of
residual elements and of nonmetallic impurities
(such as phosphorus and sulfur) that are
known to contribute to toughness losses under
neutron fluence.
Meeting tight filler metal specifications
for new plant construction involves the use
of precise manufacturing controls, plus management
of the steel melting process to avoid
use of copper-bearing scrap and to control the
content of copper and other potentially problematic
constituents in iron and other input
stocks. These tight chemistry controls also
contribute significantly to producing material
with a low initial RTNDT
tions include an initial RTNDT
. Typically, specificaequal
to or less
than about -4F (-20C) for all low-alloy steels
used for RPV fabrication, but actual values
can be substantially lower, especially for
weld filler metals. This results in seam welds
expected to experience only a limited loss of
toughness when irradiated with fast neutrons
over their service lifetime.
Comparative Assessment
To test this expectation, EPRI assessed the
relative impact of radiation-induced embrittlement
on low-alloy weld filler metals used
for RPV fabrication in the existing U.S. fleet,
in ALWRs constructed more recently overseas,
and in future plant designs. As shown in
Table 1, the study applied estimated end-oflife
(EOL) fluence values and materials specifications
to calculate the adjusted reference
temperature (ART) of horizontal and vertical
seam welds after 60 years of operation, defined
per the methodology in U.S. Nuclear
Regulatory Commission Regulatory Guide
1.99, Rev. 2 as:
ART = RTNDT + ∆RTNDT + margin
Table 1 shows that the much higher copper
content of the low-alloy weld fillers used in
two operating U.S. BWRs and three operating
U.S. PWRs resulted in significant and
sometimes dramatic upward shifts in RTNDT
relative to those estimated using the properties
of actual ALWR filler metals. The shift in
ART and, thus, EOL toughness are substantial:
For the operating BWR and PWR cases
August 2014 | POWER
in this limited data set, the predicted ARTs
after 60 years of operation exceed 120F
(49C) and 230F (110C), respectively. At temperatures
above the ART, brittle fracture is
not a concern; lower ART values, therefore,
are desired.
Modern weld fillers exposed to the same
neutron fluence have predicted ART values
that remain well below 0F (-18C), indicating
very limited embrittlement-related toughness
loss after long-term radiation exposure.
These findings demonstrate that uniform
application of restrictive specifications in
the purchase of low-alloy steel filler metal
should eliminate embrittlement concerns,
even for cases of vertical seam welds in the
beltline region of ALWRs.
Table 2 provides the estimated shift in
transition temperature for modern filler metals
applied in RPV seam welds for advanced
PWR designs, including Westinghouse's
AP1000, Mitsubishi's APWR, Korea Hydro
Nuclear Power's APR1400, and AREVA's
EPR. These estimates are based on projected
360º SUPPORT
TWENTY-FOUR/SEVEN
WE OFFER YOU A COMPLETE RANGE OF SERVICES.
AROUND THE CLOCK. AND AROUND THE WORLD.
There are many professions that demand excellence during the workweek.
That won't suffice for our industry. With nuclear power, there are no days off.
And all the days have 24 hours. That's why we have 28 strategically located
offices and over 1500 employees available to assist our clients 24/7. This is
just one example of our culture at ENERCON, and it is best summarized by our
credo: Excellence - Every Project. Every Day.
With long term contracts in place to support most nuclear power plant
operators, we have become known as the " go to company " for resources and
technical expertise.
Our recent acquisitions of TALISMAN and MARACOR have significantly
expanded our resources and capabilities in the regulatory, licensing and
PRA areas. TALISMAN specializes in providing support to licensees that are
addressing complex issues before the NRC. MARACOR focuses on supporting
the nuclear power industry to help manage plant risk activities.
Give us a call, visit our new website. We are committed to Excellence-Every
project. Every day.
info@enercon.com
enercon.com
CIRCLE 26 ON READER SERVICE CARD
www.powermag.com
51
http://www.enercon.com http://www.powermag.com
POWER August 2014

Table of Contents for the Digital Edition of POWER August 2014
