Nuclear Plant Journal, September-October 2014 NuclearPlantJournal.com
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Safety
Primary water stress corrosion
cracking (PWSCC) of Alloy 600 material
and 82/182 weld metals has had serious
consequences. A pressure boundary leak
in a bottom mounted instrumentation
(BMI) nozzles could lead to serious
degradation concerns such as nozzle
ejection and significant boric acid-
induced head wastage if through-wall
cracking were to occur. The current
requirement for examination of bottom
mounted instrumentation (BMI) nozzles
is CC N722 as required by 10CFR50.55a
which consist of direct visuals every
other outage. There are no requirements
for volumetric or surface examinations
of BMIs for the presence of part thru
wall flaws from Primary Water Stress
Corrosion Cracking (PWSCC). Prior to
October 2013, the capability to inspect
the Palo Verde BMI nozzles (CE System
80) did not exist. In February of 2013
Arizona Public Service (APS) thru the
EPRI NDE center proceeded to design,
fabricate, and ultrasonically (UT)
fingerprint a BMI mockup to replicate
the Palo Verde BMI configuration and
in October 2013 proceeded to perform
a vendor demonstration. A successful
demonstration lead to accurately
characterizing the leak and enabled APS
to efficiently interrogate the remaining
nozzles eliminating the potential for
false calls and avoiding the need for
unnecessary repairs and associated
potential radiation dose. Additionally,
APS was able to confirm the condition
of the remaining BMI nozzles and thus
ensuring that the Unit can be safely
operated.
Cost Savings
In 2003 a BMI leak occurred at
the South Texas Project (STP) nuclear
generating station. This was the first ever
BMI leak. No demonstrated technology
existed for ultrasonically testing or
repairing of BMI nozzles. As a result the
Unit was shutdown for approximately 6
months. Because of the prior work with
EPRI (i.e. capability study) APS was
able to efficiently inspect all 61 nozzles
in a timely manner and subsequently
repair the leaking nozzle. The impact to
the outage schedule was only 30 days. If
APS elected a “do nothing approach” this
would have added an additional 90 days to
the outage schedule. The cost avoidance
(estimate value using $300,000/day)
was approximately 27 million dollars.
Additionally, any false calls leading to
just one (1) additional repair would cost
an additional 2 million dollars.
Innovation
APS is the only Westinghouse CE
US plant in the United States with BMI
nozzles. Prior to the U3R17 outage
in the fall of 2013, there was no NDE
capability for characterizing a flaw in the
Palo Verde BMI nozzles. The capability
study allowed APS to develop the
probe design, scanning techniques and
determine limitations if any. Performing
the capability study well in advance, also
enabled APS to efficiently interrogate
the Unit 3 BMI nozzles, minimizing
the impact to the U3R17 outage and
eliminate false indications that could
have been interpreted as actual primary
water stress corrosion (PWSCC) cracks.
This avoided costly repair/mitigation
activities, associated radiation dose and
significant outage delays.
Productivity/Efficiency
By performing the capability
study well in advance of the U3R17
outage, APS was able to efficiently
characterize the flaw of the leaking
nozzle and subsequently interrogate all
the remaining nozzles in a timely manner.
The alternative would be a “do nothing”
approach and this would have added
approximately 90 days to the outage.
This also eliminated potential false calls
resulting in unnecessary repairs adding
additional work order cycle time, outage
delays and HP surveys.
Transferability
Although Palo Verde is the only
CE plant in the United States with BMI
nozzles, there are several CE plants
outside the US with BMI nozzles that
would benefit from this work (i.e.
capability study), namely the CE plants
in Korea. Additionally, there are several
plants in the US with BMI designs that
have known limitations, such as the
Westinghouse 2 loop and B&W design
BMI’s. Performing a capability study
would allow these plants to determine
the needed technology to design a probe,
scanning techniques and accurately
document the limitations.
Contact: Edison Fernandez, Arizona
Public Service, telephone: (623) 393-
Time of Flight Diffraction (TOFD)Ultrasonic Testing of the Palo Verde
Mock-up immersed in a tank of water.
