Identification of nonregular indication according to change of grain size/surface geometry in nuclear power plant (NPP) reactor vessel (RV)-upper head alloy 690 penetration

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Identification of nonregular indication according to change of grain size/surface geometry in nuclear power plant (NPP) reactor vessel (RV)-upper head alloy 690 penetration. In this paper, an investigation of nonregular TOFD indications acquired from RVHP tubes using experiments and computer simulation was performed in order to identify and distinguish TOFD signals by coarse grains from those by Primary Water Stress Corrosion Crack (PWSCC). For proper understanding of the nonregular TOFD indications, microstructural analysis of the RVHP tubes and prediction of signals scattered from the grains using Finite Element Method (FEM) simulation were performed.
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Nuclear Engineering and Technology
journal homepage: www.elsevier.com/locate/net
Original Article
Identication of nonregular indication according to change of grain
size/surface geometry in nuclear power plant (NPP) reactor vessel
(RV)-upper head alloy 690 penetration
Kyungcho Kim a, *, Changkuen Kim b, Hunhee Kim b, Hak-Joon Kim c, Jin-Gyum Kim a,
Myungjo Jhung a
a Korea Institute of Nuclear Safety, Daejeon 34142, Republic of Korea
b Doosan Heavy Industries and Construction, Changwon, 51711, Republic of Korea
c Sungkyunkwan University, School of Mechanical Engineering, Suwon 16419, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history:
During the fabrication process of reactor vessel head penetration (RVHP), the grain size of the tube
Received 16 August 2016
Received in revised form
17 July 2017
Accepted 20 July 2017
Available online 30 August 2017
material can be changed by hot or cold work and the inner side of the tube can also be shrunk due to
welding outside of the tube. Several nonregular time-of-ight diffraction (TOFD) signals were found
because of deformed grains. In this paper, an investigation of nonregular TOFD indications acquired from
RVHP tubes using experiments and computer simulation was performed in order to identify and
distinguish TOFD signals by coarse grains from those by Primary Water Stress Corrosion Crack (PWSCC).
Keywords:
Finite Element Method (FEM)
Primary Water Stress Corrosion Crack
(PWSCC)
For proper understanding of the nonregular TOFD indications, microstructural analysis of the RVHP tubes
and prediction of signals scattered from the grains using Finite Element Method (FEM) simulation were
performed. Prediction of ultrasonic signals from the various sizes of side drilled holes to nd equivalent
aws, determination of the size of the nonregular TOFD indications from the coarse grains, and exper-
Reactor Vessel Head Penetration
imental investigation of TOFD signals from coarse grain and shrinkage geometry to identify PWSCC
Time-Of-Flight Diffraction
signals were performed.
From the computer simulation and experimental investigation results, it was possible to obtain the
nonregular TOFD indications from the coarse grains in the alloy 690 penetration tube of RVHP; these
nonregular indications may be classied as PWSCC. By comparing the computer simulation and exper-
imental results, we were able to conrm a clear difference between the coarse grain signal and the
PWSCC signal.
© 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the
1. Introduction
of Korean NPPs in accordance with the American society of Me-
chanical Engineers (ASME) Code Case N729-1, which requires pe-
Since
there
were
Primary
Water
Stress
Corrosion
Crack
riodic inspection as to the susceptibility represented by the
(PWSCCs) found in the reactor vessel head penetration (RVHP)
effective degraded year parameter [1].
weld of the Oconee and Davis-Besse nuclear power plants (NPPs) in
Recently, the penetration tubes of the upper reactor head have
2001 and 2002, the Nuclear Regulatory Commission (USNRC) is-
been manufactured using alloy 690 to replace alloy 600 in order to
sued US NRC Bulletin 2002-01, US NRC Order EA-03-009, and US
improve corrosion resistance [2]. However, PWSCCs can be initiated
NRC Order EA-03-009 (Revison 1). According to the recommenda-
in the contact area of the primary water at high temperature and
tion of the Korean Regulatory Body and the Korea Institute of Nu-
pressure. Thus, in the contact area, the time-of-ight diffraction
clear Safety, inspections of RVHP have been performed since the
(TOFD) technique, which is claimed to have a high probability of
early 2000s. In addition, Korean utilities have inspected the RVHP
detection to detect cracks, has been periodically applied to ensure
material integrity. During the fabrication process of RVHP, the grain
size of the tube material can be changed by hot or cold work and
* Corresponding author.
E-mail address: kck@kins.re.kr (K. Kim).
the inner side of the tube can also be shrunk due to welding outside
of the tube. Thus, nonregular TOFD signals can be acquired during
1738-5733/© 2017 Korean Nuclear Society, Published by Elsevier Korea LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
K. Kim et al. / Nuclear Engineering and Technology 49 (2017) 1524e1536
1525
tube inspection from the deformed grains (coarse grain) and
signals, we investigated the relationship between the ultrasonic
shrinkage geometry. Therefore, it is necessary to investigate those
signals and the grain size. Then, using TOFD B-scan images, ultra-
nonregular TOFD signals in order to discriminate them from aw
sonic scattering signals with coarse grains were analyzed.
signals and to evaluate those aws correctly. With that purpose in
In the computer simulation and experimental results, for the
mind, nonregular signals are classied into two types in terms of
alloy 690 penetration tube of the reactor pressure vessel (RPV)
formation mechanisms and signal patterns: (1) nonregular surface
head, the changes of weld shrinkage and microstructure grain size
geometry indication, very similar to surface geometry indication
were investigated by TOFD signal evaluation.
(SGI), obtained from the inner surface of the tubes, which have big
curvatures; and (2) nonregular penetration tube indication (NPTI),
2. Materials and methods
obtained from big volumetric aws such as cracks, which are one
kind of penetration tube indication (PTI).
2.1. Mock-up
The effects on TOFD-image pattern and size of nonregular TOFD
signals (nonregular surface geometry indication and NPTI) were
The SGI mock-up block was fabricated using J-groove welding,
experimentally evaluated using mock-ups with simulated articial
which penetrated the tube to 16.91 mm thickness, as shown in
defects used to generate these nonregular signals. Also, in order to
Fig. 1 and Fig. 2; block was then cut out from the base metal of the
properly understand the nonregular TOFD signals (NPTI), a scat-
RV with some remaining part outside of the tube. Then, using a
tered signal from the coarse grains in the tube was simulated using
modied dial gage, the inner surface of the mock-up block was used
a computer.
to measure the area of shrinkage due to welding deformation.
For the experimental investigation, one SGI and three PTI
Finally, machining notches were added to the mock-up block at
mockups and a computer controlled ultrasonic testing (UT) system
suitable positions for acquisition of ultrasonic signals.
with transducer module of PCS-24 TOFD were prepared. The ma-
The applied welding method is GTAW, as shown in Fig. 1 and
terial, dimensions, and congurations of these mock-ups are the
Table 1. GTAW is widely used to obtain stable arc and high quality
same as those of Korean NPPs. The major purpose of the SGI mock-
weld metal. And the mock-up was designed with 5 degrees of
up is to investigate the effect on detection and sizing of internal
groove angle to minimize the welding amounts in the J-Groove.
aws, which depend on the shape of the inner surface. One SGI
Fig. 3 shows the shape of the shrinkage of the SGI mock-up. As
mockup of a penetration tube made of alloy 690 was fabricated
shown in Fig. 3, compared to healthy penetration nozzle, the
using gas tungsten arc welding (GTAW) to induce weld shrinkage.
measured shrinkage of the penetration tubes was 1.2e1.6 mm.
Then, several Electric Discharge Machining (EDM) notches were
Articial EDM notches in the inner and outer shrinkage areas of the
machined in the axial and circumferential directions with 10% and
welding surface deformation area were fabricated. Positions and
25% of tube thickness, respectively, in the shrinkage region. Three
sizes of the EDM notchesareshownin Fig. 4 and Table 2. The depths
PTI mock-ups were selected from the fabricated penetration tubes
of A, B, and D EDM notches were 25% of the penetration tube
made using forged alloy 690 materials; it was found that therewere
thickness; in cases F, G, J, and M, the notches were 10% of the
nonregular TOFD signals in the penetration tubes before they were
penetration tube thickness.
installed in the reactor vessel (RV) head. Using electron microscopy,
PTI is the most general indicator during any inspection of the
it was found that the PTI mock-ups cut out for microstructural
penetration nozzle material. Therefore, destructive testing of an
analysis and evaluation of grain size of the areal region produced
area inwhich a nonregular signal has been detected is one common
nonregular signals. The measured grain size and shape are used as
method to analyze the nozzle with PTI. A PTI mock-up block was
the input values of the Finite Element Method (FEM) simulation for
therefore prepared using the same material and size of the SGI
an ultrasound beam model to characterize the ultrasonic beams,
mock-up block, as shown in Fig. 5. The PTI mock-up was cut out of
including backscattering noise, attenuation, and propagation of
the area that had acquired nonregular TOFD signals from the
ultrasound, and to predict ultrasonic signals from the grains, etc.
installed RPV head. Specications of the probe used for the in-
For the computer simulation, the microstructure of the major
spection are provided in Table 3.
components of the NPPs was modeled to determine the effectof the
grain size on the ultrasound testing. In order to describe the ge-
2.2. Experimental set-up
ometry of the grain boundary, the Voronoi method [3] was used to
generatethe grainpattern. The coordinates of the generating points
The mock-up blocks were inspected using a TOFD transducer
are generated by a random number generator in MATLAB (The
assembly (WesDyne International, Madison, PA, USA) with a
Mathwork, Natick, MA, USA). Using the simulated ultrasonic
computer-controlled scanner, as shown in Fig. 6. The TOFD
Fig. 1. Manufacturing stages of surface geometry indication (SGI) mock-up.