J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Optimization of monitoring method for CGR measurement of XM-19
Hideki YUYAa, Toshio KUZUYAb and Satoshi MAYUMIbaChubu Electric Power Co., Inc., Shizuoka, JAPANb

Techno Chubu Co., Ltd., Nagoya, Japan(Received: 24 September 2014 / Accepted: 19 January 2015)

Crack growth by stress corrosion cracking (SCC) in stainless steels is an important issue not only for fission
reactors but also for fusion reactors. In Japan, the continuous operation of the plants having cracks in their in-corecomponents has become possible since 2004, which is based on a crack growth analysis by SCC crack growth rate(CGR). However, the collection of SCC CGR data of steels used in the reactors has not been completed yet. From
those backgrounds, our research project for the CGR measurement of XM-19 which is a high strength stainlesssteel used for a tie-rod of the core-shroud was started about five years ago. A potential drop method (PDM) using
reverse current was applied to the CGR monitoring. However, a large scattering of the daily data was observed inan early stage. The data scattering was suppressed later by optimization of current reverse frequency and ourmonitoring system achieved 1.5 micro-meter resolution of crack depth. CGR of XM-19 ranged 1㹼210-10 m/s
under the BWR normal water condition, which was higher than that of solution annealed 316 stainless steel.
Keywords: stress corrosion cracking, crack growth rate, potential drop method, reverse current, noise reduction,light water reactor

a world authority on SCC in thesis [3] and actually oursystem didn't work well in an early stage.
The tie-rod repair method has been used for the coreshroud of BWR (Boiling Water Reactor). The method is toset four bars around the shroud and to link top and bottomof shroud by the bar, which prevents the shroud fromsplitting at the moment of seismic load. Tie-rod iscomposed of some parts made of Inconel alloy X-750 andXM-19. Both materials are known to be highly resistant toSCC occurrence [4]. However, SCC CGR data set has notbeen collected, especially for XM-19.
From the above background, we started theexperimental study on CGR measurement about five yearsago. Five years were consumed mainly by building of
CGR monitor system. Therefore, the details are introducedwith experimental result of crack growth measurement ofXM-19 in this paper.

1. Introduction

Stress corrosion cracking (SCC) is one of maindegradation phenomena seen in the components used undera pure water environment. Japan Society of MechanicalEngineering Fitness-For-Service (JSME FFS) code has
been developed for the judgment of structural integrity ofcracked components of Light Water Reactor since 2004 [1].
JSME FFS code has a data set of SCC crack growth rate(CGR) of type 304 and 316 stainless steels for crack
growth analysis. SCC CGR is a great concern of plantowner because the future operation including continuous
use, repair and replacement of the component will be
judged by the crack growth analysis based on CGR data.
The mechanism of SCC has not been fully understoodand the theoretical formulization [2] has not been
completed because many parameters such as stress level,water purity and material condition have effects on CGR.
Moreover, SCC CGR is usually so low (at most 10-9 m/s)that the reliable data collection takes very long time. In theexperimental aspect, the use of experiment apparatus needssome practice because the apparatus is composed of manyvalves, pumps and instruments for water quality
measurement etc. The establishments of water quality
control procedure and of CGR monitoring system are themost important tasks to be adjusted before the experiment.
Especially, the system should be build by the users because
apparatus makers usually leave the system for users.
Furthermore, from the view point of industrial standard,
few guidelines for CGR measurement are available. Some
problems relating data collecting manner are suggested by

2. Experimental

2.1 CGR measurement apparatus
Figure 1 shows the schematic diagram of SCC CGR
measurement apparatus. The water reserved in a tank is
supplied to an autoclave through a pipe line with a highpressure pump and a pre-heater. The pressure of water in
autoclave is controlled by a pressure adjustable valve at thedownstream. The temperature of water in autoclave iscontrolled by a pre-heater and a band-heater of theautoclave. CGR monitoring system is composed of a leadwire, a direct current source and a voltage meter. Figure 2
shows the detail of the monitor system. The lead wire ismade of Pt because the corrosion of wire can make noise.

author’s e-mail: Yuya.Hideki@chuden.co.jp

120©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Yuya et al., Optimization of Monitoring Method for CGR Measurement of XM-19

DOion
exchange
resin

CEFlowmeterPDO

Pump

P+-CoolerHP
Pump

2.3 CGR measurement of XM-19
A 19% cold worked plate was supplied from GE in
US for CT specimens of SCC experiment. Table 1 is thechemical composition and Table 2 is the mechanicalproperties of XM-19. The plate was cut into 0.5TCT
specimen (Fig.5) and approximately 1 mm pre-crack wasintroduced in air at Kobe Material Testing Group in Japan.
After the CT specimen was installed in our autoclave, theenvironmental fatigue crack was introduced by fatigue loadin hot water. This is the manner to change crack growthfrom fatigue mode to SCC mode. The environmental
fatigue load was started from low with high stress ratio (Ȫmax/Ȫmin), followed by high with low stress ratio. Thismanner was used not to introduce plastic deformation at
crack tip of the specimen. After the environmental fatigue
loading, the loading mode was changed to constant, staticSCC mode. If the monitored crack seemed to stop underconstant load, we repeated the manner from fatigue modeto static mode.
The validity of crack growth data was judged byreferring a Forum of Japan society of corrosion
engineering [5]. The specimen size used for CGR
measurement test can be valid if the following equations
are satisfied. K value was confirmed to be acceptable up to36MPҀm for this study.

Programmed current

Voltage meter

DC supplier

Voltage (V)TimeFig.4 Optimization of the current reverse frequency for ourlaboratory

T/C

Data collectionTime䠄䡐䠅specimen

Fig.2 Crack growth monitor system
2.2 Reverse current PDM (Potential drop method)
Potential drop method (PDM) with reversed current
has been applied to the CGR measurement. PDM is themethod to detect the increase in voltage due to the increase
in resistivity. The resistivity increase is caused by decreasein cross section of specimen which is due to crack growth.
The current reverse is one of techniques to cancel theelectromotive force. The principle of cancelling is to utilize
the characteristics of electromotive force which is notaffected by current direction (Fig.3). However, thefrequency of the current reverse is important because theorigin of force is time-dependent event in our laboratory,
which is simply depicted in Fig.4. This is considered to be
due to the room temperature control problem, which may
be specific to the laboratory.

B > 0.5(K/Ȫy)2,(1)where B is specimen thickness (m), K is stress intensity(MPaҀm) and Ȫy is 0.2% proof stress (MPa) at test
temperature.
W-aӍ4/ȧ(K/Ȫy)2(2)where W is specimen width (m) and a is crack length (m).

Current reverse

I+++readings+-TimeVVTime

Fig.1 Schematic drawing of the apparatus

non-Current reverse

Time

Corrosion potential
monitor

PreHeater

I-- Time

CGR monitor
systemT/C
P

Hx+VspecimenPLong

I+

T/C

PCE

Short (good for us)

Iband heater

Autoclave

Valve

Accumulator

GasWaterreservetank: Water flow direction
DO: Dissolved Oxygen meter
CE: Conductivity meter
T/C: Thermo couple
P : Pressure gauge

pH

Bypass line-Table 1 Chemical composition of XM-19
TimeVnoiseTimeMoCNbCrFeMnNiSi

2.2

0.03

0.2

20.76

58.93

4.59

12.82

0.39

Table 2 Mechanical properties of cold worked XM-19
Time

Fig.3 Principle of current reverse method

121

TemperatureȪy0.2% (MPa)ȪULT (MPa)Ȝf (%)

24Υ

891

993

22.1

288Υ

690

790

14.5

H. Yuya et al., Optimization of Monitoring Method for CGR Measurement of XM-19

Voltage fluctuation (arb.u.)

2.5E-04

1.5E-04

1.0E-04

5.0E-05

0.0E+00

Fig.5 Schematic drawing of 0.5TCT specimen

Current [A]51800

Current [A]

27080

90100

Clack Length (mm)11.5005
11.5000

㼼1σ䠙0.00126

11.4995
11.4990

2013/5/19

Date

2013/5/20

Fig.9 The fluctuation of crack length reading prior to the
CGR measurement (Load free)
3.2 CGR of XM-19
Figure 10 shows the result of crack growth behaviorof XM-19. The water in the water reserve tank was opened
to the ambient air and the oxygen was thought to bedissolved at saturated solubility, therefore, the measureddissolved oxygen (DO) was about 7㹼8 ppm during theCGR test. In the middle of September, the apparatus wasonce shutdown due to a planned power outage. Thespecimen was unloaded and the autoclave was cooled
down to room temperature before the outage not to
introduce plastic deformation at crack tip of the specimen.
After the outage, the stress and the temperature conditions
were returned to the same condition as that before theTime [sec]560

11.5010

11.4980
2013/5/18

3250

11.4985

Fig.6 Low frequency pattern-340

11.5015193

03011.5020

Time [sec]800

20Figure 9 is the crack length fluctuation of thespecimen measured prior to CGR measurement under load
free condition. The interval of two dashed lines (statisticals1Ȫ, 68% confidence interval), which corresponds to thecrack length resolution of our monitor system, is less than
1.5ȣm. As a result of a private communication with aworld authority on SCC, the resolution of crack is about 1
ȣm at best. Considering that the resolution was 100ȣm
order in the case of Fig.6, the final resolution (1.5ȣm) ofour monitor system could be a great progress. The cause offluctuation in crack length was considered to be thefluctuation of water temperature in the autoclave due to theroom temperature control problem.

3110Fig.8 Relation of current pattern and voltage fluctuation

3.1 CGR monitor system
The voltage fluctuation under non-loading condition
was measured under various conditions to confirm the data
quality of our CGR monitor system. In an early stage ofconfirmation, we considered that the current reverse
frequency should be low because we believed that thenumber of readings should be large from a view point ofstatistics. Figure 6 is a typical low frequency pattern ofreverse current, which was applied in an early stage to ourCGR measurement. However, we noticed that too low
reverse frequency would be worse because the noise in ourmonitor system was time-dependent. Figure 7 is the
optimized high-frequency reverse pattern for our system.
The voltage readings during a few seconds after the start ofthe current discharge are cleared to eliminate the effect ofripple noise on the readings. Figure 8 shows the relation
between the voltage fluctuation and total time (whichcorresponds to total number of readings). The voltage
fluctuation in the case of high-frequency is low comparedwith that in the case of low-frequency. The result in thecase of Fig.6 is depicted in this figure, indicating that thevolume fluctuation after the improvement becomes onesixth compared with that before the improvement. The
increase in the number of readings corresponds to theincrease in total measurement time and suppresses thefluctuation, which is reasonable from statistical viewpoint.

-30

Total measurement time (min)3. Results and discussion0䕺 801 sec
䕕 30sec
䕧 25sec䕿 7sec2.0E-04

5Fig.7 High frequency pattern122

H. Yuya et al., Optimization of Monitoring Method for CGR Measurement of XM-19outage. The crack growth behavior before the outage islinear with the time and the trend is almost same as thatafter the outage, showing absence of the effect of theoutage. Two regions where crack growth behaviors became
relatively stable were selected for decision of CGRs.
Figure 11 is a part of the fracture surface of specimen aftercrack opening. This microscope observation indicates thatthe fracture surface of SCC is inter-granular type, whichenabled us to confirm the total crack length of SCC. As theresult of confirmation, the relation between the voltage
reading and the crack length was obtained and used todraw Figs.9 and 10. The rough estimation of the ratio of
actual length and measured voltage was 40ȣm/ȣV. The
crack length resolution of our CGR monitor system duringcrack growth was approximately 30nV, which corresponds
to 1.2ȣm and is the same order as that measured prior tothe CGR test.
Figure 12 is the summary of the result of this work,
which is the comparison between plots of this work and thedeposition lines of sensitized 304 and solution annealed
316 stainless steel in JSME FFS code. In addition to theplots of this work, the results of previous work (0% cold
worked XM-19) are also plotted for a reference. The waterelectrical conductivity at the inlet of autoclave was lessthan 0.2ȣS/cm, which was almost the same value as thatof a actual plant. On the other hand, the conductivity at theoutlet of autoclave ranged 0.5㹼0.6ȣS/cm. CGRs datataken in this study, therefore, may be conservative because
the higher the electrical conductivity becomes, the fasterthe CGR becomes in hot water [6]. Low conductivity lessthan 0.2 ȣ S/cm at the inlet and high conductivity
exceeding 0.5ȣS/cm at the outlet were also seen during
CGR measurement of 0% cold worked specimen.
Therefore, the discussion about the effect of cold work on
CGR is considered to be possible. The fact that the CGR of0% cold worked specimen is almost same as that ofsolution annealed 316 stainless steel, as shown in Fig.11,
seems to support this consideration. From the above
consideration, the difference in CGRs of XM-19 is thought
to be caused by cold work as pointed out by someliteratures [7].

䕺 XM-19 (19 % cold worked)

Crack growth rate (m/s)

䕻 XM-19 ( 0 % cold worked)← Clack Length

8

12.3

DO →

12.1

Term of planned
power outage

11.9

7/31 8/10 8/20 8/30

9/9

9/19 9/29 10/9 10/19 10/29 11/8 11/18 11/28

10100

4. Conclusion

The reverse current frequency in PDM was optimized
for our monitor system. As the result of optimization, thecrack length resolution became 1.5 ȣ m, which iscomparable to the apparatuses of major institutes.
SCC CGR of XM-19 was measured by the optimizedmonitor system. The CGRs were 1㹼210-10 m/s at stressintensity of 30 MPaҀm and deposited above the line of316 SS in JSME FFS code. The comparison in CGR
between 0% and 19% cold worked XM-19 showed theeffect of cold work on CGR.

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[1]

[2]
[3]
[4]
[5]

64
2

11.7

1Fig.12 Comparison between JSME and this study

141012.5

10-11Stress Intensity (MPa√m)

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10-1010-1216

12.910-9316 Stainless SteelDO (ppm)

Clack Length (mm)0.5T CT of alloy XM-196kN, 288Υ water

13.1

11.5

10-8

18

13.5
13.3

Fig.11 Fracture surface of specimen

0

[6]
[7]

Date

Fig.10 Crack growth behavior of XM-19123

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