E-Journal of Advanced Maintenance Vol. 6-4 (2015) 153-164
Japan Society of Maintenology

Evaluation of Wall Thinning of the Steel Piping after Long TermOperation at Fugen Power StationRyosuke ANDO 1,*, Teruyoshi ABE 1, Takahisa NAKAMURA 1
1

Japan Atomic Energy Agency, 65-20 Kizaki, Tsuruga , Fukui 914-8585, Japan

ABSTRACT
Wall thinning of serviced carbon steel piping of the secondary cooling system after long term operation ofAdvanced Thermal prototype Reactor (ATR) Fugen power station has been investigated as a series of evaluationof validity and availability of utilization of serviced materials on research projects focused on aging
management.
Reliability of wall thinning rates of the steel piping has been examined referring the previous inspection data.
Examinations also have been made on prediction of wall thinning rates, rationalization of management of pipe
wall thinning and verification of countermeasures against wall thinning.
KEYWORDS

ARTICLE INFORMATION

serviced materials, steel piping, wall thinning, FAC, LDI, JSME code, ATR
Fugen power station

Article history:
Received 11 August 2014
Accepted 6 January 2015

1.

Introduction

Investigation of wall thinning of steel piping of the secondary cooling system has been carriedout using serviced materials of the decommissioned Advanced Thermal prototype Reactor (ATR)
Fugen power station.
More than 1,100 measurement targets had been examined throughout 18 periodic inspections
including pre-service inspection by ultrasonic thickness measurements at Fugen.
Surveying and organizing the previous periodic inspection data, then calculating wall thinning
rates by using the method defined in JSME code for pipe wall thinning [1], there was a tendency thatthe most of the measurement points having the average thinning rate of more than 0.2mm per 10,000
hour located in condensate water or feed water system, especially elbows nearby feed water pumps
and feed water heaters where flow drift expected to be occurred had heavier corrosion.
Comparing to the categories defined by JSME such as FAC-1 and FAC-S where relatively higher
dissolved oxygen inhibits the wall thinning, whereas possibility is not deniable that substantial wall
thinning could be caused by a significant effect of flow drift on the other hand, can be seen a lot in thesecondary cooling system of Fugen, there still remains some possibilities of deterioration in reliability
at the measurement points tested fewer times even they showed relatively rapid thinning rates.
Investigation of the reliability of the data regarding thinning rates has been examined based on
the detailed measurement of the wall thinning with complementing to the previous inspection resultson the serviced steel pipes of the decommissioning Fugen. Moreover, investigations on the wall
thinning have been carried out such as accuracy improvement of the prediction formula, evaluation ofvalidity of countermeasures and rationalization of the management as follows.

2.

Wall thinning rate and wall thinning prediction formula

2.1 Investigation method
Fig. 1 shows a schematic diagram of the Fugen secondary cooling system.

It is based on the*Corresponding author, E-mail: ando.ryosuke@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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JSME code of pipe wall thinning management, all of the measurement targets such as elbows werechosen in the area of Flow Accelerated Corrosion (FAC) and Liquid Droplet Impingement (LDI)
control area, where significant wall thinning were considered tend to occur.
The investigation of the status of the pipe wall thinning has been carried out by measuring
thickness of the steel pipes using a portable ultrasonic thickness gage.
In these measurements, the following method has been adopted in order to determine the statusof the spatial distribution of the pipe wall thinning, and make obtained data contributable forelucidating the mechanism of the pipe wall thinning.

Fig.1

Schematic diagram of Fugen secondary cooling system classified to FAC and LDI control area.

The measurement was carried out by making the grid spacing of 100 mm, and then the wall
thickness was measured on a grid, namely, a cross section (a measuring point, hereafter) by using an
ultrasonic thickness gage.
The number of the measuring points in a radial direction was chosen more than 4 for the pipes
smaller than 5B in a nominal diameter, and less number was chosen either more than 8 or 100 mm inpitch for the pipes larger than 5B.
Also, in a radial direction, either lesser length of twice in diameter or 100 mm from the end of
piping parts were chosen as the measurement range at the upstream side of the elbows and tees, thatcan be the cause of the flow drift, and three-fold in diameter was chosen as the measurement range atthe downstream side commonly.
Thinning rate was calculated by subtracting the measured wall thickness at this time from the lastmeasured wall thickness and divided by the actual operating time at exactly the same grid point.
In addition, in case significant wall thinning was observed, the grid spacing was reduced to 20
mm in pitch instead centering the very same measured point to the end of the measurement area
described above.
Table 1 shows an example of the results of the wall thinning on the 148 measurement targets,
namely 28,000 measurement points in all.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Table.1 Example of the results of wall thickness measurement of Fugen serviced pipes.

The following is the summary of the investigation results.
① There is no measurement point less than the regulated minimum gage throughout entire 148
measurement targets, namely, about 28,000 measurement points examined.
② Thus, it is confirmed that the management of the wall thinning during operation at Fugen isconservative and appropriate.
2.2 Reliability of wall thinning rates
Fig. 2 shows the plots of the range and the mean values of the thinning rates of eachmeasurement target using all of the obtained data of the measurement points which are distributed ineach measurement target up to in the range of 100 to 200 points each, then the prediction results of thewall thinning using Kastner equation [2] which is used in JAEA code (PASCAL-EC) described later,
the mean value of the thinning rates of the each piping system model and the total mean value of theentire 148 measurement targets, namely, about 28,000 measurement points investigated are alsoplotted.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.2 Comparison of wall thinning rate between measured and predicted.

From the results, the following were shown on the value of the thinning rates and reliability.
①Total mean value of the thinning rates of the entire 148 measurement targets, namely,
about 28,000 measurement points investigated is less than 0.1 mm per 10,000 hour,which is less than one-fifth of the one caused by the flow accelerated corrosion (FAC),
0.5 to 1.0 mm per 10,000 hour.
②
Although the independent value of the thinning rates of the measurement points varies
a lot, the mean value of the thinning rates of the each piping system is equal to the total
mean value i.e. less than 0.1 mm per 10,000 hour.
③
The measurement points having significant variation of the thinning rates show atendency to alter in between positive to negative value widely.
④
Predicted thinning rate of each measurement target, calculated by a wall thinning
analysis code, PASCAL-EC, shows a little smaller value than the total mean value.
⑤The measurement points having significant variation of the thinning rates, particularly
showing negative values, need to obtain proper information on the cause.
It is well known that corrosion rates of carbon steel can be inhibited less than 0.1 mm per 10,000
hour by controlling the oxygen content, pH and restricting the contaminants of the ambient water sothat the passivation file on the surface the steel cannot be damaged in anticorrosion, the results of ourinvestigation seem to be rational and it is said to be confirmed that the management of the coolingwater quality of Fugen during its operation was appropriate.
2.3 Detailed measurement using three dimensional measuring equipment and LASER
microscopeThree dimensional measurements of the distribution of wall thinning of serviced steel pipes byusing a contact type three dimensional measuring equipment (3DME) and high resolution observation

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Stationof corrosive surface by using a LASER microscope have been applied as the detailed investigation.
For selection of the measurement targets for the detailed investigation, the following areconsidered.
①
Corrosion rate: The target must be located in the control area of FAC and LDI andhaving comparatively rapid or widely varying corrosion rates.
②
Heat flow condition: Comparison between vapor-liquid two phase flow and single
phase flow, and comparison of the measurement points having different corrosion rates
under the same heat flow condition, can be carried out.
③Unsteady parts such as type of materials and weld joint: Comparison between carbon
steel and low alloyed steel, and comparison between unsteady parts such as weld joint
and steady parts, can be carried out.
Thus, the measurement targets for the detailed investigation were chosen as shown in Fig. 3.

Fig.3 The view of the measurement targets detailed investigation carried out.
Fig. 4 shows an example of the results of the detailed investigation and general investigation.
As shown cross section #2 in Fig. 4 of the ultrasonic thickness test results, it is revealed that thethickness at the outer curve of the elbow tends to be thinner than the normal thickness and thicker
than both at the inner curve. By the test results of 3DME, it is revealed further that there exists
distinct difference in thickness between the outer curve and inner curve of the elbow seems to havebeen formed by bending of the pipe with cold working. Besides, the profile of thickness deviation ofan elbow shows much more clearly by 3DME which has higher resolution of 0.1 mm in pitch
compare to the unclear results by ultrasonic thickness measurement of 100 mm in pitch.
Based on the information obtained, an observation result on the inner surface of the outer curveof an elbow is shown in the same figure. The observation result confirms that a lot of groove-likerecesses and projections, like wrinkles, made by cold work exist on the inner surface of an elbow in a
vertical direction of the flow.
Furthermore, by measuring the profile of thickness deviation of the cross sections #2, #13 and#16 in the same manner, it is confirmed that there exists distinct deviation of the thickness that thethickness at the outer curve of the elbow tends to be thinner than the one at the inner curve and it
disappears in the straight portion of the pipe like cross section #16.
In addition, it is confirmed the facts that there is no wrinkle like structure observed on the innersurface like G in the cross section #13, where the effect of the cold working for pipe bending is157

R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

negligible, and that the inner corrosive surface appears to be covered by flat and substantially uniform
general corrosion.

Fig.4 An example of the results of the detailed investigation.

Fig. 5 shows the results of the detailed investigation on the welded area in the same manner. It
is shown that there exists thinner area along with the welding line in a radial direction corresponding
to the heat affected zone (HAZ). Although it is confirmed again by the LASER microscopy theexistence of the thinner area along with the weld line, no local corrosion has been confirmed. It isassumed that the thinner area along with the weld line was formed by the thermal shrinkage caused bywelding heat input.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.5 Results of the precise measurement at welded area using three dimensional wall thickness
measuring device

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

It is presumed that the significant deviation of the thinning rates near weld shown at the first
periodic inspection was due to this thickness deviation and the use of nominal thickness for thethinning rate calculation instead of the last measured thickness, and that the thinning rate became theaverage at the second periodic inspection and after because of the use of the last measured thickness.
Bides, it can be assumed that significant deviation of the thinning rates was caused by slight positional
displacement at the measurement point where the thickness alters sharply at each measurement.
There is a high probability that localized deformation and steep thickness change caused by cold
bending, welding and other processes can affect the deviation of the thinning rates. Those facts wereobserved at the all detailed measurement targets.

3.

3.1

Accuracy confirmation of wall thinning prediction formula
Flow analysis

Flow analyses have been carried out around the measurement targets for the detailed
investigation mentioned above with the steady state condition of Fugen during operation using acommercial flow analysis code, Fluent of ANSYS. Three dimensional analysis was adopted, and ineach flow analysis, the estimation area was expanded to the upstream elbow or bent of those targeted.
Fig. 6 shows an example of the results of the flow analysis on the welded area in the samemanner.

Ansys CFX-Post

Ansys CFX-Post

Fig.6 The synthetic flow velocity distribution on the surface of a wall in the vicinity of FDW-SP-48 elbow

Comparing the estimation results obtained by Fluent with the results obtained by othercommercial three dimensional flow analysis code, OpenFOAM [3], there are no significant
differences in turbulent flow energy distribution and flow velocity distribution as shown in Fig. 7.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.7 Comparison of flow analysis results between Fluent and OpenFOAM in the vicinity of FDW-SP-6
elbow

3.2

Estimation of thinning rate

Fig. 8 shows the estimation results of thinning rates of the targeted elbow, FDW-SP-6, calculated
by PASCAL-EC and DRAWTHREE-FAC [4], based on the turbulent flow energy calculated byOpenFOAM, as contour maps on the expanding plans cut off along with the outer curve of the elbow,
after converting the data into the cylindrical coordinate system.
It is confirmed that there is a certain degree of correlation between flow energy distribution andthinning rate distribution, and the estimated value by DRAWTHREE-FAC is comparable to measured
data.
However, there still exists significant difference in the thinning rate between measured andcalculated up to two-digit.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.8 Comparison of calculated wall thinning rate distributions with measured

4.

Validity of countermeasure for wall thinning

Fig.9 is a summary of the observation results of the corrosive surface of the detailed measurementtargets. FAC corrosion organization known as orange peel and Scale patterns were not observed.
There has been no corrosive surface of FAC such as scale-like or orange peer like structure observedon the corrosive surface of entire detailed measurement targets. Local corrosion also has not been
observed, only uniform and general corrosion was observed. Typical examples of the corrosionsurface of FAC reported by Ministry of Economy Trade and Industry [5] are shown for reference.
In addition, these observation results coincide with the fact that the total mean value of thethinning rates of entire 148 measurement targets, namely, about 28,000 measurement points
investigated shows less than 0.1 mm per 10,000 hour, and This fact means that the management of thewall thinning at Fugen was appropriate.

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.9 Comparison of corrosion surface observations and typical FAC surface

5.

Rationalization of wall thinning management

A web based data base has been developed in order to contribute to the rationalization of
the wall thinning management as shown Fig. 10, storing all of the wall thinning measurement
results of entire Fugen piping system, flow analysis (3D-CFD) results, and estimated resultsof thinning rates considering the parameters on environmental conditions such as temperature,
average flow velocity, flow velocity nearby inner surface of piping, water quality, and so on.
Furthermore, while the isometric figures of piping as shown in Fig. 10, CAD drawings
for the use of processing of the dimensional value, and estimation results of flow analysis
and wall thinning analysis have been stored on the data base, it is now available to browse
and use of the information stored through web..

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R. Ando, et al./ Evaluation of Wall Thinningof the Steel Piping after Long Term Operation at Fugen Power Station

Fig.10 Overlook of Fugen wall thinning database indicating the view of the secondary cooling system

6.

Conclusion

Based on the data of Fugen serviced materials, investigations have been carried out on accuracyof wall thinning prediction formulas, validity of countermeasures for wall thinning and rationalizationof pipe thinning management
As a result, it was confirmed that the management of the coolingwater quality of Fugen during operation was appropriate. These results represent the tendency of thewall thinning of the serviced material of Fugen for long operating years, therefore, they can contributeto accurate understanding of wall thinning phenomena of serviced materials.

Acknowledgement
This study was conducted as an "aging research in Fukui Prefecture in 2009 - 2011", which wascommissioned by JNES

ReferencesFor citations in the text, numbers between square brackets should be used such as [1]. All publications
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