E-Journal of Advanced Maintenance Vol.2 (2010/2011) 1-13
Japan Society of Maintenology

Pipe wall thinning management at Electricité de France (EDF)
Thomas KNOOK1,*, Matthieu PERSOZ1, Stéphane TREVIN1, Séverine FRIOL1,
Marie-Pierre MOUTRILLE1, Lionel DEJOUX2
1Electricité de France – DPIH/DTG/C2M - FAC specialty Group, 21 Avenue de l’Europe, 38040
Grenoble, France
2
Electricité de France – DPN/UNIE/GMAP - Maintenance Group, 1 Place Pleyel, 93282 Saint-Denis
Cedex, France(Received; March 16th, 2010)
Abstract.
Among the various degradation modes that cause pipe wall thinning in the secondary system of Nuclear
Power Plants (Corrosion, Galvanic Corrosion, Environmental Corrosion, Flow Accelerated Corrosion,
Cavitation, Droplet impingement, Erosion and Abrasion), FAC is the one that is the more widespread in the
installation and that requires constant efforts to fight. The wall thickness loss due to FAC can effectively occur
in any carbon steel piping containing hot water or wet steam and can lead to pipe ruptures with dramaticconsequences.
Electricité de France operates 58 Pressurized Water Reactors that were put in service between 1977 and 1997.
The degradations and leaks due to FAC that EDF has experienced and the numerous accidents that have beenreported abroad, lead EDF to develop a global strategy to control FAC on piping and to prevent any dramatic
rupture. This global strategy is based on a National Maintenance Rule which has been written by the corporate
engineering level of EDF called “RNM” and applied by every Nuclear Power Plant operator of the EDF fleet.
The RNM is mainly based on the use of the FAC-prediction software “BRT-CICERO™” as well as on specificactions for the lines or elements that are not modeled in the software.

KEYWORDS: Flow Accelerated Corrosion, FAC, Degradation mode, Piping, Carbon steel, Maintenance,
Prediction software, BRT-CICERO™, Non Destructive Examination, TOFD, Computed Radiography,
Chromium.

1.

Introduction

Among the various degradation modes that causes pipe wall thinning in the secondary system of
Nuclear Power Plants (Corrosion, Galvanic Corrosion, Environmental Corrosion, Flow Accelerated
Corrosion, Cavitation, Droplet impingement, Erosion and Abrasion) [1], [3], [4], FAC is the one thatis the more widespread in the installation [2], [5], [10], [11] and that requires constant efforts to fight.
The wall thickness loss due to FAC can effectively occur in any carbon steel piping containing hot
water or wet steam and can lead to pipe ruptures with dramatic consequences.
Electricité de France operates 58 Pressurized Water Reactors that were put in service between 1977
and 1997. The degradations and leaks due to FAC that EDF has experienced and the numerous
accidents that have been reported abroad, lead EDF to develop a global strategy to control FAC on
piping and to prevent any dramatic rupture. This global strategy is based on a National Maintenance
Rule which has been written by the corporate engineering level of EDF called “RNM” and applied byevery Nuclear Power Plant operator of the EDF fleet. The RNM [12] is mainly based on the use of the
FAC-prediction software “BRT-CICERO™” as well as on specific actions for the lines or elementsthat are not modeled in the software.

*

Corresponding author, Tel : 00 33 476 20 23 19, E-mail : thomas.knook@edf.fr
ISSN – 1883 – 9894/10/11 © 2010 – JSM and the authors. All rights reserved.

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

In the present article following items are developed :
2.
3.
4.
5.
6.
7.

The French regulatory requirements on pressure piping,
The organization at EDF to manage the risk of pipe wall thinning,
The BRT-CICERO™ software and its validation,
Small bore piping and low energy lines in BRT-CICERO™,
The Quality Assurance requirements,
The Non Destructive Examinations (chromium measurement, Digital Radiography, weld root
wear detection and dimensioning by TOFD),
8. Consideration of the feedback experience,
9. Conclusion

2.

The French regulatory requirements on pressure piping

A new pressure vessel law was issued in France on March 15th 2000 for non nuclear pressurized
vessels and piping. The components that are submitted to this law depend on the type of gas they
contain (steam / water or other gas), on their design pressure (PS) and volume (or nominal diameter
DN for piping). All steam / water piping with PS > 0.5 bar and DN > 100 mm (4 inch) and PS×DN >3500 bar are submitted to this law.
Basically, the regulator states that the components must be maintained in a good state of work
permanently. The operator is responsible for the components security and must have the skills to
operate and maintain them.
Regulated piping are submitted to “periodic inspections” “as often as necessary”. In practice, theperiod is based on a criticality analysis that is a function of three parameters : 1) the occurrence of thepossible degradation modes (FAC, wall thinning after cavitation but also cracking, fatigue, etc.), 2)
the degradation kinetics, and 3) the risks for human beings. The higher is the criticality, the shorter theinspection period must be.
Inside the NPP, a “Recognized Inspection Service” (RIS) has the delegation from the French
Administration to ensure the security of the pressurized components. This service is liable for theapplication of the law, has to prove his capability to carry out his mission in front of the
Administration and can impose all type of requirements or actions from the operator to ensure thecomponents security (including the shutdown of the plant).
Each regulated piping must have an Inspection Plan (IP), written by the RIS , that specifies the different
actions to be performed and their periodicity. The IP takes into account the different degradation
mechanisms that can occur. In addition to periodic inspections, steam / water piping with PS > 0,5 bar and
DN > 250 mm (10 inch) and PS×DN > 5000 bar are submitted to periodic qualifications, that consist in acomplete internal and external inspection. This periodic qualification must take place every 10-12 years.
3.

The organization at EDF to manage the risk of pipe wall thinning

As EDF operates a fleet of standardized Nuclear Power Plants, the maintenance rules are edited by acorporate engineering unit called UNIE (Unité Nationale d’Ingénierie de l’Exploitation). The 58 NPPs
have the obligation to apply the national maintenance rules issued by this unit on their equipments.
Since the mid 90’s at EDF the pipe wall thinning due to FAC, and the associated maintenancestrategy is based on the use of a prediction software developed in house with the cooperation ofmultiple engineering units. This prediction software is called BRT-CICERO™. After this software
enabled the operator to detect a severe damage on an expander downstream of a regulated valve of theunit 2 of Fessenheim in year 2000, the corporate level of EDF decided to make mandatory the use of
BRT-CICERO™ for all the 58 NPPs. This decision was called “DP 109”. Many position papers were
issued afterwards on how to use the software on site, how to model the piping and input the referencedata, how to handle the results, when and how to perform the inspections and how to take into accountthe domestic and international feedback. These papers accompanied the “DP 109” but became too
numerous and difficult to master in their totality. Because of this diffuse “guideline”, it was therefore2

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

decided in 2008 to produce a new single guideline which gives all the prescriptions andrecommendations for the surveillance of the conventional secondary piping and valves. This guideline,
issued in April 2009, is called the National Maintenance Rule on conventional piping or “RNM” [12].
The RNM tackles all the activities of FAC surveillance on piping, valves and welds. The mainprinciples are the following :• After having shown the performances of the predictive software BRT-CICERO™ to theregulation body, EDF has obtained a derogation to the March 15th, 2000 pressure vessel law, thatenables the operator to inspect for FAC damage only the piping elements indicated by thesoftware (heavy costs of scaffolding and insulation removal are saved this way),• Each pipe component modeled in BRT-CICERO™ and predicted under the design thickness atthe outage N+1 must be either inspected (chromium and thickness measurements) at the outage
N or justified by a written analysis submitted to the RIS for approval ; different methods are
possible such as using several wall thickness measurements made in the past, etc.,• Because of the difference in operational conditions it is not possible to establish a justification of
non-inspection based on informations from a sister plant,• Pipe elements located downstream of regulating valves cannot be justified using the above
methods and they must be inspected if they are predicted under the design thickness,• If a pipe component is measured under its design thickness, the component must be either
replaced or justified for at most one cycle by a mechanical calculation towards the load ofpressure and taking into account the predicted wall thinning during this cycle,• During a thickness measurement by ultrasonic (UT), all specific pipe components must be
meshed entirely (elbows, bends, reducers, tees, etc.),• The inside wall of double-wall venturis is inspected by televisual inspection,• For each component inspected by UT, an analysis of the thickness values along the mesh must be
done in order to undertake an inspection of the weld in case of a thickness decrease at a meshextremity (see § 7),• As the current version of BRT-CICERO™ calculates wear rates on piping elements only and not
on welds, and as the degradation of a weld root may not be seen by the analysis of the wall
thickness measurement file of the component, especially if the component contains chromium
and not the weld, a special inspection program is required for the welds that are assessed to be
FAC sensitive (downstream of regulating valves for instance) and with a priority on components
whose base metal has a chromium content higher than 0.1 %,• All the dissimilar welds on the regulated pipe lines must be listed and inspected ; the inspection
concerns the carbon steel part of the connection,• All carbon steel valves on regulated pipe lines must be inspected visually (VT), except those
whose shell has already been inspected with no FAC damage after 10 years of operation,• For the lines susceptible to FAC that are not yet modeled in BRT-CICERO™ (low energy lines,
partial time operating lines, small bore piping, etc.), an inspection program must be establishedthat includes the most sensitive areas from an engineering point of view (areas locateddownstream of reg. valves and diaphragms, tees, series of elbows, etc.).

4.

The BRT-CICERO™ software and its validation

The BRT-CICERO™ software has been developed by EDF since the early 90’s and is based on theenhanced Berge’s model [6], [8], [9], [13], [14]. This prediction tool allows to calculate the wall
thickness loss in simulated pipes taking into account all influencing parameters like water chemistry,
temperature, flow-rates, steam quality (for 2 phase flow lines), oxygen level (more specifically for
BWR), operating rates but also the shape, dimensions and pressure drop (through regulating valves)of the individual pipe elements and the chromium content in the carbon steel.
The use of the software allows the operator to establish a list of most affected components basedon the calculation of the FAC speed in each individual pipe component. The tool allows the ranking of
the pipe components regarding FAC speed, predicted remaining wall thickness or margin calculations.

3T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

The importance of chromium content of steel in the calculation model [7] allows the software to
take into account this very important intrinsic parameter that can explain a high FAC speed incomponents in conditions of hydraulic flow turbulent but weakly disrupted like straight pipe elements
downstream straight pipes. On the EDF fleet several hundred of straight pipes downstream otherstraight pipes were already predicted with higher FAC rates than the upstream pipe.
The Figure 1 shows an example of a tube downstream a straight tube with a higher predicted FAC
rate than the upstream tube and a lower minimum measured wall thickness.

Chromium = 0.071%
Chromium = 0.041%
Calculated FAC rate = 0.1 mm/10000Hrs
Calculated FAC rate = 0.14 mm/10000Hrs
Minimum measured wall thickness = 13,2 mm Minimum measured wall thickness = 12,4 mm

Fig. 1. Tube downstream straight tube

The wall thickness management is based on the use of the prediction tool BRT-CICERO™ but
feed back experience from other powerplants in France and worldwide is also taken into account for
the final definition of an inspection program. The use of the prediction tool enables the reduction of
the inspection volume to approximately 75 to 150 pipe components each second outage. An operator
that can not reduce the inspection volume can also use the software as a ranking tool in order toperform the inspection on the most critical components first.
At EDF the inspection performed on components predicted with a thickness lower or equal to theminimum required thickness is wall thickness measurements according to a grid. On each inspected
element a chromium measurement is systematically performed also. All inspection data are input into
the BRT-CICERO™ data base and taken into account for future FAC calculations.
Specific FAC degradations in the weld root areas conducted EDF to validate specific NDE test
methods like the ultrasonic method TOFD or digital radiography.
4.1 The BRT-CICERO™ model
The algorithm for kinetics calculation derives from Berge and Saint-Paul model [6], [8] and is
modified according to the Sanchez-Caldera model [13], [14] described into the equation (1).
V=

θ x (Ceq - C∞)(1 / K * + (1- f) x (1 / k + δ / D))(1)

Where :
Ceq : solubility of ferrous ion in water in equilibrium with magnetite reduction [14],C∞ : ferrous ion concentration in bulk water (considering close to 0),
K*: kinetics coefficient of formation of ferrous hydroxyde (Fe + 2H2O → Fe(OH)2 + H2),
f : rate of ferrous ion soluble and transformed in magnetite at Metal-Oxide interface (f is mainly
equal to 0.5 [1]),k : mass transfer coefficient,θ : porosity of oxide layer,
δ : oxide thickness,
D: diffusion coefficient of ferrous ion in water.

4

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

The model takes into account the whole parameters which have an effect on the intensity of the
FAC phenomenon.
The chemistry of water is taken into account through Ceq content of soluble ferrous ion in
equilibrium at magnetite/water interface and through C∞ content of soluble ferrous ion concentration inbulk water in circulation. The value of Ceq depends on the hydrogen pressure pH2, the hot pH and theconstants of equilibrium of the various soluble ferro-iron species. Ceq is calculated according to [14] :Ceq = (pH2)1/3 . [K1 [H+]2+ K2[H+] + K3 + K4/[H+]](2)

where : K1, K2, K3 et K4 are the respective constants of equilibrium of the species Fe2+, Fe(OH)+,
Fe(OH) 2 and Fe(OH)3- .

The thermohydraulic conditions of each pipe element are taken into account by the coefficient ofmass transfer k that is equal to the coefficient of mass transfer of a straight tube of the same diameterkST multiplied by a geometry factor (Geo) according to the modified Colburn formulation :(3)

k = Geo x kST

with kST determined from the modified Colburn’s relation and validation with a test loop called CIROCO.
0.2

0.4

kST = 0.0193(D/d).(e/d) .Re.Sc(4)

With :
e = roughness in meters (in BRT-CICERO™ this value is set to 4x10-5 m).
d = internal diameter (hydraulic diameter) of the component in meters.
Re: Reynolds number.
Sc: Schmidt number.
To take into account the shape of the components, a geometry factor is introduced for each component,
whose value is dependent on the shape and type of the component. The minimum value of the geometry
factor is 1, for straight tubes. Moreover, the hydrodynamic effect of one component on downstream
components is taking into account by calculating a decreasing factor along the downstream line.
The pH, hydrogen and temperature effects were checked with the CIROCO loop and allparameters are the results of an optimization between pH calculation [17] and the mixed function ofhydrogen pressure versus temperature as described in equilibrium (2). The paper [16] describes thegood correlation between hot pH calculation and FAC rate monitored with CIROCO loop.
Finally, the effect of chromium is well known for many years and is a paramount parameter toreduce the FAC rate [7]. All these parameters allow to propose a simplified model which is :V = 2k . f (Cr ). C eq

(5)where f(Cr) is an empirical function including porosity, hydrogen pressure and chromium content
effects according to the temperature.
4.2 The BRT-CICERO™ validation

The performance of the BRT-CICERO™ prediction tool is periodically reviewed to ensurecompliance with the French vessel legal and safety requirements. One way to check the performanceof BRT-CICERO™ is to compare the components thickness calculated with BRT-CICERO™ with themeasured components thickness. At the time being (end 2009) 14,439 thickness measurements aretaken into account and the analysis is still in progress.
After treatment of abnormal points, the curve in Figure 2 is obtanied. Some points appear to bepredicted with a high degree of conservatism : this is generally due to a lack of information on thechromium content or to an initial thickness much higher than specified, which is often the case for5

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

thick components (headers, tees, large bore reducers). Some cases can also be due to a tooconservative geometry factor.

80

Measured Thickness (mm)

60

dd

40

20

0
-20

0

20

40
Calculated Thickness (mm)6080

Fig.2. Comparison between measured and calculated thickness for pipe components
with BRT-CICERO ™ version 3

5.

Small bore piping and low energy lines in BRT-CICERO™

In the mid-90’s, when EDF started to use the FAC prediction software BRT-CICERO™, only themain lines were modeled in the plant databases. These main lines are those with the highest energy
and therefore with the highest level of risk in case of pipe failure. However, the introduction of smallbore and/or low energy lines in BRT-CICERO™ was identified as an important issue of improvement
when EDF evaluated its own FAC surveillance strategy after the Mihama accident. Furthermore, a
few ruptures on small bore piping were at the origin of production loss, which is another reason forincreasing the surveillance.
Therefore a project was launched at EDF to get modeled in the BRT-CICERO™ data bases thecomplementary lines for the 6 types of reactor : 900 MW (CP0, CP1 and CP2 type), 1300 MW (P4and P’4 type), and N4 1450 MW. All the lines of the secondary circuit whose operating temperature isgreater than 75 °C and whose nominal diameter DN is greater than 32 (1’¼) have been listed. This
represents several hundreds of lines for each type of reactor. The second step of the work consists in
identifying which among those lines really need to be modeled. Are for instance excluded the lines :• that have a very low operating rate (for example lines that only operate during the start up
and shut down stages, lines to pressure relief valves, by-pass lines to atmosphere, etc.),• in which the flow rates is very low (equilibrium lines for example),• in which circulates dry steam (main and live steam lines),• that have been replaced by stainless steel, etc.

The remaining lines have to be modeled. As they are not main lines (the main lines are alreadyinput in BRT-CICERO™), it is sometimes difficult to determine reliable values for their operation
parameters (temperature, pressure, flow rate, steam quality, operation rate). When data are missing or
are susceptible to uncertainties, conservative assumptions are made in order to set “envelope” values.
After the operational parameters are determined, the geometry of the lines is input in BRT-CICERO™.
The precise isometric drawings of each line are needed, in order to get all the required values : pipe
length, diameter and thickness, accessories characteristics such as tee angles, bending radius,
expander / reducer lengths, orifice diameter of diaphragms, etc. The manufacturing documents mustalso be used in order to find the steel grades, the manufacturing tolerances and information on valves(especially the pressure drop).

6T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

Modeling auxiliary lines of the secondary circuit is a long job whose hardest part is the data
research in the documentation and the setting of the most reliable values for the different parameters
needed for the FAC calculations. The completion of the BRT-CICERO™ data bases was howevercompleted in end 2009 for the eighteen CP1 900 MW units. Eighty-four new lines totalizing morethan 1200 pipe elements have been added per unit, which should greatly improve their maintenanceand the storage of NDE results (wall thickness and chromium measurements). The same work for theother sixteen 900 MW units is under progress and should be finished by mid-2010. For the 1300 and
1450 MW units, the work is planned to be completed by the end of 2010.
6.

The Quality Assurance requirements

The use of a prediction software for maintenance purposes requires a high level of quality on thedata and on the way the software is used. From the quality of the input data depends the quality of thepredictions. That’s why the RNM requires that 100 % of the input data in the BRT-CICERO™ databases needed to be verified by each NPP before end 2009. All the chemistry data (amine contents bycycle), the operating parameters of the lines (temperature, pressure, steam quality, flow rates) and thepipe component characteristics (dimensions, diameter, nominal thickness, steel grade, design
thickness, etc.) have therefore been verified using the reference sources (chemistry application, design
documentation, isometric drawings, etc.). This verification is double-checked by another person, thatfills the field “validated by” available for each line in BRT-CICERO™.
The RNM and other statutory documents like the modeling procedure give some rules of use andmodeling in the software, on the way to take into account the manufacturing tolerances and themeasurement uncertainties for example.
Training sessions on BRT-CICERO™ are organized by a specialized training unit of EDF on abasis of 5 to 6 sessions per year. Only people that have followed this training session with successhave the ability to use BRT-CICERO™.
The management of the plant must be strongly involved to have all these verifications and rules
applied by the staff. Audits of the plants on the FAC subject are regularly performed by the corporate
level.
7.

The Non Destructive Examinations

7.1 Chromium measurements

One of the influencing parameters on the FAC rate of carbon steel is the chromium content. The
value of the chromium content in low alloy steel varies from 0.00 to 0.30% according to the grade. The
influence on the FAC rate has been demonstrated by the EDF R&D department on the FAC test loop
“CIROCO” for chromium values from 0.04% and up (and [7]). The problem is that the exact Cr content
values are not always indicated by the manufacturer. The manufacturer must only guarantee they are
lower than the maximum acceptable values (0.25 or 0.30% depending on the grade for example).
In order to measure an accurate chromium content in the components, EDF qualified ameasurement method based on the XRF technique (X-Ray Fluorescence technique). Because theuncertainty of the Cr content can change the FAC rate, EDF defined the needed accuracy for thechromium content measurements according to the FAC rate calculations made by BRT-CICERO™.
The required accuracy levels are :•••For Cr contents from 0 to 0.03 % (w/w) the boundary accuracy is ± 30%
For Cr contents from 0.03 to 0.077 % the boundary accuracy is ± 15%
For Cr contents from 0.077% the boundary accuracy is ± 10%

The qualification of the XRF devices used for on site chromium measurements is performed on
calibration blocks with certified chromium contents (see Figure 3).

7

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

Fig.3. Calibration blocks used for the qualification

The chromium measurements are made by EDF with annually qualified devices and appropriate
procedures. More than 2800 components on the EDF fleet have been measured during the year 2009
for example. Some of the measurements are made by contractors with qualified devices. The use of an
XRF measurement device is shown in Figure 4.

Fig. 4. XRF measurement device positioned on a pipe

7.2 Wall thickness measurements by ultrasonicDuring inspections on critical elements predicted at design thickness at cycle N+2, the standard
procedure of wall thinning loss measurement is performed by standard ultrasound technique. The UT
measurements are performed on the elements to be inspected according to a predefined grid. As theform of the degradation caused by FAC are large areas, the measurements are performed on large
mesh grids. The size of the grid depends on the size and form of the elements. For a component
without welds, 8 longitudinal lines are defined and the mesh pitch is approximately one third of thecomponent diameter. In the presence of discontinuities or in case of local disturbing areas, the gridmesh can be reduced. In order to assure the repeatability of the measurements, the grid mesh ismarked permanently on the components. The surface preparation is performed by grinding and therequired surface roughness of 6.3 µm or better (12,5µm for castings).
7.3 Wall thickness measurements by Digital Radiography

In some cases the use of standard ultrasonic technique can not be applied. For example during
operation or in presence of the insulation. In order to dispose other wall thickness measurement
methods, EDF validated the use of Digital Radiography (DR). The advantage of this technique is thatit can be used on insulated pipe elements even during operation.
Digital Radiography is a non-destructive inspection technique based on the same principles asclassic radiography. The difference lies in the detector which in the case of classic radiography iscomposed of a silver film, and in the case of digital radiography of a photo sensible panel.

8T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

The limits of digital radiography for thickness measurements is directly related to the energy of theradiation source being used (‘X’ ray tubes, sources of Se, Ir or Co).
7.3.1 Validation Methodology
For the validation of the DR technique, a test programme has been drawn up. This programme
consisted in measuring the thickness of test tubes and comparing the results obtained with the realvalues. The determination of the precision of the method is carried out by measurements on various
test tubes, depending on the thickness range of use. Supplementary tests on real components were also
carried out to check the test parameters on components with realistic defects and the limits of use.
The thickness measurements validated by EDF were carried out using the "Rhythm" software Trade
mark of General Electric (see Figure 5) which is composed of a thickness measurement module basedon the "wtScope" software developed and validated by the Federal Institute for Materials Research andTesting (BAM) in Germany. The thickness measurement module can be used for all types of detectors
such as digitised radiation silver film, photo-sensible image plates and digital detectors.

Thickness measured

Detection of
external wall

Detection ofinternal wallFig. 5. Wall thickness measurement module of Rhythm DR software

Lmax

E

7.3.2 The L-max parameter
The diameter and thickness of the parts to be inspected have an influence on the precision of the
thickness measurement and are to be taken into consideration during an inspection. The essential
parameter for making the shots is the total length penetrated by the X or γ rays.
This variable, called L-max (Figure 6), is determined in the following manner :

D/2

D/2-E

L max = 2 × E

D
Fig. 6. L-Max

9

D
−1
E

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

7.3.3 Results of validation tests
The test results show that thickness inspection using the DR technique is possible in theconfigurations indicated by standard EN 14785.
The essential parameter to be controlled using this technique is the total length of the material
penetrated by the X or γ rays. This length "L-max" must be considered as an essential parameter
which makes it possible to determine, on the one hand, the feasibility of an inspection with respect tothe diameter and thickness of the part to be inspected depending on the source used and, on the other
hand, the measurement uncertainty.
Another important variable is the exposure time, which determines the signal/noise ratio.
Increasing the exposure times reduces the measurement uncertainty.
The presence of water is an important parameter to be taken into consideration for inspections during
operation. The presence of water does not lower the L-Max value itself (water density is lower thanthat of steel) but the detection of the internal wall can cause difficulties if the L-Max value is close to
its limits due to diffuse radiation (radiation scattering). The presence of water will limit the use of DR
and was taken into account during the tests.
The resolution of the detector does not have a notable influence on the measurement quality and"fast" detectors are suitable for use for all thickness measurements. The shot configuration has only aminor influence on the measurement uncertainty.
The following table shows the L-Max limiting values on the basis of the precision required and thesource used.
Configuration

Measurement
Se75
accuracy (mm)L-Max
With water+/- 0.5*
Without water+/- 0.5
50 mm
With water+/- 0.7*
Without water+/- 0.755 mm* Maximum limits not yet determined.

Ir192
L-Max
75 mm
95 mm
95 mm
100 mm

Co60
L-Max
120 mm *120 mm *120 mm *120 mm *Table 1 L-Max limits for different source types

7.4 The welds inspection

The degradation of welds by FAC and the solutions to control them were identified as important
issues when EDF evaluated its own FAC surveillance strategy after the Mihama accident. Different
cases of weld root wear occurring in EDF facilities have been analysed. It was established that theform of the degradations are closely linked to the chromium content of the weld deposit material but
also to the chromium content of the base metal in the adjacent components. The typical forms of theobserved degradations are shown in a simplified manner in the Figure 7.
.
High Cr level

Cr < 0,04%

High Cr level

High Cr level1
High Cr level

Cr < 0,04%

Low Cr level

17%Cr

Low Cr level3
Cr < 0,04%

medium Cr level

Stainless4

2

Fig. 7. Simplified forms of weld root wear degradation

Inspections to detect and dimension this type of weld root degradation can be carried out using
different non-destructive examination methods including Digital Radiography (DR) or ultrasounds
based on the TOFD method. These two methods were tested since 2005 and validated at the end of
2009 by EDF DTG. The use of DR or TOFD depends on the geometry of the components, the10

T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

configuration on site, and the intervention conditions. The inspection method by TOFD is preferred
for weld root wear detection and dimensioning. For wall thickness measurements on small bore piping
and piping under insulation, DR can be applied.
7.4.1 Weld root wear detection and dimensioning by TOFDThe NDE technique referred to as Time Of Flight Diffraction (TOFD) is based on the principle ofdiffracted ultrasound waves. TOFD uses longitudinal waves (LW), the principle of this technique isillustrated in the Figure 8.
ReceiverRes. Th
Volume filledwith ultrasounds

Th

Transmitter

Res. Th
Res. Th = Residual thickness
Th = Thickness

US Time path in µs

A
Lateral surface
wave

C
B
Upper diffraction Lower diffraction
wave
wave

D
Back wall wave
reflexion/diffraction

Fig. 8. Principle of TOFD

The couple of transducers (transmitter and receiver) "fills-in" the volume to be tested withultrasound waves. Among the transmitted ultrasound waves we can identify : the lateral surface wave(A), passing just under the surface from the transmitter towards the receiver taking the shortest path(straight line), the diffraction waves (B and C) generated by the upper and lower ends of a defect, andat the opposite side the reflection (or diffraction) wave D.
The TOFD method makes it possible to dimension defects with a high accuracy because thetechnique is capable of "measuring" their height and length. Sizing is carried out directly on the highfrequency signals.
TOFD is an ultrasound technique and is therefore sensitive to the metallurgical structure of thewelds. Accordingly, the technique is not appropriate for the testing of austenitic steel welds.
Another advantage of this technique is that various complex geometrical shapes of components can beinspected. In this case additional analyses or simulations are necessary (welds near expanders for
instance) and the accuracy is reduced a little.
7.4.2 Validation Methodology
The use of the TOFD technique was validated through a test programme that consisted inmeasuring the remaining ligament on test blocks with artificial defects of known sizes, and comparing
the measurements obtained with the real sizes. The precision of the method is determined from tests
on these blocks. Supplemental tests on real components with real weld root degradation defects were
performed to verify the test parameters on real defects. The tests on the elements with real defects
confirmed the results obtained on the test blocks.
The whole test programme confirmed the feasibility of the remaining ligament inspection at theroot of the weld and Heat Affected Zone by means of the TOFD technique. The measurement
precision depends essentially on the position of the couple of transducers compared to the centre of
the defect and the tested thickness. To summarise, and for wall thickness up to 44 mm, the applicableaccuracy to the remaining ligament measurement at the weld root is indicated in the following table.

11T. Knook et al./ Pipe wall thinning management at Electricité de France (EDF)

Recommended configurations:Thickness(mm)
4 to 7.5
7.5 to 13.5
13.5 to 25
25 to 44

Frequency
RecommendedMHz(CEN/EN Standard)
10 - 15
10 - 15
5 - 103-5

Recommended
beam angleα(CEN/EN Standard)
60° - 70°
60° - 70°
60°
60°

PCS : Distance
betweentransducers
(mm)
32
55
70
100

Total
uncertainty.
(mm)

Covered zone(mm) with 3 scans± 0.20± 0.30± 0.35± 0.45

28
50
50
80

Table 2 Recommended configuration for the TOFD inspection

8.

Consideration of the feedback experience

The operational feedback and its treatment is considered by EDF as an important way to improve
the FAC surveillance strategy, especially on lines that are not yet modeled in the prediction software
BRT-CICERO™. That’s why the RNM guideline requires that significant events like leaks or
measurements below the minimum acceptable thickness must be formally reported to the corporate
level and to the other NPPs of EDF.
International feedback events are analyzed by the EDF corporate level and can lead to special
actions on the EDF NPPs, such as the evaluation of the conservatism of BRT-CICERO™ on tubes
behind diaphragms after the Mihama accident.
Each year, EDF also organizes a feedback meeting that gathers all the EDF NPPs specialists, inorder to share the national and international feedback and to address different issues on the software,
the outages management, the inspection volumes, the chromium measurements, the NDE techniques,etc.
9.

Conclusion

Flow Accelerated Corrosion occurred and will continue to occur in most of the 441 Nuclear Power
Plants that are in operation worldwide. As all these plants are ageing (221 units are in operation for
more than 20 years and 81 over 30 years), it becomes more and more important for the operators tomanage this degradation mode in order to prevent accidents or leakages that can lead to personal
injuries or unscheduled shutdowns.
The use of prediction tools like BRT-CICERO™ can help the operator to identify the most criticalcomponents and so to schedule the inspection and maintenance activities in a most efficient manner.
International collaboration in the field of FAC is also very important so we can learn from each otherand take into account the lessons learned. The international conference on Flow Accelerated
Corrosion organized by EDF next May in Lyon France (www.fac2010.net) is one of the actions toimprove the overall knowledge on FAC.
References[1]

B, Chexal, J, Horowitz, R, Jones, B, Dooley, C, Wood, M, Bouchacourt, F,N, Remy, F,
Nordmann, P, Saint Paul, Electric Power Research Institute, Flow-Accelerated Corrosion inPower Plants, TR-106611-R1, 1998.
[2] M. Douglas, Effective Monitoring and Control of FAC, Wano Seminar, January 2008, Haiyan
Zhejang, China[3] J.P. Mercier, Nuclear power plant maintenance, Kirk Editions, Paris, 1988.
[4] P. Mousset, L’expertise métallurgique appliquée aux centrales thermiques, Kirk Editions, Paris,
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[14] L. E. Sanchez-Caldera – Ph.D Dissertation, Massachusetts Institute of Thechnology, June 1984.
[15] F,H, Sweeton, C,F, Baes, Journal of Chemical Thermodynamics, Vol, 2, p, 479, 1970.
[16] E.-M. Pavageau, O. de Bouvier, S. Trévin, J.-L. Bretelle, L. Dejoux, 14th International
Conference on Environmental Degradation of materials in nuclear Power Systems-Water
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[17] K, Fruzetti, EPRI Final Report, November 2003, TR 1003609.

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