E-Journal of Advanced Maintenance Vol.1 (2009) 68-76
Japan Society of Maintenology

Investigations of Multi-frequency Algorithms for Detection of Defects in
FBR Magnetic Steam Generators Tubes Covered with Sodium
Ovidiu MIHALACHE,* Shinya MIYAHARA, Toshihiko YAMAGUCHI, Takuya
YAMASHITA
1

Japan Atomic Energy Agency, Fast Breeder Reactor Plant Engineering Research Center, 1 Shiraki,
Tsuruga-shi, Fukui-ken, 919-1279, Japan(Received; January 20th, 2009)

Abstract. In the present paper it is presented the performance of multi-frequency eddy currents algorithms tocope with the eddy current signal from both sodium structures located on the outer surface of magnetic steam
generator tubes surface as well as support plates of steam generator tubes. The algorithm improves the signal tonoise ratio of defects detection even when these are located under steam generator support plates. Experimental
measurements of defects signals using eddy currents, when there is or there is no sodium on the outer tube
surface, are used to validate reliable computer simulation models in order to investigate and optimize themulti-frequency algorithms. The parameters of the most reliable multi-frequency algorithm, based on two or
three excitation frequencies and using both simulated and experimental remote field eddy current signals, arefound out for both cases when sodium adhere or not to the SG tube surface and a defect is placed or not undersodium bands or SG supports plates.

KEYWORDS: steam generator, fast breeder reactor, multi-frequency, remote field eddy current, sodium,support Plate, finite element method

1.

Introduction

In the magnetic heat exchanger tubes of Fast Breeder Reactors (FBRs) liquid sodium is used as
an intermediate heat-transfer fluid. A high pressure water flows inside of tubes while a low pressure
sodium flows runs inside the vessel containing the steam generator (SG) tubes. Due to the highchemical reactivity of sodium with water, penetration of SG tubes wall has to be avoided by all means,
and therefore earlier defect detection before wall penetration during the in-service inspection (ISI) is
required to increase the safety of SG tube components.
Remote Field Eddy Current Technique (RF-ECT) is a special method able to detect with equalsensitivity both inner and outer defects in ferromagnetic SG tubes [1-2]. The RF-ECT inspection
system includes both an electromagnetic excitation device and a detection coil that measures theelectromagnetic disturbance due to a defect in a tube. ISI is performed at room temperature after
sodium from SG vessel was drained and cooled down from several hundreds degrees to room
temperature. After draining, sodium adheres to the outer tube surface in a thin layer. In a previous
work it was estimated that sodium thickness layer was between 10 and 50 μm using comparison
between eddy current measurements and accurate finite element simulations [3, 4]. Because solid
sodium is also highly electrical conductive, but with a relative magnetic permeability close to one, itspresence on the outer tube surface affects the electromagnetic disturbance signal from existing cracks
located on the outer tube surface. When ECT inspection analysis is conducted from inside of the SG
tubes, due to the inability to access the outer SG tube surface, the noise in the signal due toirregularities in sodium layer thickness or sodium drops or bands near SG support plates can mask thesignal from a defect, making difficult to interpret the results in real ISI conditions. Previously
numerical simulations, for both magnetic and non-magnetic SG tubes, showed that by using a*

Corresponding author, E-mail: Ovidiu Mihalache, mihalache.ovidiu@jaea.go.jp
ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

68

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

multi-frequency ECT algorithm approach it is possible to eliminate the uncertainties from sodium
signal even when there is a possibility that sodium drops or bands appear next to support plates (SP)[5-6]. However, the reliability of numerical simulations could be confirmed only with experimental
measurements in the absence of sodium, raising the question of whether or not the algorithm changes
or could be trusted in the presence of sodium layer and what is its performance using experimental
data.
The multi-frequency techniques is a well-known method which was previously applied in thesignal processing of eddy current signal from tubes and pipes in nuclear power plants. Earlier work
shown in [7-9], proved the feasibility of the method but only for water cooled reactors and using
non-ferromagnetic SG tubes and bobbin type coils excited by classical eddy current probes.
In the present paper, the authors investigates the feasibility of multi-frequency remote field eddy
current algorithms to suppress SG support plates influences in the presence of unknown sodium layer
thickness and sodium structures located nearby. Optimized multi-frequency algorithms areinvestigated using first numerical finite element simulations, calibrated with experimental
measurements, in order to enhance the detection of outer tubes defects for various sodium layer
thicknesses. The parameters of the multi-frequency algorithm are determined in the case when sodium
adheres or not to the outer SG tube surface using numerical finite element simulations. The
multi-frequency algorithms are checked and verified using directly experimental measurements fromdefects under SP when these are covered by thin sodium layer.
The paper is organized in the following way. First, an experimental set-up is described toinvestigate the eddy current effect due to sodium structures located on the outer surface of magnetic
SG tubes. Next, eddy current (EC) finite element (FE) simulations are used to model the sodium effectand to evaluate the maximum noise arising from sodium structures and SP covered by sodium.
Simulations are validated with experimental measurements form defects and SP when there is or not
sodium on the outer SG tube surface. Suppression of SP and sodium signal is investigated using
accurate numerical multi-frequency algorithms to evaluate the most optimized method and then thealgorithms are applied directly to experimental data to increase detection of defects even when these
are located under SG support plates.
2.

Experimental Set-Up to Investigate Sodium Effect on the ECT Signals

An experiment was set-up to investigate what is the eddy current effect of small layers of sodium
adhering to steam generator tubes. Two SG tubes, one made of austenitic stainless steel and the otherbeing ferromagnetic, were introduced in a cylindrical tank, as it is shown in Fig. 1. Several SPs wereattached to the tubes in both upper and lower part. In the tube were machined two outer grooves (OD)
20% from the tube wall thickness (tw), 10 mm wide and one of the support plates was located justabove one defect. A view of the OD in the helical part of the SG tubes is shown in Fig. 2a. Liquidsodium at 2000 C was first introduced in the tank and then its temperature was quickly raised to 5000
C. Sodium was kept in the tank at high temperature for two hours and then its temperature was again
decreased to 2000 C. At this point, sodium was drained and the tank was cooled down in one day tothe room temperature. The RF-ECT signals from the magnetic SG tubes, as well as from the support
plates were recorded both before sodium was introduced in the tank and after sodium was drained andthe tank temperature was around 300 C. A fiberscope camera was introduced in the tank in order tovisualize the sodium deposits and sodium adherence to the magnetic SG tubes as well to the SP.
Fig. 2b shows the sodium deposits near the outer groove. The defect, as well as the tube arecovered by a thin sodium layer with a variable layer thickness. Fig. 2c shows a view of a sodium dropon SG tube formed after sodium draining. An estimation of the raw-image from fiber-scope camera
evaluated this drop to 4-5 mm height and 6-8 mm diameter.
Fig. 3 shows a view of the straight SP (support plate located on the straight part of SG tube)
before and after sodium adherence. An outer groove 20%tw, 10 mm wide, not visible in the pictures islocated under SP. Images taken with the fiberscope shows that sodium adheres to the SP and alsosodium enters in the space gap between SP and SG tube. However it is difficult to quantitatively
estimate how much sodium fills this gap and if sodium fills or not the defect under SP.

69

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

Fig. 1. View of the steam generator tubes and support plates inside of sodium tank

a)

b)

c)

Fig. 2. a) Groove 20%tw, 10 mm wide, on helical part of SG tube;b) Sodium structures around defect; c) Visualization of a sodium drop located on the top of SG tubea)b)Fig. 3. a) Configuration of straight support plate (groove 20%tw 10 mm is located under SP); b)
Adherence of sodium to the straight SP70

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

3.

Finite Element Simulation for Sodium and Steam Generator Support Plate RF-ECT Signal

Numerical simulations were conducted using a finite element code [4] which can model theelectromagnetic interaction between electromagnetic field produced by the RF-ECT coils system andferromagnetic SG tube and calculate the electromagnetic disturbance due to a defect in the tube. The
RF-ECT probe consists in two excitation systems, far from each other with a differential detections
system, based on the bobbin coil type, located between the two excitation devices. The current isflowing in the same direction in both excitation coils.
The presence of sodium layer on the outer tube surface modifies the electromagnetic path signal
during eddy current measurements due to the sodium high electrical conductivity (23.8x106 S/m). In aprevious paper [3], the authors found out, by matching numerical remote field ECT simulations withexperimental measurements, that the medium thickness of the sodium layer deposit on the outer SG
tube surface is less than 50 μm. However, the true thickness of the sodium layer cannot be known inadvance, as could be seen in the pictures taken with the fiberscope during experiment.
In numerical analysis, sodium layer thickness was set to be between 0.01 and 0.1 mm in order toevaluate the maximum noise which could came from the sodium layer. The geometrical model of the
SP covered by sodium and located on the straight part of SG tubes, of sodium bands covering outer
SG defects and sodium bands located next to SP is presented in Fig. 4. Sodium can fills completely or
partially the defect as it is located under SP.

a)

b)

c)

Fig. 4. Geometrical model for: a) straight SP covered by a thin sodium layer; b) defect under sodium
band; c) sodium structures near SP

A numerical evaluation of the RF-ECT signal of sodium layer and sodium band effect is
illustrated in Fig. 5 . As sodium layer thickness increases the outer defect ECT signal decreases. Also,
the sodium band signal is similar with the defect signal. During numerical simulations were
investigated several sizes (height up to 5 mm, up to 40 mm wide) of sodium bands in order to cover a
wider range of problems. The variation of signal coming only from sodium bands is shown in Fig.
5b.

a)

b)

Fig. 5. Simulation of RF-ECT signal from: a) OD20%tw, 10mm partially filled with sodium; b) a sodium
band with different sizes

71

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

Numerical simulations were validated with experimental measurements of outer groove signal inboth cases when sodium adhere or not to the outer SG tube surface (see Fig. 6). While the agreement
between simulation and measured data was very good in the absence of sodium, a small discrepancies
was observed in the presence of sodium. This is because simulations took into account only a constantsodium layer thickness. However, the results are in good agreement and confirmed that the medium
sodium layer thickness size is 30 μm, which was in agreement with previous simulation results that
determined a maximum 50 μm sodium layer thickness [3].

a)b)Fig. 6. Comparison between simulations and measurements in absence of sodium layer for OD 20% tw,
10mm in a helical tube: a) without sodium; b) SG tube covered by sodium

In Fig. 7a it is shown the comparison between numerical simulation of straight SP and measured
data when there is no sodium on the outer SG surface. In Fig. 7b, a defect under SP was added in the
numerical analysis. The small discrepancies between simulation and measured data in Fig. 7b could
be due to the unknown gap between SP and defect, because the SPs simulated in Fig. 7a and Fig 8a
are two different SPs located on different part of the tube, one SP with a defect under it and the otherone without any defect.
Fig. 8 shows the same simulations like in Fig. 7 but in the presence of sodium. A very good
agreement was obtained, despite the complexity of the real model, confirming the validity of thenumerical electromagnetic model for all parts involved in the analysis: magnetic SG tubes, SP,
unknown distribution of sodium layer. However, in the visual inspection of the experiment could not
be seen any large sodium structure formation near SP. But this does not rule out that a sodium
structure could not be formed near SP and consequently this situation was taken into consideration innumerical simulations.

a)b)Fig. 7. Comparison between simulations and measurements in absence of sodium layer for : a) straightSP; b) OD 20%tw under straight SP72

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodiuma)b)Fig. 8. Comparison between simulations and measurements in the presence of sodium: a) straight SP; b)
OD 20%tw under straight SP

4.

Multi-Frequency Algorithm to Suppress Both SG Support Plate and Sodium Signal

The multi-frequency algorithm is based on the assumption that by combining the RF-ECT signal
at several frequencies (low and high frequencies), the signal from sodium structures and support
plates can be reduced while the signal from existing covered defects is enhanced. The algorithm canbe successful only in the case when sodium bands signal and defect signal have a different patternvariation with frequencies. In the particular case of the studied RF-ECT probe and SG tube parameters
the authors found through numerical simulations that this assumption remains valid.
Let’s suppose we have the RF-ECT defect signal Si at several frequencies νi. Then, the "j"multi-frequency algorithm compute the mix signal Sj by combining the Si signals using the linear
parameter αi as in Eq. 1, where Qi is the linear operator defined by Eq. 2.

Sj =

n

∑α ⋅ Q [S (ν )]
i

j

i

i

i

(1)

i =1

⎛ cosθ i − sin θ i ⎞⎟Qi = ⎜⎜cosθ i ⎟⎠⎝ sin θ i

(2)

The "k" noise in the signal (in our case SP and sodium signal) can be written in a similar way(see Eq. 3), where "k" represents different sources of noise as: sodium layer, sodium drop, support
plate and others.
n∑α ⋅ Q [ N (ν )](3)⎧min N k⎪⎪⎛ Sj ⎞⎨⎟⎜
max⎪⎜ Nk ⎟
⎪⎩
⎠
⎝

(4)

Nk =

i

k

i

i

i

i =1

The optimum parameters of the multi-frequency algorithms , (αi, θi) are found out by minimizing
the norm of all known noise and maximizing the signal/noise ratio as is shown in Eq. 4.

73

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

Using numerical simulations of the defect signal and sodium layer and bands signals theparameters of the multi-frequency algorithm are calculated in that way to reduce the sodium bands
signal. These parameters are determined in the case of “sodium approximation” (there is sodium layer
outside of SG tube). The reason of doing this approximation is that the sodium signal will beconsidered as an additional noise source, since it is not possible to have knowledge of the sodium
layer thickness or size of the sodium drops or bands.
In numerical simulations were investigated all frequencies from 50 to 500 Hz in a 50 Hz step.
The best mixing frequencies were found to be in the range 100-200 Hz for this particular ECT sensor
and SG tube parameters. For other sensors or SG tube geometries, numerical simulations confirmedthat parameter of the multi-frequency algorithm modifies as well. A numerical analysis has to be
conducted for every specific ECT sensor configuration to find out the optimum working parameters.
An example of the algorithm is illustrated in Fig. 9 using numerical simulations. The signal from a
defect in several position (see Fig.4b) under a sodium band is enhanced while the sodium band anlayer signal is reduced to a central spot.

Fig. 9. Simulation of multi-frequency analysis to suppress sodium band and enhance defect signal

Using numerical simulations it was found out that for ferromagnetic SG tubes it is not possible toextrapolate the same multi-frequency algorithm to the suppression of the steam generator supports
plates and a different multi-frequency tuned algorithm it is required.
In Fig. 10 it is presented the application of multi-frequency algorithm to the “no sodium”
condition – there is no sodium on the outer SG tube surface. Fig. 10a is the result obtained using
numerical simulations while the signal in Fig. 10b-c is the result obtained using measured data.
Figures 10a-c show the best results among all combined multi-frequency algorithms, using thefollowing frequency combinations: (150Hz-450Hz), (150Hz-600Hz), (450hz-600Hz) and(150Hz-450Hz-600Hz)). The 2-frequency mix combination (450Hz-600Hz) showed the best
signal/noise ration in suppressing the SP signal in the absence of sodium. The signal from an outerdefect under straight SP of magnetic SG tube is enhanced. The algorithm can cope well with the noisein the experimental data and only a three-frequency algorithm (shown in Fig. 10c) increased thesignal/noise ratio.
In Fig. 11 it is shown the result of application of multi-frequency algorithm when there is sodium
on the outer SG tube surface and the defect is located under SP. Despite there is no knowledge about
how sodium fills the gap between SP and tube and if the defect is filled completely or partially withsodium, the algorithm can enhance the defect signal and suppress SP and sodium signal. The
parameters of the multi-frequency algorithm changes as a thin sodium layer adheres to the outer SG
tube surface. It was found that in this case a (150Hz-450Hz) algorithm can cope better with the noisefrom sodium (Fig 11b).
By validating numerical simulations with measured data, in the presence of sodium, the newmulti-frequency algorithm parameters are determined and a three-frequency algorithm is shown in Fig
11c to give also an enhanced signal/noise ratio compared to the two-frequency algorithm approach.

74

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

a)

b)

c)

Fig. 10. Application of multi-frequency algorithm in the absence of sodium to suppress support platesignal and enhance defect signal to: a) simulated data; b) experimental data (two-frequencies algorithm);c) experimental data (three-frequencies algorithm)

a)

b)

c)

Fig. 11. Application of multi-frequency algorithm in the presence of sodium to suppress support plate andsodium signal and enhance defect signal to: a) simulated data; b) experimental data (two-frequencies
algorithm); c) experimental data (three-frequencies algorithm)

Through numerical simulations it was studied how the sodium layer affects the ECT signal. By
validating numerical simulations with measured data, the performances of linear multi-frequency
algorithm was investigated to suppress both sodium and SP signal in order to reveal defects under SPof magnetic SG tubes.
5.

Conclusion

In the paper were presented validations of RF-ECT signal from defects under support plates ofmagnetic steam generator tubes with experimental measurements in both cases when sodium adheres
or not to the outer SG tube surface. Multi-frequency RF-ECT algorithms were calibrated withnumerical simulation in order to optimize the parameters of multi-frequency algorithms and thenapplied directly to experimentally measured data. Detection of outer groove under the straight SP ofmagnetic SG tubes could be enhanced in both cases where there is or there is no sodium on the outersurface of SG tubes. A three-frequency algorithm, validated with experimental data, was able to better
suppress both SP and sodium signal and reveal a hidden defect under the straight SP. The optimized
parameters change as the analysis moves from the no sodium to sodium approach.

75

O. Mihalache et al./Investigations of Multi-frequency Algorithms for Detection of Defects
in FBR Magnetic Steam Generators Tubes Covered with Sodium

References[1] ***, Nondestructive Testing Handbook (2nd edition), 4, Electromagnetic Testing, 1986.
[2] D. L. Atherton, S. Sullivan and M. Daly, A remote field eddy current tool for inspecting nuclear
reactor pressure tubes, Br. J. Nondestructive Testing 30, 1988, pp 22-28.
[3] O. Mihalache, K. Yokoyama, M. Ueda and T. Yamashita, Evaluation of the effect of sodium insteam generator tubes using remote field, Studies in Applied Electromagnetics and Mechanics,
24, 2004, pp.223-230.
[4] O. Mihalache, Advanced remote field computational analysis of steam generators tubes, IOS
Press, Electromagnetic Nondestructive Evaluation VII, Studies in Applied Electromagnetics andMechanics 26, 2006, pp 220-227.
[5] O. Mihalache, T. Yamaguchi, M. Ueda and T. Yamashita, Analysis of defect detection in steam
generator tubes of FBR under support plates and in the presence of sodium using multi-frequency
eddy currents algorithm, The 15th International Conference on Nuclear Engineering, Nagoya,Japan, April 22-26, 2007.
[6] O. Mihalache, T. Yamaguchi and T. Yamashita, Numerical simulations of a multi-frequency
eddy current algorithm for improved defect detection in magnetic SG tubes covered by sodium in
FBRs, The 4th Conference of Japan Society of Maintenology, Fukui, Japan, July 2-3, 2007, pp.
59-62.
[7] S. Kumano, N. Kawase, K. Kawata and A. Kurokawa, Signal processing of rotating pancake
eddy current signal for steam generator tubes, Proceedings of the 13th International Conference
on NDE in the Nuclear and Pressure Vessel Industries, Kyoto, Japan, 22-25 May, 1995, pp.
413-420.
[8] V. Ganugapati, K. Arunchalam, P. Ramuhalli, L.Udpa and S.Udpa, A polynomial mixing
algorithm for suppressing TSP signals in bobbin coil eddy current data, Electromagnetic
Nondestructive Evaluation (VII), Studies in Applied Electromagnetics and Mechanics 26, IOS
Press, 2006, pp. 213-219.
[9] Zhenmao Chen, Naoki Chigusa, Hiromu Isaka and Kenzo Miya, Data Processing of Corrosion
Noise Polluted ECT Signals for Heat Exchanger Tubes of Cu-Ni Alloy, Electromagnetic
Nondestructive Evaluation (VIII), Studies in Applied Electromagnetics and Mechanics, 24, IOS
Press, 2004, pp 128-135.

76