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Extension of French Methodology and Technology to the Inspection of WER Steam Generators Robert Lévy, Intercontrôle/Framatome, France Contact

INTRODUCTION

In France, the Safety of Nuclear Installations has been ranked as a matter of highest concern for almost forty years now.
In PWR reactors, the steam generators including a boundary between the primary and secondary circuit, their inspection was, from the beginning, made part of the permanent programme for maintenance designed by the Utility at national level.
The actual boundary being in fact the tube bundle in the steam generator, the feasibility study of the inspection of such elements was attributed to the French Atomic Energy Commission (CEA). The study was based on the requirement that the technique selected for the inspection should allow access to information on the status of the whole length of every single tube in the steam generator.
The eddy current testing method which allows fast and easy inspection of tubes, being already known in the tube fabrication industry, its application to the case of vertical U-bend tubes supported by metallic plates was studied in depth. The multifrequency technique was found to be the best way to solve the problems linked to the structure. An instrument and a differential bobbin coil probe were then designed.
A subsidiary of the CEA, INTERCONTROLE, was created to bring to the field the works carried out in the laboratories. Intercontrôle has therefore been involved in the inspection of steam generator tubes since the very first pre-service inspection in France, in 1975.

METHODOLOGY

The methodology applied for the inspection of steam generator tubes could be summarised in the compliance with four essential rules : reliability, accuracy, reproducibility, traceability.

Reliability


Reliable methods
The eddy current testing method is based on physical principles which have been known for more than one century now !
The multifrequency technique, based on the principle of energising the sensor at alternating voltages with different frequencies, had been originally applied by the CEA to the elimination of the unwanted signal of support plates that would distort the signal of any defect occurring at a support plate level. The first pre-service inspection revealed an additional and unsuspected unwanted signal : the permanent metallurgic noise due to the fabrication of the tube. The multifrequency technique solved the problem very easily, without having to modify the sensor (probe), thus confirming the validity of the choice initially made.

Reliable tools
The complete line of equipment necessary to carry out the inspection of a complete tube bundle on the whole tube length was designed in collaboration with the CEA and Framatome.
The probe has allowed the inspection of the whole tube length, starting from one end, and going through the U-bend even in the case of very tight bends (rows 1,2,3) of the steam generator, since the very first inspection. This unique feature enabled the operators to put the equipment on the platform and leave it in position for the whole duration of the inspection.
The pusher-puller now carries out fully automatic cycles with an intelligent use of the eddy current signal.
The manipulator, improved through the years, is now adaptable to all the existing configurations of PWR reactors. The new design reduces the interventions of operators and therefore also reduces the doses they receive.

Reliable operators
When the company started to grow fast in order to face the very rapidly increasing need for inspections, the importance of training became obvious.
A full training program was designed, together with a personnel qualification procedure and these documents were approved by the Utility. In-house certification levels were defined according to the level of skill and responsibility of the operators.
At the highest level, experts took part in the first national certification in eddy currents delivered by the French non destructive testing committee (COFREND), and obtained level 3 certification.

Accuracy


Tailored probes
The bobbin coil probe having been used for the first inspections, the need for comparison between successive inspections from year to year, made it a permanent tool of inspection. Nevertheless, this permanent use was never understood as a limitation and, when it became necessary to use more adequate inspection tools adapted to local situations, probes delivering a more accurate diagnosis were developed. Rotating probes allow exceptional performances in the detection and sizing of cracks ; when their use is limited to the area of interest - generally identified thanks to the signals of the axial probe - there is no increase in the global duration of the inspection.
Specific problems linked to the ageing of the steam generators have then given rise to the development of specific probes. Rotating probes were thus developed for the inspection of the tube portion at the level of support plates (Figure 1) or in the U-bend.

Figure 1 : Characterisation of corrosion on the outer wall of a steam generator tube, at the level of a support plate, using an eddy current rotating probe.

Automation of data acquisition and data analysis
After the reduction of human factor by automation of full sequences of data acquisition, a very beneficial improvement was brought by the on-line verification of data quality.
This verification is carried out by a specific module of the acquisition and analysis software AIDA. Implemented since 1989, this module has made possible an increase in both the acquisition and the analysis performances of the teams.
During the acquisition stage, the verification of the quality of data is done on a number of parameters of the complete set of data. If data is acceptable, the operator is informed and a set of print-outs is automatically issued. If the data is rejected, the operator receives a message and the tube concerned is immediately re-tested without any loss of time for repositioning the manipulator.
Since all data reaching the analysis stage has been accepted by the quality check module, the automatic analysis carried out by the software is possible on complete batches of tubes. All the results obtained during this phase must be validated by a qualified analyst before being included in the final report.

Reproducibility
The qualification of the techniques developed in order to solve a new problem is generally carried out on test blocks with representative defects but the most reliable evidences are brought by destructive examination of pulled tubes.
The adequate conditions for the implementation of a qualified technique are then defined by procedures, the revisions of which generally include a feedback from the experience during actual inspections.

Traceability
When monitoring a structure so important as the steam generator, no data can be considered of no importance. Recordings which were carried out on the most reliable medium when inspections started (magnetic tape) have drastically evolved with the introduction of data digitisation and the use of computers and their associated media. Since the very first inspection, each set of data acquired was fully identified in terms of date, site, unit, steam generator and tube co-ordinates.
The importance of the availability of data acquired during previous inspections was revealed when the first stress corrosion cracks appeared at the roll transition in the French steam generators. A characteristic distortion of the roll transition signal obtained with the bobbin coil could be associated with the presence of cracks. All the recordings of all roll transition signals in all the steam generators were replayed by special teams of analysts who could precisely identify all the tubes that would require additional inspection using the rotating probe. A status of all defective tubes was very rapidly obtained and was a very strong support to the Utility in the selection of plugging criteria for this specific type of defect.

TECHNOLOGY

A complete line of equipment has been developed and periodically upgraded. This equipment is now qualified and extensively used for the inspection of PWR steam generator tubes.

The eddy current instrument
The multifrequency eddy current instrument included in each line of equipment is the Harmonic 210. Operating in the range of frequencies between 10 kHz and 1MHz, it enables to excite a probe at four frequencies and to carry out absolute or differential measurements.
The instrument is controlled by a microprocessor and the configuration is programmable by the operator. Data is available at the outputs of the instrument in digital mode.
The instrument features a DC current supply to feed a special built-in coil of the axial probe, used to saturate local variations of magnetic permeability.

Eddy current probes
All the probes necessary to the various inspections carried out on steam generator tubes are designed, assembled and verified by the company.
The experience in this field is not limited to PWR steam generators but also covers the probes necessary for the inspection of condensers, navy steam generators and heat exchangers.

Data acquisition shelter
All the equipment for verifying, recording and storing acquisition data, monitoring and controlling the manipulator and the probe pusher is installed in a pre-wired "shelter" transportable from site to site.

Data analysis software
This development implemented by the inspection teams on all French nuclear sites is not application specific. Its modular concept makes it adaptable to both the data acquisition and data analysis of signals obtained from any bobbin coil probe during the inspection of a tube.

EXTENSION TO THE IN-SERVICE INSPECTION OF VVER REACTORS

Both the methodology and the technology described in the previous sections were transferred to the in-service inspection of steam generator tubes in VVER nuclear power plants.

The methodology
In 1989, Intercontrôle carried out the feasibility study of the inspection of VVER steam generator tubes in order to supply an inspection system in Russia. The basic principles successfully applied for fifteen years were kept as guidelines.
Multifrequency techniques were selected and the rules of detection and analysis applied in France for Inconel tubes were transferred to the case of stainless steel tubes of smaller diameter and larger wall thickness ; optimised excitation frequencies were calculated. Thanks to the experience gained throughout the very numerous inspections carried out and the lessons learnt from site, industrial solutions were found for all the problems related to the new type of structure to be examined.
The essential elements to be included in data acquisition procedures and data analysis procedures were defined for the operators.

The technology
An original manipulator adapted to the cylindrical geometry of the collector was fabricated, transferring from the French design of manipulator the concept of the grippers to move from one tube to another.
The eddy current instrument used for inspections in France was perfectly adapted for the case of VVER stations.
The experience gained in designing probes was used for the design of an axial probe capable of inspecting horizontal tubes with tight bends (Figure 2). A rotating probe was also developed for the inspection of the collector ligament, a VVER specific problem.
Figure 2 : Multifrequency examination of a VVER steam generator tube on its whole length with a single travel of the bobbin coil probe.
Thus, from the beginning, the tools developed over several years were instantly made available to be implemented on the VVER steam generators.
The data analysis software was also adapted to the "events" likely to occur in the tubes of VVER steam generators. A Russian version of the software was developed, (Figure 3 ).
Figure 3 : Typical displays obtained with the automatic analysis software AIDA for eddy current data from a bobbin coil probe (Russian version).
Two complete lines of this equipment were delivered in Russia.

Later, in collaboration with Siemens (Germany) who supplied a different manipulator, eddy current instruments, data acquisition and data analysis software were also delivered to the NPP of Rovno in Ukraine.
Reliable tools for reliable methods had thus been transferred ; another step was still necessary.

Training
The need for training in eddy current testing was extremely urgent since this inspection method was not used in Russia nor in Ukraine for steam generator tubes.
For Russia, training was part of the initial contract for delivery of the equipment. Technical assistance was then provided on site by French level 3 experts who participated in the first inspections carried out with the new equipment.
In Ukraine, in addition to the training delivered with the delivery of the equipment and to technical assistance on site, training sessions were included in the programme of a TACIS project funded by the European Union ; operators were taught for level one and level two qualification.
This training was then renewed twice thanks to two contracts funded by the French Ministry of Foreign Affairs, thus allowing to increase the number of operators made familiar with eddy current testing of tubes and to upgrade the training of operators previously trained.
More qualified operators are still needed, but the process has been initiated.

THE RESULTS

When considering the results obtained in Russia and in Ukraine since eddy current testing is performed in VVER steam generator tubes, it becomes obvious that proven methodology is easily and successfully transferable.
First of all, there is now a growing amount of data, available on reliable media, which enables the utilities to have a better knowledge of the status of the steam generators.
Then the experience of the operators constantly increases and this makes them feel more confident : the quality of the work they carry out is a striking evidence. As an example, the operators of Rovno NPP in Ukraine transport their inspection equipment on the sites of Khmelnitsky and South Ukraine and they have, up to now, efficiently inspected 70 000 tubes.
Last but not least, the corrective maintenance adequately performed contributes to the safe operation of the nuclear power plants ; since the implementation of eddy current testing of the tubes, no forced outage due to a leaking tube has been recorded.

CONCLUSIONS

The methodology adopted in France for the inspection of the tube bundles in 57 PWR nuclear power plants operated since 1975 by one Utility, EDF, can be considered as an example to be followed since no tube rupture has ever been recorded.
When Intercontrôle has had to transfer the experience gained in the inspection of millions of tubes to the case of VVER steam generators, the same criteria were selected and the same level of technology was applied.
The result today is self-standing : the existence of autonomous teams of operators in places where the method was unknown five years ago ! An unquestionable evidence of the validity of all the choices which were made in the framework of the French methodology and which were transferred and implemented.

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