|NDT.net - October 1999, Vol. 4 No. 10|
Electromagnetic Non-Destructive Evaluation II,|
Proceedings of the 4th International Workshop on E'NDE,
Chatou France, September 1998.
ISBN: 90 5199 375 7 - published by IOS Press
|TABLE OF CONTENTS|
Figure 1 shows the reference cost curves used to forecast the price of 1 kWh produced by an up-to-date power plant, burning different types of fuels.
You can clearly see that the benefit drawn between 1975 and 1990 from the low price of nuclear energy is going to disappear. Therefore, the main objectives of nuclear operators for remaining competitive are :
|Figure 1 : Forecast reference cost per kWh|
Figure 2 : Location of main degradations in Steam Generators tubes
Substantial degradation in these tubes is due to the choice of the Inconel 600 alloy as the tube material. This choice was made for the great majority of PWR plants throughout the world. The stress corrosion cracks (IGSCC), appear from the primary side in the deformed zones like the U bends or the transition at the top of the tube sheet. But also it was initiated from the secondary side in the areas of concentration of pollutants, which are the deposits above the tube sheet or in the interstices between the tube and the tube support sheet. Generalised micro-cracking (Intergranular Attack IGA) due to corrosion has also been found under the deposits accompanying the IGSCC. Metal losses through wear have been observed at the anti-vibration bars (AVB) level in the top of the bundle and on the periphery in contact with loosing objects fallen into the SG. Figure 2 shows the zone where each type of defects appears.
Inspection of the tubes is done with an axial probe with two coils over the whole length of the tube. 320,000 tubes were examined in this way in 1996. In addition, to measure the length of cracks and be able to characterise them more readily, several rotating probes with two differential coils with ferrite cores are employed locally.
More than 300,000 tubes were examined in the same year.
Identification of the defects from the ECT probe signals is difficult. As can be seen above, the signals are always mixed :
The policy followed by EDF has been not to plug the tubes on a defect signal associated with a theoretical depth without knowing the nature of that defect and assessing its behaviour and its harmfulness. Hence, besides the very numerous laboratory simulation studies conducted to learn how to recognise the type of defect, some tubes of the SG in operation have been pulled out for checking and validating the diagnosis made by END. Up to the end of 1996, 375 tubes were pulled out which represented approximately 2 milliard Francs for investigations, without counting the loss of production due to the duration of the removal operation.
This whole procedure has enabled EDF to define a plugging policy suited to each degradation and closely linked to the ECT signal identified.
Figure 3 : Location of main degradations in RCCA rodlets
Wear and tear, due to vibrations induced by the circulation of the fluid in the core, appears in the areas of friction by the rod against its guide. Locally it can lead to the puncturing or indeed the fatigue rupturing of the rod. For certain clusters, the end of which is inserted into a dense neutron flux, the irradiation causes swelling of the absorbent which expands the cladding until it cracks. The swelling also leads to a slowing down in the descent of the cluster or its jamming. Figure 3 shows the location of the defects.
The examination of each rodlet by an encircling ECT probe allows the presence of defects to be discerned. 24 coils enable the simultaneous inspection of a complete cluster. In 1996, the number of clusters examined was 32,000.
The hot laboratory investigation of several rods has enabled a relationship to be established between the mean section of wear and the ECT signal. However, some kinds of wear in the form of a V were poorly estimated. Similarly, in the swollen area, the cracking may be masked by the swelling signal. The solution to these two problems could not be found by the eddy current method. Therefore, profile measurement by a rotating ultrasonic probe is used each time the ECT signal in any area of the rod exceeds a predetermined threshold.
The establishment of criteria linked to the ECT signal has enabled a policy to be defined for scrapping a cluster or for using it for one or two cycles after its neutral position has been modified.
The strategy therefore has consisted in :
Thanks to this strategy, productivity gains have been obtained by placing performances in competition on the basis of quality and rapidity in data acquisition. Similarly the reliability of the diagnosis over a long period has enabled savings to be made in expenses, for example :
As has been noted above, to reach a final diagnosis, two or more ECT probes are employed, or indeed another method, that of ultrasonics, on the same component. To reduce the examination times, and therefore costs, we are going to try using multi-technique "probes" simultaneously. The data processing systems should be available commercially and should enable a diagnosis to be produced in real time. Despite the use of different techniques and different service providers, it must be possible to compare easily the results from one campaign to another, hence the obligation to work with a common data format. These three notions (shown in bold) constitute the main lines of progress along which EDF is counting on moving in the future with its partners.
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