[en] Until 2010, more than 60 steam generator (SG) tubes have been removed and analysed in the EDF hot laboratory of CEIDRE/Chinon. This article is particularly related to three recent events that lead to the extraction of several tubes dedicated to laboratory destructive examinations. The first event that constitutes a first occurrence on the EDF Park, concerns the detection of a circumferential crack on the external surface of a tube located at tube support plate elevation. After this observation, several tubes have been extracted from Bugey 3 and Fessenheim 2 nuclear power plants with steam generators equipped with 600 MA bundle. The other two events concern the consequences of chemical cleaning of the tube bundle steam generators. The examples chosen are from Cruas 4 et Chinon B2 units whose tubes were extracted following non destructive testing performed immediately after or at the completion of cycle following the chemical cleaning. In the case of Cruas 4, Eddy Current Testing (ET) were performed for requalification of steam Generators after chemical cleaning. They allowed the detection of an indication located at the bottom of tube for a large number of tubes; the ET signal was similar to that corresponding to 'deposit' corrosion. Moreover, inspections of Chinon-B2 SGs at the end of the operation cycle following the chemical cleaning, showed the presence of conductor deposits at the bottom of some tubes. The first part of this document presents the major results of laboratory examinations of the pulled tubes of Bugey 3 and Fessenheim 2 and their analysis. Hypothesis concerning damage mechanisms of the tubes are also proposed. The second part of the paper relates the results of the laboratory examinations of the pulled tubes of Cruas 4 and Chinon B 2 after chemical cleaning and their analysis. (authors)

[en] In 2006, EDF identified on several PWR units some broached hole blockages on the upper Steam Generator (SG) Tube Support Plates (TSP). The consequences on SGs circulation ratio and the risk of the SG cracking tubes led EDF to implement a program of counteractions. TSP blockage often occurs in association with secondary fouling due to the surface degradation of the materials on the secondary circuit. Thus, EDF has defined a two-level strategy to improve the SGs operating conditions: -) the implementation of curative remedies to remove oxides deposits in SGs e.g. chemical cleaning, -) the removal of the copper alloys materials of the secondary circuit to implement a high pH AVT (9.60) as a preventive remedy along with soft chemical cleaning to maintain satisfactory SGs performances and safety operating conditions. Within 2007-2009, EDF applied SG chemical cleaning on 12 PWR plants (six with the AREVA inhibitor free high temperature process HTCC and six others with the low temperature EPRI/SGOG process by Westinghouse). Both the HTCC and the EPRI/SGOG processes led to good results regarding the deposit removal and the TSP blockage reduction. Feedback about the corrosion of carbon steel surfaces is more favourable for the EPRI/SGOG process than for the HTCC process, which led locally to a loss of thickness of several hundred microns. Moreover, the excessive release of gaseous chemicals (ammonia) in the environment was also an important issue for the AREVA process. Therefore four new applications of the EPRI/SGOG cleaning process are foreseen in 2010-2011. Meanwhile, EDF is carrying out a program to remove of all copper alloy exchangers in the secondary circuit, in order to implement a high pH AVT, which offers proven benefits on SGs fouling and TSP blockage, as well as an efficient way to mitigate carbon steel flow accelerated corrosion damages Since the removal of brass condenser is not yet possible for some units for environmental or sanitary reasons, EDF hopes to implement an intermediate pH AVT. This is until a solution is found to improve the situation and prevent cleaned SGs to quickly being confronted with the same issues of oxides deposits. (authors)

[en] The materials constituting the condenser tubes of French nuclear power plants display a great diversity and are subject to different degradations, known from the operational feedback from experience. Copper alloys (mainly brass), which were bound to disappear in renovated condensers, are still significantly present, due to their unique bacteriostatic ability. Brass tubes lifetime is still governed in general by steady abrasion, as evaluated by eddy current nondestructive testing. However, an atypical NDE (non-destructive evaluation) behavior has led to spot a new damage: localized under-deposit pitting corrosion on the raw water side, caused by the particular quality of water chemistry and heavy scaling of the tube surface. This damage is likely to overcome steady abrasion for tube life prediction. Prevention includes a tighter look at NDE indications and improved descaling solutions (chemical or mechanical). Other specific damages have been reported from operation feedback: the main one was accidental stress corrosion cracking, which has occurred on some recently renovated brass condenser tube bundles. Thanks to a metallurgical and mechanical study, its cause was found in the manufacturing process. This experience has resulted in tightened specifications for brass tubes manufacturing. Stainless steel and titanium still appear more damage-resistant and represent a safe solution when no microorganism issue is present. The degradation feedback, confirmed by NDE inspections, is very low in French power plants. However, titanium hydriding still represents an issue when cathodic protection is present. Furthermore, some other damages have been reported on titanium, like isolated steam erosion. Vibration fatigue damage has been observed on stainless steel tubes, but it is more in relationship with the condenser design than with the material itself. (authors)

[en] In 2006, EDF identified on several PWR units broached hole blockage on the upper Steam Generator (SG) Tube Support Plates (TSP). TSP blockage often occurs in association with secondary fouling. The units with copper alloys materials are more affected due the applied low pH_25^oC (9.20) all volatile treatment (AVT). Carbon steels materials are less protected against flow accelerated corrosion (FAC) and therefore more corrosion products enter the SGs through the final feed water (FFW). In parallel of chemical cleanings to remove oxides deposits in SGs, EDF has defined a strategy to improve operating conditions. It mainly relies on the removal of copper alloys materials to implement a high pH AVT (9.60) as a preventive remedy. However for some plants, copper alloys removal is not straightforward due to environmental constraints. EDF must indeed manage the implementation of a biocide treatment needed in closed loop cooling systems (as copper has a bacteriostatic effect on micro-organisms) and more generally must comply with discharge authorisations for chemical conditioning reagents or biocide reagent. An alternative conditioning was tested on the Dampierre 4 unit in 2007/2008 during 6 months to assess if operating at 9.40 was acceptable regarding the impacts on copper alloys materials. The perspective would be to implement it in the units where no biocide treatment can be applied on a short term. In parallel, other chemical conditionings or additives will be implemented or tested. First of all, EDF will carry out a trial test with APA in order to assess its efficiency on the removal of oxides deposits through SG blowdown. On the other hand, AVT with high pH ethanolamine (ETA) will be implemented as an alternative of ammonia and morpholine conditioning on some chosen plants. Ethanolamine is selected as a way to mitigate FAC kinetics in two-phase flow areas (reheaters or moisture heater separator) or to limit liquid releases. This paper provides the lessons of the Dampierre 4 experiment and focuses on EDF strategy to remove copper alloys and increase the feedwater pH. Mid-term perspectives for EDF conclude this paper with the APA test and the implementation of ETA. (author)