[en] When fast heavy ions penetrate in matter, they slow down essentially by depositing their energy on the electrons. This can lead to strong electronic excitation densities in the solid and then to structural modifications. In this work, calcium fluoride (CaF₂) was used to look further into the damage induced by irradiation with fast heavy ions in ionic crystals. Four techniques were mainly employed to characterise this damage. These techniques of analysis are wide angle X-ray diffraction, surface profilometry, Rutherford backscattering spectrometry and UV-visible optical absorption spectroscopy. The results of this work show that CaF₂ answers in a multiple way to the electronic excitations. For stopping powers higher than approximately 5 keV/nm, a polygonization seems to occur. This causes a structural disorder, a swelling of 0.27 % and the formation of fractures in the material. A second damage mechanism is caused above approximately 13 keV/nm and results in a loss of the initial crystalline structure. However, optical centres appear whatever the ion stopping power, which indicates that these defects cannot be the cause of the two above mentioned damage mechanisms. According to a thermal spike model, the two thresholds can be linked to melting and sublimation energy of the material, respectively. (author)

[en] Pure single crystals of CaF₂ were irradiated with different heavy ion species having energies of several hundred MeV. The induced damage was characterised by means of optical absorption spectroscopy, Rutherford backscattering spectrometry in channeling geometry (RBS-C), and out-of-plane swelling measurements using surface profilometry. For all ion species and energies, the optical spectra of the irradiated CaF₂ samples exhibit a wide absorption band with a maximum around 550 nm, typical for small calcium aggregates. The absolute absorbance depends on the fluence and on the electronic energy loss (S_e) of the ions, but there is no indication for a new band or a specific defect at high S_e values. In contrast to this, profilometry clearly indicates that swelling occurs only above a critical electronic energy loss of about 5 keV nm^-1. Furthermore, structural damage as measured by RBS-C starts to become significant above the same threshold. Therefore, swelling is obviously not directly related to the defects detected by optical absorption but is most likely linked to the structural disorder evidenced by the channeling technique

[en] Compressed pellets of 40 nm sized CaF₂ powder were exposed to Pb ions of about 4 MeV/u. The structural change of the irradiated crystals was monitored by in situ X-ray diffraction experiments. The evolution of the diffraction spectra as a function of the ion fluence gives evidence for two different phenomena: (1) the area of the diffraction peaks slowly decreases due to the loss of the crystalline phase of CaF₂, and (2) the width of the peaks broadens at a much faster rate. We assume that this latter observation can be ascribed to a grain breaking process

[en] During the last decade, in different countries, several occurrences of external corrosion have been identified on steam generator (SG) tube bundles equipped with thermally treated 600 alloy. In France, this feedback leads EDF to enhance the SG inspection program. Nevertheless, until now, no damage of this type was reported. Recently, during in-service inspection at the Cattenom plant on a SG equipped with alloy 600TT tubes, Eddy current tests have highlighted a signal that could be related to external corrosion. The tube was removed and sent to the EDF hot laboratory for destructive examinations. Various exams were performed at different scales to characterize the causes of this NDT signal, the material properties and the residual stresses. The assessments carried out on the tube conclude that the source of the damage is external intergranular stress corrosion cracking, also called ODSCC (Outside Diameter Stress Corrosion Cracking) making it the first occurrence on the tube bundles made of alloy 600TT in the French fleet. This first case of 600 TT ODSCC in France is an unexpected and particular one, because of its altitude in the full mechanical rolling area. This is reinforced by the low number of occurrences noted to date (only one after nearly 30 years of operation of alloy 600TT tube bundles). International (Biblis) OPEX had identified recent IGSCC with cracks initiated and propagated in the tubesheet. For this case, the scenario considered requires highly restrictive conditions (tube in the sludge zone and on the periphery of the tube bundle, including the tube lane) and may explain the singular nature of the Cattenom tube

[en] In many different insulators, the irradiation with heavy ions in the MeV to GeV energy regime induces pronounced volume expansion. This swelling increases with fluence, projected range and electronic stopping power of the ions. From the relative volume change per single ion as a function of the mean energy loss, a critical swelling threshold can be deduced. In amorphizable materials such as quartz and garnets (e.g. Y₃Fe₅O₁₂ and Gd₃Ga₅O₁₂), swelling is directly linked to the crystalline-amorphous phase change in each ion track, whereas for non-amorphizable ionic crystals (e.g. LiF and CaF₂), the correlation to known defects is not yet clear

[en] On 13 May 2008, after a primary circuit pressure testing on Saint-Alban reactor 2, a tube on the hot leg of the steam generator (SG) that had been plugged was found without a plug. First investigations revealed that 4 other plugs were concerned; one of them in a different reactor: Penly 2. Therefore, EDF declared the mis positioning of steam generator tube plugs as a generic defect. All the plugs have been recovered and analyzed in the hot laboratory of Chinon. We will propose an explanation for the plug failure based on the analysis performed in the hot laboratory of EDF in Chinon. Linked with the destructive examination, Areva and EDF have gone through a wide phase of experiments aiming to identify the root cause. A lot of parameters have been tested to detect the more relevant ones. Several SG plugs exhibited an installation defect. This defect presents a very low statistic (about ten non-compliant plugs out of over 22.000 plugs installed throughout the French nuclear pool). However, the potential impact in terms of safety justified the extensive investigation program that was conducted. Upon completion of the program, the lubrication phase of the plugs at the manufacturing stage seems to be the most relevant parameter to guarantee correct installation, as is often the case in tribology. This phase was therefore adjusted accordingly. Additionally, the compliance of each installed plug is validated by analysis of the installation curve

[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] To improve the understanding of ageing mechanisms and to support the asset management of steam generators (SGs), EDF in collaboration with the Materials Ageing Institute (MAI) is undertaking a major project to investigate two retired steam generators. The two selected SGs, one from Cruas unit 4 and the other from Paluel unit 2, are recirculating AREVA designs with thermally treated alloy 600 tube bundles and presenting about 30 years of operation. Topics of interest concern wear mechanisms, vibration fatigue, secondary side corrosion of tubes, denting, clogging, fouling, alloy 690 tube plugs, SG internals, tube ends, secondary side chemistry, zinc injection and NDT (non-destructive testing) probes developments by reliability studies. For each topic, the paper will describe the broad guidelines of the destructive examinations program. Most of these examinations will be performed in the new hot laboratory of EDF called LIDEC

[en] A circumferential fatigue crack was observed on a steam generator tube of the unit 2 of the Fessenheim plant. The results of destructive testing and the examination of the fracture surface show that the circumferential crack is linked to a large number of cycles with a very low stress intensity factor. Other aggravating factors like inter-granular corrosion have played a role in the initiating phase of fatigue cracking. The damage has been exacerbated by the lack of support of the tube at the level of the anti-vibration bars. (A.C.)

[en] In February 2008, a primary-to-secondary leak occurred at Fessenheim Unit No.2 on a steam generator. A circumferential fatigue crack was observed at the upper tube support plate level of the R12C62 tube although the stability ratio evaluation performed to take into account some prior international events, concluded that this tube had no risk of fluid-elastic instability. A new tube pull process was developed and performed by AREVA in 2011 just before the SG replacement. The extraction at the uppermost TSP elevation was a first occurrence in the French EDF PWR. Destructive examinations were carried out in the EDF hot laboratory of CEIDRE/Chinon in order to characterize damage mechanisms at the initiation and propagation stage. The document relates the major results of laboratory examinations leading us to exclude the fluid-elastic instability scenario as previously reported in North-Anna (1987) and Mihama (1991) tube rupture incidents. Results analysis with particular focus on the fracture surface description using Scanning Electron microscopy observations and metallurgical investigations provide new elements concerning the aggravating factors of fatigue damage. Fracture surface investigations reveal that the circumferential crack was due to high cycle fatigue with a very low stress intensity factor. Some aggravating factors like intergranular corrosion appeared to be critical for the fatigue cracking initiation stage. The deterioration was also largely promoted by the lack of tube support at the Anti-Vibration Bars