Skip to main content

NEWS

14. 2. 2017

No deviations detected in the chemical composition of materials for forged parts of steam generators in Krško NPP

The meeting of the Western European Nuclear Regulators Association (WENRA) informed the involved counterparts about the issue of the steam generator manufacturer AREVA (ex Siemens - Framatome) regarding components of forged parts for steam generators supplied by the French Creusot Forge. It was presented that in some of the forged components (e.g. the primary chamber of steam generators) suspected discrepancies in the chemical composition of the material, such as increased carbon segregation on the surface of the forging, could lead to a fast fracture of the component. Apart from certain parts of reactor vessels, the carbon segregation phenomenon on the surface of forgings was also detected on some parts of steam generators, which were made by the same manufacturer. French regulatory authority (ASN) requested additional non-destructive inspection and analysis of these components and also a more detailed examination of the documentation on the manufacturing of components and inspection after the manufacture, where some data falsification was suspected. Routine inspections have been conducted at AREVA and also directly at the manufacturer of forged components Creusot Forge. Components manufactured in Creusot Forge have been also installed into steam generators in other countries, not only in France.


SNSA notified the Krško NPP on the 8th November 2016 about the issue regarding forged components of AREVA steam generators and asked the Krško NPP to check whether the replaced steam generators parts manufactured by Siemens-Framatome could possibly be made in the Creusot Forge factory. The Krško NPP replied that they have been already aware of these issues. They also stated that forged parts of Siemens-Framatome steam generators were constructed in Japan Steel Works factory (JSW below) and were manufactured in accordance with the requirements of the specifications of the Krško NPP. The certificates of the JSW proved that the carbon content in the primary chambers of the steam generators forgings is 0.21% and 0.22%, which is within the required limits in relation to the standard ASME II material SA 508 Class 3A, which are between 0.15% - 0.25%. The Krško NPP also states that all archived records and certificates of the materials of the forgings from the manufacturing of the steam generators are available in the Krško NPP.


In 2000, SNSA issued under the administrative procedure "Design and manufacturing of steam generators for the Krško NPP" a license amendment, where in the first point of the operative part it is stated that steam generators in the Krsko NPP are designed and constructed in accordance with the documents of the Siemens-Framatome and meet the criteria of the quality of the built equipment and materials for nuclear power plants. The Krško NPP started the procedure for obtaining independent expert opinions on the quality of the new steam generators from several authorized organizations in order to demonstrate the quality of the new steam generators in accordance with applicable legislation at that time and in order to obtain an operating permit for steam generator replacement. The scope of welding and non-destructive tests was covered by the Welding Institute of Slovenia; quality and appropriateness of the use of materials by the Institute for Metals and Technology; quality assurance, measurement and verification of the quality of the steam generators by the Institute for Metal Structures and by the Faculty for Mechanical Engineering in Ljubljana; they also covered the scope of the verification and quality assurance of mechanical equipment during production and acceptance testing and field of design control of the components of a nuclear steam supply system (NSSS).


The explanation in the license amendment confirms that the authorized organization the Institute for Metals and Technologies (IMT) was carrying out quality control of the materials for the vital parts of the steam generator with an emphasis on strength, technology, corrosion chemical and microstructural properties of materials. The expert opinion, based on a review of the technical documentation, presence in different stages of the manufacture of individual sets of steam generators and own investigations, concludes that installed materials meet the requirements of ASME II standards and purchase specifications.


As part of the documentation for the manufacture of steam generators, SNSA also owns an expert opinion from the authorized organization of the Institute for Metal and Technology (IMT). The latter, as a part of the project, carried out their specific analysis of the test sample of the weld between the dividing plate of the alloy Inconel 690 and ASME SA 508 CL 3a, from which the primary chambers of the steam generator are made. The welded joint has been made from the same materials as those that are installed in the steam generator (archival material). The IMT report shows that within the regarded welded joint analysis an independent chemical analysis of the materials of the welded joint was carried out as well. The results of this analysis show that the carbon content in the test sample of SA 508 Class 3A of the base material, from which the primary chamber is made is 0.191%. This is additional confirmation that the material, which is used, meets the requirements of the ASME II (Figure 1).


Figure 1: Table of the chemical content of SA 508 Class 3A material from the IMT report

 

 

On the 12th January 2017, SNSA carried out an inspection at the Krško NPP with the aim of verifying the appropriateness of the material of steam generators forgings in the Krško NPP and the records and certificates of materials, which are in the Krško NPP archives. Before the inspection, SNSA had reviewed the Siemens-Framatome project documentation and concluded that the steam generators installed in 2000 had been designed and manufactured in accordance with the requirements of ASME standards and purchasing specifications of the Krško NPP. Similar conclusions were also drawn by authorized organizations, which accompanied the project of the steam generators manufacturing.


The Krško NPP has further explained that within the framework of the steam generators replacement modification, special attention was devoted to verify the quality of the new steam generators. As part of quality assurance (QA), the Krško NPP has performed a lot of QA audits at the manufacturer of the steam generators Siemens-Framatome and suppliers of individual components. During the manufacturing of individual parts of steam generators, the Krško NPP and authorized organizations conducted a comprehensive quality control (QC) at the manufacturer of the steam generators Siemens-Framatome and also at suppliers of individual parts including suppliers of forgings JSW. For the purpose of auditing and control of the production of new Framatome-Siemens steam generators, the Krško NPP also organized a special training for the members of the supervisory team, which supervised the manufacturing. The documentation of training was presented to the SNSA as well.  The annex of training documents lists the names of all subcontractors of the manufacturing steam generators project and the French factory Creusot Forge is not listed.


The Krško NPP has presented to the SNSA information copies of archived original JSW specifications certificates on the chemical composition and mechanical properties of the material SA 508 Class 3A from which primary chambers of the steam generators are made/forged. As the Krško NPP letter already noted, it is also evident from JSW certificates that the carbon content in forgings of primary chambers of steam generators is 0.21% and 0.22%, which is within the required limits according to the ASME standard (Figures 2 and 3). SNSA has also reviewed part of relevant reports of the QA control and final report of QC control of the manufacturing of parts of the steam generators in JSW. The report shows that the producer JSW is on the reference list as the subcontractor in the Krško NPP - Siemens-Framatome contract. JSW is also a holder of the ASME (N-Stamp) certificate for materials and products and therefore also competent for the production of parts for nuclear power plants. For the supervision of forgings manufacturing in JSW a total of 136 reports were written by the Krško NPP supervisors. It was concluded from the final report that there were no anomalies or irregularities in the material and by the manufacturing of components, which could have an impact on the quality of forgings.

 

Figure 2: Chemical composition of primary chamber of the steam generator (SG1) material from the JSW certificate

 

 

Figure 3: Chemical composition of primary chamber of the steam generator (SG2) material from the JSW certificate

 

 

The Krško NPP has confirmed the SNSA’s presumption that all important forged parts of the steam generator were manufactured by JSW Steel Plant and meet the requirements of the purchase specifications and ASME standards with regard to material ASME SA 508 II Class 3A. The Krško NPP has also confirmed during the SNSA inspection that they have been already aware of the above-mentioned issue of installed materials of the AREVA steam generators and that they have addressed it in the corrective program. Within this activity, the Krško NPP conducted a review of archival JSW specifications certifications on the chemical composition and mechanical properties of the material SA 508 Class 3A from which the primary chambers of the steam generators are made. Based on the results of the examinations mentioned, they are within the required values of the ASME II and the fact that the Krško NPP has been present at all important activities of the steam generators forgings manufacturing, which was evidenced with all examined QA and QC documentation on SNSA inspection, the Krško NPP was not obligated to carry out additional activities. The Krško NPP and also the SNSA, of course, intend to continue with active monitoring of the open issues and activities after the events in France.