Replacement of steam generators for Embalse NGS - the steam generator cartridge design and manufacturing issues, localization and site assembly challenges

Embalse Nuclear Generating Station (Central Nuclear Embalse) was placed in service in 1983 and the outage for refurbishment is foreseen for 2011/2012. Embalse is equipped with four vertical inverted 'U' tube-type Steam Generators (SG) with integral preheater, I-800 tubes and carbon steel internals. Between 2002-2006, the owner assessed the potential for SG life extension; Nucleoelectrica Argentina S.A. (NA-SA) and AECL and a number of actions were completed towards meeting this objective (i.e.: primary divider plate replacement, additional U Bend support and inspection port installation). However, degradation of the tube supports (carbon steel broached plate) and U-bend supports due to Flow-accelerated corrosion (FAC) compromised the possibility for life extension of these Steam Generators. This issue, coupled with the plan to increase the plant power output during the life extension of the station, resulted in the strategic decision by NA-SA, to replace the Steam Generators. Several options were considered for SG replacement: In-situ replacement of the SG tube bundle, the original steam drum to be re-used; Removal and replacement of the entire SG (including the steam drum); and, Replacement of the bottom portion of the SG, i.e. the shell, the tube bundle, the tube sheet, the primary head and its internals and the primary nozzles with a factory assembled cartridge (collectively called the 'SG cartridge'). In this option, the original steam drum would be retained for the extended life. The final decision, based on the recommendations from the Life Assessment Study performed during the Pre-project Condition Assessment Process, is to replace only the Steam Generator cartridges. NA-SA requested AECL's support for the preparation of the Technical Specification for the replacement cartridges, allowing for the higher plant output. This paper presents the design basis for the technical requirements covered in the Technical Specification. The specified requirements include: Increased heat transfer area; Critical components with higher corrosion resistance; And enhanced design features to improve and facilitate steam generator maintenance and inspection. As part of the localization program for major components required for Central Nuclear Embalse life extension program, NA-SA is also seeking AECL support to evaluate the capabilities of local suppliers of steam generator tubing and the manufacture of the steam generators. (author)