Maintenance and Life Assessment of Steam Generators at Embalse Nuclear Station

Abstract

The Embalse Nuclear Generating Station (ENGS) has four vertical I-800 U-tubes Steam Generators (SGs) manufactured by Babcock & Wilcox (B&W). They are one of the most important compo-nents from the point of view of safety and cost-related elements for potential life extensions in case of a replacement thereof. A Life Management program has been started covering the entire plant and starting with the Life Assessment (LA) of this component which consists in a systematic way to evaluate aging mechanisms focused on the plant refurbishment and life extension. Because of this, maintenance-based ageing assessment from beginning of operation is analyzed and current LA-frame maintenance and inspections programs are carried out in order to maintain a high availability of the SGs then to enable the planning for the plant life extension. The most important taken actions have been the Eddy Current (EC) In Service Inspection program which performs 100% of the tubes of two SG every 1.5 years started in 1992, the mechanical cleaning by blasting of the internal tube surface, the sludge removal from the secondary side tubesheet, the divider plate replacement, the installation of antivibration bars (AVB's), installation of TSP inspection ports and an exhaustive inspection of the secondary internals as a preliminary result of the Life Assessment started during early 2000. The most relevant aging mechanism up to 2004 was the Flow Accelerated Corrosion (FAC) of U-bend supports and consequent fretting of tubes. The eddy current inspections allowed the fretting degradation to be detected and mitigated by installing AVB's. Currently, efficiency of this mitigat-ing action is being performed by vibration measurements and visual inspections. However, other degradation mechanism that could have origin due to the U-bend FAC like loose part damage (LPD) is being to be analyzed since could be an issue in the future. At present, FAC degradation on the cold leg side and sludge deposition on the hot leg side of the carbon steel Tube Support Plates is the main ageing issue at the point that SGs life extension could be compromised. During the LA-frame secondary side inspections of oct-nov 2005, many internals of SGs 2 and 4 were inspected for the first time and some important and minor degradation issues that are being to be assessed were found: possible crevice corrosion at the thermal plate-tube gap; incipient FAC at the steam separa-tors and loose part/foreign object detection. The results of the 2005 inspections and the conclusions of the TSP-related studies which include structural studies for accidents (Seismic and Steam Line Brake) and feed water chemical improve-ment to reduce FAC rate, will allow to complete the Life Assessment and the recommendations needed for refurbishment and life extension (repair/replacement of subcomponents, specific inspec-tions, modifications of the In Service Inspection and Maintenance programs, etc) The ENGS SGs shows a low percentage of exchange-surface area loss due to tube plugging, but they must be monitored continually and systematically if a life extension is intended. However, replacement-retubing op-tions during the refurbishment outage should be being taken into consideration until the complete SG life prognosis is finished.