NDTsi 2001 Session: Major Component Inspection I-IV
Visual Inspection of Brackets for Emergency Core Spray System in Barsebäck Unit 2
P. Efsing, J. Berglund, C. Sandelin, Barsebäck Kraft, Sweden; A. Werner, Westinghouse Atom TRC, Sweden
ABSTRACT
During the normal re-fueling outage in 1999, several defect brackets connecting the Emergency Core Spray System (ECSS) with the core lid were detected in some external recirculation loop reactors of ASEA type in Sweden. As a consequence of these defects, in this case Barsebäck Unit 2, four brackets were completely ruptured and one partially. One project was formed in order to enable the inspection of the brackets with a qualified inspection system to the next scheduled re-fueling outage, in the summer of 2000. The brackets are made from Inconel X.750 plates, they are welded to the lid and the stainless steel piping system with Nickel based Inconel 182 weld metal. At an initial stage, the inspections were expected to include only the use of Visual Techniques. However it was shown at an early stage to be difficult to validate characterisation and length sizing capabilities for such a system within given standards and tolerances. The anticipated defect sizes and widths that was included in the qualification process was described in a defect review process, which concluded that defects may be found in both the plate material as obvious from the initial inspection results, and in the welds. Since there must exist tensile stresses in order to drive crack propagation, theoretical studies were made in order to determine the most probable locations for defects. These studies were also part of the safety evaluation process, both during the first cycle after the defects were detected, and after the subsequent inspection during the next outage. If the most likely cause for the defects is manufacturing residual stresses, the defects can be expected to be found either in the weld metal, or at the positions they were found during the first inspections. In the safety evaluation of the system, a simplified fracture mechanical analysis demonstrated that even with very conservative assumptions regarding the crack propagation rate in Inconel X-750, safe operation could be maintained during a full operational cycle of the stations. The unit operates under 1-year cycles. As part of the refined safety analysis performed during the cycle to enable the appropriate documentation for further operation, a full elasto-plastic fracture mechanical evaluation of the conclusive results of the simplified method was performed to demonstrate the conservatism. The results imply that defects in the vicinity of 7-10 mm, inclusive tolerances must be detected, in order to maintain the function of a single bracket. It could be noted that stress analysis showed that there was a suitable degree of robustness in the design, such that several more brackets could be ruptured without compromising the function of the system or endangering the spray geometry, given at least one bracket per finger is intact. The proposed and qualified inspection system contained two inspection methods; Visual Technique (VT) for detection of defects, and Eddy Current (ET) for characterisation and length sizing of any detected defects. The capability for the VT was designed for detecting defects in the vicinity of 5 mm length and with a width of >20 ?m. These sizes were determined from the defect review process. The ET-inspection is qualified for characterising and length sizing of defects from 5 mm in the base material and 10 mm in the weld material, with a tolerance of +/-3 mm and +/-5 mm respectively. With the renewed inspection results from the second outage by hand, it can be concluded that the inspections carried out without qualification and under a tight time schedule during the initial year, was sufficient and that the safety analysis report did not leave out any important defects.
Publication Source: 3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, November 14-16, 2001, Seville Spain. Publisher: Tecnatom s.a. - [Homepage]
