Role of Weld root Geometry - Backing rings in Flow Accelerated Corrosion (FAC): Radiography testing is an alternate NDT method for detection - An experience at Tarapur Atomic Power Station-12; Nuclear Power Corporation of India Limited, India

摘要

Tarapur Atomic Power Station (TAPS) is one of the vintage nuclear reactors operating in the world and belongs to earlier generation of Boiling Water Reactors (BWRs) has completed 40 years of successful, commercial and safe operation. TAPS was designed to generate 210MWe out of which 70% was from reactor primary steam and the remaining from Secondary Steam Generators (SSGs). These SSGs were isolated in the year 1984 and units are operating at re-rated capacity of 160Mwe since then. Of late it has been identified that Flow Accelerated Corrosion (FAC) is one of the degradation mechanisms with High Energy system piping components. The sudden failure of high energy (Enthalpy) piping components lead to loss of human lives as well as property loss in addition to the unplanned down times. The power industry is now reviewing their effectiveness of detection and monitoring programmes to identify & mitigate the consequences. The recent pipe rupture incidents in overseas NPPs in secondary cycle piping have alarmed Indian NPPs. This issue was reviewed in detail at TAPS-1&2 and corrective & measures have been formulated for timely detection of degradations. During these four decades TAPS has engineered many inspection methodologies to detect various degradation mechanisms such as IGSCC (Inter Granular Stress Corrosion Cracking), TGSCC (Trans-Granular Stress Corrosion Cracking), Erosion-Corrosion (EC), Stress Corrosion Cracking (SCC) and Flow Accelerated Corrosion (FAC). These degradation mechanisms were timely detected using various inspection methodologies developed time to time. The expertise gained from the Operation & Maintenance (O&M) of reactors for the past four decades has strengthened the capabilities of station inspection personnel to develop various inspection methodologies & their application in the filed. One such inspection methodology was developed by the station QA group which has potential to detect the degradation of high energy system piping components vulnerable to FAC, especially the weld root inaccuracies. This inspection technique is established at site NDT facility with in-house expertise and effectively utilized to detect the piping components degradation due to FAC. This paper gives a brief insight into the various inspection methodologies followed at TAPS for timely detection of degradation mechanisms avoiding catastrophic failure of High Energy system piping components. TAPS has effectively utilized these inspection methodologies and implemented to assess such component's integrity have also been briefly discussed.