PWR steam generator tube denting at top of tubesheet

Steam generator (SG) tube denting at the top-of-tubesheet (TTS) has been experienced in many PWRs. The onset of TTS denting was prevalent from 1982 to 1993, and a total of 50 plants reported having experienced TTS denting as of the end of 1993. In recent years, PWR SG tube denting has been observed in the vicinity of the top of the tubesheet (TS) at several units, some with replacement SGs. The configuration of the tube-tubesheet interface at the TTS is similar to the original drilled hole tube support plate (TSP) design with a small annular crevice between the tube and the corrosion-susceptible tubesheet material. Subsequently, there is significant interest in identifying the root causes of TTS denting and the preventative measures that may be effective in mitigating its spread through the tube population. Note that the advanced tube materials used in replacement SGs (i.e., Alloys 690 and 800) have shown stress corrosion cracking (SCC) susceptibility under significant deformation in the laboratory. In the case of Alloy 800, some plants have experienced outside-diameter stress corrosion cracking (ODSCC) at dented TTS locations. A recent EPRI study investigated the TTS denting experiences in order to provide a fundamental understanding of some of the causal factors of TTS denting, such as the corrosion of the tubesheet material, the effects of TS sludge on the chemical and physical conditions in the TTS crevices, and the residual tube stresses that result from TTS dents. Predictive models identifying the impact of TTS denting on SG maintenance and operating lifetime as well as cost-benefit analyses of proposed mitigation strategies were developed. This paper will discuss the potential use of secondary chemistry metrics to assess the risk of the occurrence of TTS denting. (author)