Simulation of Fret for Life Assessment of Feeder Pipes of PHWR

Abstract

Abstract In case of KAPS and KGS, different ingots were used for manufacturing the coolant channels. Due to difference in the creep related material properties of different ingots, the coolant channels in these reactors experienced different creep rate. Due to the differential creep rates, especially in the neighboring channels, the coolant channels came into contact with each other. This is a serious deviation from as installed conditions wherein the gaps and spacing between closely assembled coolant channel components are decided based on accurate assessment of known loads and forces. Differential creep results in closing of gaps between neighboring feeder pipes or between feeder pipe and gray-loc clamp. Depending on the orientation of the clamp and direction of the feeder pipe at that location, worst contact could be between the first elbow of feeder pipe and the sharp edge of the crown nut on clamp. Several such contacts have been detected in KAPS and KGS. Seriousness of the contact emerges from the fact that the coolant channel and the feeder pipes vibrate due to coolant flow with high velocity. With time the contact conditions like the contact force and location(s) of contact change and this has significant influence on the life and performance of the coolant channel as a whole. Since feeder pipes are constantly vibrating, the contacting locations are likely to be dented or damaged. Whenever and wherever possible, NPCIL has tried to remove the contact locally. Since the process of creep is ongoing, more and more feeder pipes are likely to come in contact and those in which the local corrections for removing the contact are made could also need correction as the time progresses. In order to assess the severity of damage, a full scale setup was erected. Contact was established between feeder pipe and the hard locking nut of gray-loc clamp. Based on previous vibration data collected from feeder pipes and end fittings, and the ASME‘s O&M guidelines on acceptable level of piping vibration in nuclear piping, vibration was induced in the contacting feeder pipes. The test was continued for more than 10 07 cycles. On the basis of damage seen on the feeder pipe due to contact, R6 method has been employed to determine the critical crack size in the feeder pipe with a postulated axial part-through semi-elliptical crack. The failure assessment diagram was constructed in the K-L plane. The lowest critical crack depth was calculated as 4.67 mm as against observed depth of dent of 1.1 mm. The paper deals with the critical assessment of contact between neighboring coolant channels caused by differential creep and assessment of safety margin available before corrective action need to be initiated.