Feeder pipes are integral part of Primary Heat Transport (PHT) system in Pressurized Heavy Water Reactors (PHWRs). The feeder pipes and the headers and pipes connecting to the steam generator are the only components in the PHT of PHWRs that are made from plain carbon steel [1,2]. Flow Accelerated Corrosion (FAC) of feeder piping has been recognized as one of the main concern affecting safety and availability of nuclear power plants. FAC is also a main degradation concern for all the high energy piping/components in the secondary circuit of nuclear reactors. On the secondary circuit components, a few cases of failures due to FAC have been reported worldwide [3-6]. FAC is described as corrosion enhanced by mass transfer, between (a dissolving oxide film on) the base material and a flowing fluid that is unsaturated in the dissolving species. Corro¬sion proceeds rapidly by electrochemical means and the corrosion rate accelerates as the velocity of the fluid increases. Dissolution of protective magnetite oxide layer into flowing stream of water causes wall thinning (metal loss) of carbon steel [3-11]. If the thin-ning remains undetected and the remaining thickness is less than
the critical thickness required to withstand the operating pressure, a ductile fracture occurs [3], Flow rates may change due to change in geometry (elbows/bends, etc.) or due to local disturbance in flow (e.g. flow obstruction by flow measurement devices or a protrusion on the inner surfaces of pipe/welds). Therefore, the local flow rate changes will cause localized changes in thinning rate [12]. The maximum rate of FAC is the key factor in determining the decision to replace the FAC affected pipes. The location of maximum rate of thinning due to FAC is also a key factor so as to allow thinning mea¬surement to be focused on that location. Cracking of some of the outlet feeders has been reported at regions of tensile stresses but the exact mechanism remains unclear with hydrogen from the FAC affected surfaces and materials aging issues expected to be playing a role [13].
Wall thinning of carbon steel feeder pipes due to FAC led to se¬vere thinning in an Indian PHWR (Rajasthan Atomic Power Station unit #2). Based on an exhaustive wall thickness management pro¬gram, an en-mass feeder pipe replacement was carried out in this reactor after 15.67 EFPY of reactor operation. A detailed investiga¬tion has been carried out on seven feeder pipes selected from those removed from the reactor. The objective of this investigation was to examine and understand the nature and extent of thinning caused by FAC in carbon steel feeder pipes of Indian PHWRs. The key findings of this investigation are presented in this paper.