Feeder Piping Nde – Current Capabilities and Future Direction

The primary heat transport system of a typical CANDU®-6 nuclear power reactor contains 760 feeder pipes. These feeders carry the coolant between the inlet or outlet headers and the individual fuel channels. Inspection requirements that developed in feeders in the 1990’s led to rapid development of NDE technology to evaluate feeder integrity. The NDE technology had to deal with the complex and variable geometry of feeders and access constraints inherent in feeder inspections. The first feeder inspection requirement was ability to detect Flow-Accelerated Corrosion (FAC). This led to an evolution in wall thickness measurement techniques for feeders that is continuing. The leading edge technique for wall thickness measurement is the METAR Crawler that scans feeder bends with 14 wall thickness probes. This system uses motors to push the bracelet along the feeder bend. Second, cracking was detected in a feeder in 1997. This led to development of a specialized, manual inspection technique to deal with access problems. This technique has been extended to use a Hydro Quebec developed drive system to move the probes over a raster scan. The presentation will explain feeder degradation modes and the NDE developed to evaluate feeder integrity. Introduction: The primary heat transport system of a CANDU-6 nuclear power reactor contains 760 feeder pipes. Feeders are made of A106B steel. These feeders carry the coolant, heavy water, between the inlet or outlet headers and the individual channels containing the nuclear fuel. Therefore, the feeder piping network is part of the pressure boundary and any leak is a major concern for the plant operation. The feeder system is divided into two sections: the lower section connects to the fuel channel and pipe size ranges from 1.5 in up to 2.5 in, and the upper section that connects to the headers with pipe size between 2.0 and 4.0 in. The junction between the two sections is made by a field weld all other welds were done on the shop floor and the crown and root caps were grounded flush wherever possible. Two aging problems were discovered in the mid 90`s: thinning and cracking. Results: Feeder Thinning. Concerns about the feeders were prompted by the discovery of considerable amounts of magnetite precipitated in the cold leg of steam generators. Some corrosion was expected in the design of the Primary Heat Transport System (PHTS) but the amount of material removed from the steam generators surpassed predictions and excessive feeder thinning became a concern. Considerable feeder thinning was observed for the first time at the Point Lepreau reactor in 1995. The degradation mechanism identified is Flow Assisted Corrosion (FAC). Excessive thinning occurs on the inside of the feeder pipes, especially on the outlet elbows close to the exit of the pressure tube. This prompted generating stations to implement an inspection program to assess the extent of the problem. Feeder thinning measurement began using an ultrasonic thickness gauge. Measurement showed that the thinnest spot is located on the extrados of the bends. For repeatability of measurement over the years, templates with a grid spacing of 19 mm were deployed at some plants. Using templates greatly improves positioning accuracy and results in better thinning rate trending but it is a slow inspection process and can be applied only to a small number of feeders for monitoring purpose. The first development to improve inspection speed and coverage was achieved by Ontario-Hydro`s SIMD. Engineers developed a four-transducer probe array contoured to the outside surface of the feeder. ® CANDU – CANada Deuterium Uranium is a registered trademark of Atomic Energy of Canada Limited (AECL)