Application of Micro Focal X-ray Technique for Radiography of Tube to Tubesheet Welds of Fbr Steam Generator

Fast Breeder Reactor (FBR) steam generator assembly, is a shell and tube heat exchanger. In this equipment, steam is generated by the transfer of heat from sodium on shell side to water on tube side. Due to inherent dangers of sodiumwater reaction, the integrity and leak tightness of weld joints separating sodium and water/steam is very important. The tube to tube sheet (TTS) weldsin steam generators separate the sodium and water,aresuch critical weld joints. To ensure integrity of the TTS joint, suitable recordable NDT technique was required to be established. Due to the stringent acceptance criteria, intricate geometry and limited access; performing NDT withconventional techniques was not feasible. These limitations were overcome using rod-anode Micro FocalX-ray Radiography technique. The micro focal X-ray unit, with smaller focal spot size in the range of 15 to 50 μm, enables great detail to be recorded in the projection radiographs resulting through the object magnification and resolution. Sensitivity up to the order of 32 microns is achieved. This paper describes the basic principle of Micro Focal X-ray and its application on TTS joints of FBR. Nearly 25000 joints of TTS welds have been successfully radiographed using Micro Focal rodanode X-ray technique. the sodium and water, are such critical weld joints.To ensure integrity of the TTS joint, suitable recordable NDT technique was required to be established.The acceptance criteria for various flaws in the TTS welds are shown in table below Major material of construction of steam generators is 9Cr 1Mo modified. Tubes are 2.3mm thick and tube sheets are 180 mm thick with integral machined spigots. Detail of joint configuration is shown in Figure 1 below. Welding of these TTS weld joints is being done by internal bore welding without filler wire using GTAW process. The possible defects in this joint are cracks, lack of fusion, incomplete penetration, tungsten inclusion, undercut, porosities, and deviation in weld profile viz concavities and convexities. These intricate TTS weld joints are required to be examined by recordable non-destructive examination to ensure that it is free from weld defects. TheTTS joints being thin and the only joints in the steam generator separating water and sodium, the integrity of the weld joints required is of the highest order.Therefore detection of micro defects to the order of 50 microns and detectable wire Image Quality Indicator (IQI) wire diameter of 32 microns was required. With artificial low energy source (Thulium170)the sensitivity requirements achieved were not sufficient to establish integrity of these TTS joints, hence it was necessary to apply micro focal rod anode X-ray technique.Use of Micro Focal Radiography enhances radiographic sensitivity.Coupled with projection radiography it facilitates accurate measurement of flaw size due to magnification. It utilizes rod-anode and focal spot of 15 to 50 microns.