Manufacture of stainless steel hexagonal channels for FBRs

Full text: Stainless Steel Hexagonal Channels are one of the most critical core components of Prototype Fast Breeder Reactor (PFBR). The hexagonal channels have to withstand extremely hostile conditions of high fluence of fast neutrons and higher operating temperatures compared to thermal reactors. Consequently their fabrication and quality control methodology are characterized by close monitoring of processes and stringent quality checks. Hitherto, hexagonal channels for Fast Breeder Test Reactor (FBTR) were fabricated by rolling of sheets, forming into two similar halves and finally joining them by TIG welding. However, this manufacturing route involves certain inherent limitations like non-uniform microstructure and inferior mechanical properties and corrosion resistance at the weld and HAZ. Also the weld requires 100% radiography. Some channels were also produced by drawing through suitably designed dies. A unique process has been adopted at NFC for the fabrication of tubes with hexagonal cross-section through cold pilgering. Suitable pass schedules were designed for production of hexagonal tubes to meet the stringent dimensional specifications. Stainless Steel tubes in D9 grade were chosen for the hexagonal wrapper tubes in view of its high resistance to void swelling and excellent creep property. The initial developmental trials were carried out using blanks of SS 304L, having similar mechanical properties as D9. After successful establishment of the route, further trials were carried out on D9 blanks. The Hexcans so manufactured were subjected to a variety of tests such as metallography, room and high temperature tensile testing and IGC testing for characterizing the material and process flow sheet. This paper briefly describes the various problems faced during trials for manufacturing of seamless pilgered hexcans and remedial measures taken to overcome them