Welding and Fabrication of Stainless Steel Equipment for 500MWe Prototype Fast Breeder Reactor

Abstract

Austenitic stainless steel is the major structural material for the primary and secondary sodium systems (except for the steam generators) for the currently operating and planned fast reactors all over the world. The boundaries of sodium systems of Prototype Fast Breeder Reactor (PFBR) is designed so as to have an extremely low probability of leakage, rapidly propagating failure and gross rupture under the static & dynamic loads expected during various operating conditions.The degradation of material properties (e.g. effect of sodium, temperature and irradiation), transients, residual stresses, flaw size etc. are the important considerations, which shall be taken into account. The principal material of construction for PFBR is austenitic stainless steel of grade 316LN/304LN. The scope of welding and fabrication of PFBR components is too large due to versatile types of systems with varieties of components with complex constructional features. High operating temperature of various systems causing high stresses are to be minimized by designing thin walled structure. Most of the Nuclear Steam Supply System (NSSS) components are thin walled and require manufacturing in separate nuclear clean hall conditions to assure the quality.The welding with stringent tolerances along with high distortion in stainless steels due to high thermal expansion and low thermal conductivity makes the fabrication extremely challenging.The welding standards and acceptance criteria of PFBR equipment is stringent compared to other industrial specification. Manufacture of over dimensional components (diameter greater than 12m and thickness upto 40mm) such as MainVessel, Safety Vessel, Inner Vessel involves die pressing of large size dished end & conical petals. The solution annealing of cold worked petals is a mandatory requirement if strain exceeds 10%. Innovative welding techniques and many trials were conducted on mock up for establishing the process parameters. The forming techniques, bending methods and welding procedures were qualified with stringent non-destructive and destructive examinations and testing before taking up the actual job. Thermal Baffle has two large concentric cylindrical shells, inner and outer shells of about 12.4m diameter and fabrication is a challenging task. PFBR also involves dissimilar joint welding between carbon steel (A48P2) and austenitic stainless steel (316LN) at integration location of roof slab & main vessel. This welding is carried out by combination of Gas Tungsten Arc Welding (GTAW) & Shielded Metal Arc Welding (SMAW) processes using ER 309L & E 309-16 welding consumables with controlled heat input to minimize the dilution of carbon & distortion. The weld between primary pipe & grid plate cannot be accessed for in-service inspection and therefore requires extra-ordinary skilled welders. Space constraints & lack of accessibility makes the welding & inspection challenging. This paper highlights the welding and fabrication aspects of few major, over dimensional and critical equipment of 500MWe Prototype Fast Breeder Reactor. Keywords: Stainless Steel, Main Vessel, Safety Vessel, Inner Vessel, Grid Plate, PFBR, SS welding, distortion.