Hot Bending of Large Size Seamless Primary Pipe for Sodium Cooled Fast Breeder Reactor

Abstract

In a pool-type fast neutron reactor, the primary pipe supplies cold sodium from pump to grid plate. The primary pipe of Prototype Fast Breeder Reactor (PFBR) was made from pressing the plates by die and punch and joining the two halves by welding in longitudinal direction. Though weld joints are extensively used and acceptable, it is desirable to reduce the number of weld joints, which increases the reliability of the component. In this context, it is proposed to use seamless pipe for primary pipes of future FBRs. However, the large-sized seamless bends needed for meeting this objective are not readily available and hence, a technology development exercise was taken-up to bend a large size seamless austenitic stainless steel pipe (Outer diameter 457 mm and nominal thickness 14.27 mm) to a bend radius of 2 times its diameter for a bend angle of 110°. The objective of the development was to demonstrate the manufacturing feasibility of seamless pipe bend, with controlled dimensions, before its adoption in the design. This paper discusses the details of development activities carried out in various stages towards bending of a large size seamless pipe. The hot bending attempts through die and punch method involving large number of trials as well as through induction bending technique are detailed out. After many unsuccessful trials of hot bending on carbon steel pipe with various die and punch options (viz. end support die, full-length support die) and several challenges with induction bending, primary pipe bending has been successfully demonstrated, meeting the specification requirements. The paper also highlights various qualification tests, which the primary pipe bend has met. The successful completion of hot bending of large diameter seamless pipe has demonstrated the indigenous manufacturing capability of such bends and has given confidence for its incorporation in the primary pipes of future FBR.