Manufacturing experiences of large stroke welded disc bellows for nuclear applications

Prototype Fast Breeder Reactor (PFBR) is a 500Mwe, Mixed Oxide (MOX) fueled and sodium cooled fast breeder reactor, which is in advanced stage of its commissioning. PFBR employs inclined fuel transfer machine (IFTM) for ex-vessel transfer during reactor fuel handling. Nested ripple type welded disc bellows are used as primary leak tight barrier in the shield plug of IFTM. The welded disc bellows are developed and manufactured indigenously. The material of construction of these bellows is AM 350, which is precipitation hardened stainless steel. The stroke to length ratio of the bellows is 0.75. Manufacturing of the welded disc bellows (in AM 350) with such a high stroke to length ratio is a first of its kind work in India. The material of the bellows needs to be heat treated to get intended mechanical properties. During initial trails several challenges are overcome with respect to manufacturing, heat treatment and quality assurance of the bellows. The bellows consist of 400 numbers of weld joints. Premature failures of the bellows at the welds joining two discs are observed during initial trails which are due to the condition of the raw material. The problem is solved by solution annealing of the raw material and ensuring the material is in its ‘H’ grade. Parameters such as welding current, weld size and the profile of the discs are arrived by series of mockups. Procedure and fixtures for carrying out helium leak testing during various stages of the manufacturing are developed. Microscopic examination of the weld joint is carried out to check the width and depth of penetration of weld. Heat treatment of the long and flexible bellows is challenging and during initial trails, permanent deformation of the bellows is observed. The problem is solved by designing proper fixtures for heat treatment. Cycle life testing of the bellows is carried out and the bellows are qualified for reactor application. This paper details the technical challenges faced during the indigenous development and the solution employed to overcome the problems.