Qualification of Critical Weld Joints for Dynamically Balanced Nuclear Component

Abstract

Centrifugal extractors (CE) are widely used in reprocessing of spent fuel of fast breeder reactors due to various advantages. The extraction is carried out from a solution of 4-6N nitric acid, organic compounds, and elements containing fission products. Centrifugal forces generated from the rotating bowl (RB) are utilized for this purpose. RB is a thin hollow shell with complex internals. It rotates along a longitudinal axis inside a stationary bowl by maintaining a minimal specified annular gap ranging 3-5 mm during operation. A high-grade dynamic balancing is essential for each fabricated RB to keep generated vibrations within limits of safe operations. The Cylindricity of RB is controlled up to 0.1 mm and a surface finish of N8 is maintained for many reasons including the requirement of dynamic balancing. Despite maintaining all parameters, it is sometimes observed that a considerable quantity of fabricated RB gets rejected due to the non-feasibility of possible corrections during dynamic balancing of fabricated RB. The post-weld examination of internal surfaces is difficult and any type of post-weld repair of fabricated RB remains unproductive. A welding procedure has been adopted during manufacturing of AISI 304L material, 40 mm ID x 1.5 mm (thk) RB with specified dynamicbalancing requirement of grade 2.5 as per ISO1940-1 (2003) at 3500 RPM and corrosion rate of a maximum of 15 mills per year as per practice C of ASTM A 262. The welding procedure qualification has been carried out using gas tungsten arc welding (GTAW). The welding variables such as groove design, heat input, welding position, welding sequence, fixtures, clean fit-up, etc are found as important to monitor apart from other commonly used to meet the acceptance criteria as per ASME sec IX.