Application of Medium Frequency Weld Controller for Resistance Welding Process in PHWR Fuel Manfacturing

Abstract

Nuclear Fuel Complex (NFC) has been continuously engaged, for the past three decades, in the manufacture and supply of 19-element natural uranium dioxide fuel bundles to all the Pressurized Heavy Water Reactors (PHWRs) in India. Starting with classical wire-wrap-design fuel bundles in early seventies, changeover to production of split-wart design fuel bundles was made in mid eighties. Several innovative production processes and quality control systems were developed and employed successfully in the production of more than 230,000 number of fuel bundles of split-wart type. The unique feature of these bundles is the attachment of spacers and bearing pads to the fuel elements by employing simple, cost-effective and environment-friendly resistance welding techniques in place of conventional Beryllium brazing process. Direct-energy and stored-energy power sources are employed for accomplishing bearing pad and spacer pad welds. The process has been well standardized with respect to selection of electrode material, weld current, weld time, pulse width etc. to obtain good quality welds consistently. In order to further improve the quality of appendage welds, especially to minimize metal expulsion defect and spread of weld strength values, experiments were carried out by employing medium frequency weld controllers. Remarkable improvements in reducing the heataffected zone, scatter in weld strength values and spark tendency were observed in the experiments carried out with medium frequency controllers. The paper highlights the comparative behavior of appendage welds carried out with direct-energy, stored-energy and medium frequency power sources. The factors governing the superior quality appendage welds that can be obtained with medium frequency weld controllers are also discussed in the paper.