Optimising Welding and Assembling Processing for Manufacturing PHWR Fuel Element Bundle

Abstract

In PHWR fuel fabrication, end-cap joint formed by Zircaloy fuel tube and cap is one of the most critical welds as it is expected to offer a hermetically sealed joint to contain the radioactive fission products. In view of their highly demanding function during reactor operation, these welds have to be produced to a high degree of reliability by careful selection of process and parameters. PHWR fuel bundle is manufactured by joining end plates to elements at both ends. Resistance projection welding technique is used to weld the element ends to end plates. This being the final operation in PHWR fuel fabrication route, it plays very important role with respect to bundle dimensions and integrity. Jigs and Fixtures are used to assemble fuel elements and end plates. The quality of these fixtures affect the bundle dimensions, inter element spacing and orientation of fuel elements/endplates. While welding Zircaloy material, properties like coefficient of thermal expansion, thermal conductivity and thin oxide layers have to be considered. Generally high conductive material requires pre-heating before welding, while posttreatment of the weld is carried out if the metallurgical properties are changing in the Heat Affected Zone (HAZ). In resistance welding, selecting a suitable weld cycle pattern involves optimization of current, time, number of on/off cycles and current slope. Different current cycle patterns offer distinct advantages and certain disadvantages too with respect to weld bonding, sparking, HAZ etc. State-of-the-art technology is being used to have better control on weld parameters and monitor them as well for further analysis. The paper discusses the effect of welding parameters including different weld cycle patterns like on/off cycle, up-slope cycle and constant current cycle. Improvements carried out to ensure dimensional integrity of the bundle are also dealt with in the paper.