Characterization of Zr - 2.5%Nb - 0.5%Cu Garter Spring With Rectangular Wire for 700 MWE PHWRS

Abstract

Garter spring assembly made of rectangular wire with modified corrugated girdle wire design is developed for the upcoming 700MWe Pressurized Heavy Water Reactors (PHWRs). The garter spring assemblies are used to maintain gap between Pressure tube to Calandria tube in the coolant channel. Girdle wire positioned centrally through the spring and resistance welded to facilitate detection of the spring during in-service inspection using eddy current test. The new design of corrugated girdle wire is adopted in the 700 MWe PHWRs to accommodate the expansion of pressure tube during service due to diametral creep. Zr-2.5 Nb-0.5 Cu is used as material of construction for garter spring and girdle wire is made out of Zircaloy-4. The process of fabrication for garter spring assembly involves combination of hot extrusion and cold swaging followed by drawing with intermediate annealing operations. Finally the hook formed springs are solutionzed, oil quenched followed by age hardened to achieve the desired properties. The girdle wire processed through a combination of hot extrusion and cold swaging followed by drawing with intermediate annealing operations. Finally the corrugation operation is carried out on plain girdle wire and resistance welded to form finished assembly. In this paper, qualification approach of garter spring assembly employing combination of destructive test such tensile test, hardness, metallography, corrosion test & chemical analysis and non-destructive tests such as ultrasonic examination, liquid penetrant examination & visual examination was brought out. Customized automated test was developed for measuring spring characteristics such as deflection measurement and measurement of residual extension. Characterization of garter spring material w.r.t texture, strength, metallograhy at different stages of fabrication starting from raw material to finished hook formed spring was also brought out. At the same time it was aimed to understand the hydriding behavior and orientation of hydrides at different stages such as billet, rod, rectangular wire, hook and finished spring material. The understand the effect of hydrides on mechanical properties such as strength, elongation and hardness was also evaluated in the spring material as well as on girdle wire. A comparative study w.r.t. metallurgical & mechanical properties was made between as-fabricated & hydrided condition.