Creep performance of Zr-1Nb-0.75Sn-0.1Fe cladding tubes with optimized Sn content

Abstract

Creep properties of stress relieved Zr-1Nb-0.75Sn-0.1Fe alloy with a lower Sn content were studied. Zr-1Nb-0.75Sn-0.1Fe alloy was found to have stress exponents of 6-7, independent of stress level, unlike Zr-1Nb-1Sn-0.1Fe alloy, in which the transition of stress exponent with increase of stress was observed. The constancy of stress exponent, without the transition observed in Zr-1Nb-0.75Sn-0.1Fe alloy with lower Sn content, is associated with the decrease of Sn atoms. The activation energies for creep defor mation were calculated to be between 210 and 260 kJ/mol for the Zr-1Nb-0.75Sn-0.1Fe alloy with a lower Sn content. The slightly smaller creep activation energy in Zr-1Nb-0.75Sn-0.1Fe, compared with that of Zr-1Nb-1Sn-0.1Fe alloy, is thought to be attributed to the lower Sn content. The creep data obtained at different temperatures and stress fell close to a single line, suggesting the creep life of Zr-1Nb-0.75Sn-0.1Fe alloy with a lower Sn content is well expressed by the Larson-Miller Parameter.

Effect of Heat Treatment on the Creep Properties of Zr-1Nb-0.12O Nuclear Cladding Tubes

“…In order to examine the creep characteristics in the circumferential direction, the Zr alloy rings with the width of 4 mm were cut vertically against the axial direction of tubes. In this study, a special grip with two halfcylinders with 1 mm wide space between them was used to strain the ring specimen with maintaining a circular configuration of the ring in contact with the grips all the way during the tests [1][2][3]5]. Constant stress creep testers were used to investigate the effect of grain size on creep deformation behaviors.…”

“…Zr alloy cladding tubes play important roles not only as the first barrier preventing nuclear fission products in the UO 2 pellet from being released into the reactor coolant, but also as the conductor transferring nuclear fission-induced heat effectively to the coolant. From the viewpoint of the nuclear fuel integrity and the reactor safety [1][2][3], high temperature creep behavior of Zr cladding tubes along with corrosion behavior is one of the key performance parameters to be evaluated since they need to sustain the load arising from the pellet-clad mechanical interaction as well as the load caused by the reactor coolant over pressure acting on the outer surface of the Zr cladding tubes in the corrosive environment [2] It was also found that the higher heat treatment temperature after the final pilgering step lowered the mechanical strength, but enhanced the corrosion and growth resistance [5]. Some cladding tubes are being heat treated in the stress relieved condition after cold pilgering to render the better mechanical strength and creep strength while other tubes are being heat treated in the recrystallized condition to render the better corrosion resistance and growth resistance at the expense of the mechanical strength.…”