Tensile hoop stress-, hydrogen content- and cooling rate-dependent hydride reorientation behaviors of Zr alloy cladding tubes

Abstract 250 ppm hydrogen-charged (250 ppm-H) and 500 ppm-H Zr alloy cladding tubes oxidized to 2.5 μm at their inner and outer surfaces were employed to examine the effect of tensile hoop stress, hydrogen content, cooling rate and oxygen content on the radial hydride precipitation behaviors during cool-down processes. The oxidized cladding tube specimens were heated up to 400 °C and then cooled down to room temperature at cooling rates of 0.3, 2.0 or 8.0 °C/min under tensile hoop stresses of 80, 100 or 150 MPa. The amount of the radial hydrides precipitated and their growth during the cool-down processes were found to be strongly dependent upon the hydrogen content, tensile hoop stress, cooling rate and oxygen content. The lower hydrogen content, the higher tensile hoop stress, the slower cooling rate and the lower oxygen content generated the larger fraction of radial hydrides and the longer radial hydride length. These phenomena may be explained by the solubility difference between room temperature and 400 °C, the effect of tensile hoop stress on the radial hydride nucleation energy, cooling-rate-dependent hydrogen super-saturation and high temperature residence time, and the effect of undissolved circumferential hydrides on the radial hydride precipitation.