Effect of radial hydrides on mechanical properties of Zircaloy-4 cladding

Radial hydrides can cause a noticeable decrease in the circumferential ductility of Zircaloy fuel cladding. One of the factors affecting the orientation of hydride precipitates is the stress, which entices hydrides to precipitate out with their platelets perpendicular to the tensile stress axis during cooling. Under a proper combination of cladding temperature and hoop stress of spent fuel cladding under dry storage, radial hydrides are possible to form by a hydride reorientation process. This paper is intended to study the hydride reorientation behavior and the effects of radial hydrides on the mechanical properties of Zircaloy fuel cladding. Unirradiated, stress-relief annealed Zircaloy-4 cladding segments were uniformly hydrides to different target hydrogen levels between 100 to 600 ppm. Then, the hydrided specimen was thermally cycled in an autoclave under a constant hoop stress. The extent of hydride reorientation was proportional to the number of thermal cycles. The final ratio of radial to circumferential hydrides, however, was influenced to some degree by the initial hydrogen concentration. The effects of radial hydrides on mechanical properties of Zircaloy cladding were evaluated by uniaxial tension tests at room temperature. The results indicate that the effect of radial hydrides on the axial ductility of the cladding tube is insignificant, even if most hydride platelets in the 320-ppmH specimen were reoriented into radial hydrides. (author)