A Study On The Hydride Precipitation Mechanism And Fatigue Cracking In A Zr-Sn-Nb Alloy Cladding Tube

Zirconium alloy cladding tube serves as the first barrier for the reactor.An in-service hydrogen pick-up problem of the fuel cladding inevitably occurs through corrosion reactions with the coolant,and circumferential hydride is formed.The cladding tubes are subjected to thermo-mechanical cycling effect during spent fuel reprocessing.The hydride undergoes reorientation in the form of radial hydride.Zirconium alloy cladding tubes are susceptible to hydrogen-induced degradation,leading to reduced ductility and fracture toughness,especially radial hydrides.In this study,a multiscale method is used to study the precipitation of hydrides in zirconium alloys for cladding tubes and their effects on plastic deformation mechanism and fatigue cracking behavior.In addition,the effects of hydride orientation on the fatigue failure mechanisms of different types of cladding tubes(equiaxed microstructure and fiber-like microstructure cladding tubes)were systematically investigated.The deformation mechanisms of cladding tube were determined under different stress states(axial tension,axial compression and hoop tension)based on the analysis of the Schmid factors and slip traces.Prismatic slip,pyramidal slip and {10-11}compression twin accommodated the deformation under axial tension.Prismatic slip and {10-12} tension twinning were activated under axial compression.Prismatic and pyramidal slips worked together to coordinate deformation.Microstructure and crystallographic analysis of circumferential and reoriented radial hydrides were performed by optical microscopy,scanning electron microscopy,electron back scatter diffraction and transmission electron microscopy.Both crystallographic nature and local mismatch coordination mechanism of the reoriented radial hydrides in the material remained the same as that of the circumferential hydrides.The hoop stress only changed the macroscopic hydride growth direction.The hydride precipitation characteristics and fatigue cracking behavior of cladding tubes with different microstructures were studied.The morphology of hydride precipitation was determined by the texture characteristics and grain shape of the cladding tube.When longitudinal direction of hydrides was parallel to the direction of the cyclic stress,the hydride firstly undergone mode I fracture,and then occurred mode II crack growth.The local crack propagation length can reach 83 ?m.When longitudinal direction of hydrides was perpendicular to the direction of the cyclic stress,the hydride undergone mode I fracture and crack propagation.The axial and hoop interrupted fatigue tests of the cladding tube showed that microcracks fist originated from the transgranular hydride or intergranular hydride.Fatigue cracks nucleated and propagated along slip bands in the Zr matrix at late stages of fatigue close to the failure were an important factor leading to fatigue failure of hydrided cladding tubes at the low hydrogen content(200-300 ppm).The effects of hydride orientation on the hoop fatigue failure mechanism of the cladding tube were systematically studied.The cracks propagated along the radial hydride-matrix interface.The hydride with radial orientation provided a favorable path for hydride fatigue crack growth,resulting in reduced fatigue life where the hydride is reduced by about 41%for the completely reoriented sample.The grain boundary stress caused by the difference in crystal orientation of the matrix adjacent grains around the intergranular hydride was considered.The critical fracture conditions(?t=?a+?i+?e+?GB)of the hydride with different orientations were analyzed.The crack propagation behavior in hydrides was evaluated based on the energy release rates of different materials and interfaces(KIC Interface/KIC Matrix(?)G?/G?).