Study On Shear Damage Constitutive Model And Mixed Mode Fracture Of Simulated Spent Fuel Assemblies

Spent fuel assembly(SFA)is the name given to nuclear fuel that has been irradiated and removed from the nuclear reactor.So far,there are mainly two ways to manage SFA,namely,the once-through cycle and closed fuel cycle.Compared with the former one,the closed fuel cycle involves the separation out of the recyclable nuclear elements,which creates high economic,social and environmental values.During the closed fuel cycle process,SFA should be cut into segments with dozens of millimeters in length to accomplish the subsequent chemical extraction process.However,both ductile and brittle materials are included in SFA,resulting in the complexity of the fracture mechanism for SFA during the shearing process.To the best knowledge of the author,in-depth researches on the shearing mechanism of SFA is extremely scarce,and the general numerical models fail to simulate the crack propagation and damage evolution of SFA accurately.Therefore,this paper takes the simulated spent fuel assembly(SSFA,Its material properties are similar to those of SFA but it is non-radioactive)as the research object.The shearing process of the cladding tube,pellet,core tube,and SSFA has been simulated successively by applying damage constitutive mechanics theory.Through the comparison of experiments and simulation results,the fracture mechanism of the above-mentioned components in the shearing process is revealed.The main contributions are concluded as follow:(1)The mechanical response of cladding tube during the shearing process is investigated.Based on the existing GTN modified model,two kinds of ductile fracture criterion are proposed: structural correction form and stress triaxiality correction form.By applying these two rules,the defect that the simulation results of the original model would early or delayed fracture is improved.The FEM model of shearing cladding tube is established and the values of parameters in the above-mentioned model are determined by the sensitivity analysis of parameters and finite element fitting method.Subsequently,experiments are conducted.The deformation,crack evolution,and fracture morphology of cladding tube during the shearing process are revealed through comparative experiments and simulation results,proving the effectiveness and accuracy of the proposed model.(2)The mechanical behavior of brittle materials under the uniaxial stress state is studied.Based on the existing model,a damage constitutive model for brittle materials that expresses the whole loading process(elastic/plastic characteristics,rigidity degeneration with damage,strength softening,and fracture)is proposed.The relationship between the damage parameters,fracture mechanism,and strain rate effect is established.Compared with the original model,the strain rate effect is indicated in detail,and the uncertainty of the critical value for the material is eliminated.The shearing process of the pellet is simulated,and experiments are conducted.Both simulation and experiment results prove that the proposed model is able to accurately predict the shearing process and fracture mechanism of the pellet.(3)Core tube shearing tests are conducted,and the fracture mechanism of the outer cladding tube and inner pellet is investigated,proving the fracture of the pellet has a slight influence on the shearing process of core tube.Therefore,an equivalent shearing scheme that ignores the influence of pellet is proposed.The workload of the proposed scheme is reduced by half and the computing efficiency is improved by about 2～4 times at the expense of only about 10% of the prediction accuracy.The FEM models for shearing of core tube and equivalent shearing scheme are established.By comparing the respective results,the obtained load-displacement curve and fracture morphology match the experiment results,proving the effectiveness of the model proposed in previous sections.Meanwhile,the fracture mechanism of core tube shearing is clarified.(4)The test platform for SSFA is built on an existing horizontal shearing machine to perform the shearing experiments.By combining the models and key technology obtained in previous sections,the whole shearing process of SSFA is simulated,and the experimental results match well with the simulations: the simulated load-displacement curves are closed to the experimental curve,and the error of the maximum shearing force is 1.8%.Moreover,the shearing process is divided into five stages according to the sequence of fracture for different layers: I-Start shearing,II-Fracture phase for the first shearing zone,III-Fracture phase for the second shearing zone,IV-Fracture phase for the third shearing zone,and V-Final fracture.Compared with the holder and shear tool,the fixed tool is the weak point of the device.The maximum stress of the fixed tool in the earlier phase of shearing process is located at the semi-circular opening area that corresponds to the core tube,and it appears at the semi-circular opening area that corresponds to the crack tip of the wrapper tube in the later phase,which changes with the displacement of the shear tool.(5)To reduce the requirement for professional knowledge of users,a numerical simulation system of shearing SFA that contains the key technologies proposed above is developed.