Numerical Method Study for Zircaloy Clad Ballooning Behavior Simulation Considering Anisotropic Creep

As the discharge burnup of nuclear fuel is increasing in light-water reactor (LWR) nuclear power plant for better economic performance, safety-related issues of high-burnup fuel is drawing much attention in the nuclear energy community. It has been widely reported that during a loss-of-coolant accident (LOCA), highly irradiated fuel pellet is prone to fragmentation, relocation and dispersion. This phenomenon brings about the question of whether current safety criteria is still applicable or how the safety margin would be affected by the high-burnup fuel, since current safety criteria was established several decades ago based on low-burnup irradiation experiment database. During a LOCA, Zr clad tube exhibits the mechanical damage behavior of ballooning under the condition of high temperature and pressure difference across the tube thickness, which would facilitate fuel relocation and generate intense heating to the clad by the high-temperature fuel fragments in the ballooned region. This paper aims to develop a robust and efficient algorithm in the framework of finite element method (FEM) to study the Zr clad tube ballooning in a high-fidelity manner. High-temperature creep has been recognized as one of the key influencing factors for the ballooning. For the purpose of simplifying calculation, the creep of Zr clad tube was often treated as isotropic in the literature, i.e.KG-*3, based on the von Mises criterion, which could lead in nonnegligible error in the prediction. In view of this, an algorithm for stress update and consistent tangential stiffness computation was derived based on Hill’s criterion, considering the fact that the creep of Zr clad in the α-phase region is anisotropic. Together with temperature- and irradiation-dependent thermo-mechanical properties, the α-β phase transition model as well as empirical burst limit models, an analysis tool for the ballooning simulation of Zr clad was implemented within the finite element software ABAQUS. The analysis tool was verified by the analytical solution of the thick-walled cylinder tube creep, and validated by comparing with PUZRY series ballooning and burs test. The results show that anisotropic creep algorithm is obviously more suitable to predicting α-phase Zr clad ballooning deformation. Besides, there is a need of developing more mechanistic creep model for the α+β mixed phase, because the simple method of weighted average of creep rate according to the β phase volume fraction leads to large discrepancy between the prediction and experiment data. It is also found that, for the condition of large pressurization rate of the clad tube, creep alone is not enough to explain the clad ballooning.