Pellet cladding mechanical interaction modeling using the extended finite element method

Fracturing of ceramic light water reactor (LWR) fuel has multiple important effects on fuel performance. One particularly important concern is that cracks in the fuel cause elevated stresses in the cladding when pellet cladding mechanical interaction (PCMI) occurs. Modeling the effects of these cracks on the cladding stress is important for avoiding conditions when these elevated stresses could cause cladding failure. This can be readily done in fuel performance codes based on the finite element method by creating finite element meshes that incorporate discrete cracks defined a priori. The drawback of this approach, however, is that the crack geometry must be pre-determined rather than computed by the computational model. The extended finite element method (XFEM) is a powerful method to represent arbitrary propagating discrete cracks in finite element models. The use of XFEM has been previously demonstrated for modeling propagating discrete cracks in the BISON fuel performance code. This paper demonstrates an initial application of XFEM to model stress concentrations induced by fuel fractures at the fuel/cladding interface during PCMI. This is demonstrated on a study of a pre-defined stationary crack in a 2D cross-section model of a LWR fuel rod. The results from a model with a discrete crack defined with XFEM compare favorably with the results from a model with the same crack geometry defined in the finite element mesh. This study focuses on benchmarking the use of XFEM for PCMI modeling with a stationary crack, but this technique will be readily extended in the future to consider PCMI in conjunction with arbitrary, physics-driven crack propagation.