An azimuthal contact and free-centerline boundary condition for cracked fuel pellets in 2D axisymmetric models

A fuel pellet deformation model comprised of plane-stress, azimuthal contact, and free-centerline displacement boundary condition is proposed to improve the prediction of Pellet-Cladding Mechanical Interaction (PCMI) in two-dimensional (2D) axisymmetric fuel codes at low power. This modification is motivated by case 3 of the EGRFP-PCMI modelling benchmark, in which none of the participants consistently correctly predicted the linear power at which pellet-cladding contact starts, nor the relationship of this threshold power with pellet length. In the benchmark, the 2D fuel codes with discrete fuel pellets did not significantly outperform the one-dimensional codes in predicting PCMI and the effects of pellet length. In the present work, we analyze the predictions of the Canadian Fuel And Sheath modelling Tool (CFAST) in the benchmark study and argue that the zero radial displacement solid-mechanics boundary condition commonly used in 2D axisymmetric models limits pellet hourglassing and is inconsistent with the expected pellet cracking. This hypothesis is confirmed by a three-dimensional (3D) thermomechanical model with cracked fuel pellets compared against experimental measurements of clad elongation. Based on these findings, the modified 2D axisymmetric model is proposed and compared against the 3D model and the experimental measurements of cladding elongation at low power.