A fracture mechanics-based model for assessing the mechanical failure of nuclear fuel rods due to rock fall

One of the potential failure mechanisms of nuclear fuel rods that will be disposed in the proposed Yucca Mountain repository is mechanical failure of the degraded cladding tubes caused by rock fall impacting on waste packages. In this paper, the development of a fracture mechanics-based model for treating rock fall-induced failure of nuclear fuel cladding tubes is described. The rock fall model has been developed by treating the forces acting on the fuel-rod cladding due to an impacting rock in terms of either a point or distributed load. The stress intensity factors of cracks in the cladding rods assembled in a 17x 17 array are analyzed by performing appropriate elastic bending analyses. The critical stress at the onset of cladding failure is predicted on the basis of a fracture criterion that is applicable for both large and small cracks. The proposed model is used toassess the critical size and weight of rock fall required to cause cladding failure as a function of the impact location, loading condition, and the geometry of rock fall, as well as the initial defect size, hydrogen content, and fracture toughness of the cladding at the time of rock fall. The result indicates that the critical rock radii for cladding failure are well within the range of potential rock falls estimated based on the joint spacings of rocks at the repository site. These theoretical predictions remain to be verified by laboratory and field data.