Analysis of crack patterns in fast reactor fuel pellets

Two dimensional r-theta finite element analysis is used to calcu the thermal stress in complete fast reactor fuel pellets, completely cracked pellet segments and partially cracked pellet segments. The residual stress levels are used to assess whether further cracking is probable, and linear-elastic fracture mechanics are used to study crack propagation. The results of the analyses show that propagation of cracks with a circumferential component becomes increasingly probable as the pellet cracks into segments with smaller angles. By making simplifying assumptions for the degree of relaxation of stress and crack healing in pellets after prolonged irradiation, the number and orientation of cooling cracks is predicted. Included are cross-sections of irradiated hollow and solid fuel pellets, subjected to different rating levels. The crack patterns are interpreted in the light of the above analyses. All the major features seen in the sections may be explained. The work was carried out in support of better representation of fuel cracking in one-dimensional fuel behaviour codes. (author)