A Methodology for the Evaluation of Fuel Rod Failures under Transportation Accidents

Recent studies on long-term behavior of high-burnup spent fuel have shown that under normal conditions of storage, challenges to cladding integrity from various postulated damage mechanisms, such as delayed hydride cracking, stress-corrosion cracking and long-term creep, would not lead to any significant safety concerns [1,2] during dry storage, and regulatory rules have subsequently been established to ensure that a compatible level of safety is maintained [3]. However, similar safety assurances for spent fuel transportation have not yet been developed, and further studies are currently being conducted to evaluate the conditions under which transportation-related safety issues can be resolved. One of the issues presently under evaluation is the ability and the extent of the fuel assemblies to maintain non-reconfigured geometry during transportation accidents. This evaluation may determine whether, or not, the shielding, confinement, and criticality safety evaluations can be performed assuming initial fuel assembly geometries. The degree to which spent fuel re-configuration could occur during a transportation accident would depend to a large degree on the number of fuel rod failures and the type and geometry of the failure modes. Such information can only be developed analytically, as there is no direct experimental data that can provide guidance on the level of damage that can be expected. To this end, the paper focuses on the development of a modeling and analysis methodology that deals with this general problem on a generic basis. First consideration is given to defining accident loading that is equivalent to the bounding, although analytically intractable, hypothetical transportation accident of a 9-meter drop onto essentially unyielding surface, which is effectively a condition for impact-limiters design. Second, an analytically robust material constitutive model, an essential element in a successful structural analysis, is required. A material behavior model, with embedded failure criteria, for cladding containing various concentrations of circumferentially and radially oriented hydrides has been developed and implemented in a finite element code. The characterization of hydrides-dependent properties of high-burnup fuel cladding is the main feature of this constitutive model. The third element in the overall process is to utilize this material model and its host finite element code in the structural analysis of a transportation cask subjected to bounding accident loading to calculate fuel rod failures and failure mode configurations. This requires detailed modeling of the transport cask and its internal structure, which include canister, basket, fuel assembly grids and fuel rods. The overall methodology is described in the paper.