PNNL

Abstract

U.S. Nuclear Regulatory Commission (NRC) rules require that used nuclear fuel (UNF) rods maintain their integrity during handling, transportation, and storage to ensure maintenance of the fuel retaining boundary, safety against criticality, and long term fuel retrievability for processing and disposal. Consequently, understanding the mechanical performance of UNF rods under cumulative loading stemming from handling, normal conditions of transport (NCT), and normal conditions of storage (NCS) is necessary as their performance under these conditions establishes part of their safety basis. The U.S. Department of Energy (DOE) commissioned a multi-laboratory team consisting of subject matter experts at Pacific Northwest National Laboratory, Idaho National Laboratory, Sandia National Laboratory, and Oak Ridge National Laboratory to develop a methodology to examine the structural performance and potential for failure of UNF under NCT and to perform a demonstration of this methodology for a typical UNF transportation campaign. This team prepared a Research, Development, and Demonstration (RD&D) Plan that describes a methodology, including development and use of analytical models, to evaluate loading and associated mechanical responses of UNF rods and key structural components during NCT. The initial scope of this plan has now been fully executed. The demonstration of the methodology laid out in the RD&D plan is focused on structural performance evaluation of Westinghouse Electric 17×17 OFA pressurized water reactor fuel assemblies with a discharge burnup range of 30 58 GWd/MTU (assembly average), loaded in a representative high-capacity (=32 fuel rod assemblies) transportation package and transported on a 3000 mile rail journey. In general, the modeling and simulation approach consists of three levels; the cask level, the fuel assembly level, and the fuel rod level. This modeling approach utilized finite element analysis sub-modeling techniques to accurately model the complete spent nuclear fuel transport system on the railcar (cask restraint structure, cask, basket, assembly, and fuel rods). The sub-modeling approach allows for more detailed finite element models of individual system components, faster analysis run times for the individual sub-models, and flexibility when updating or modifying the sub-models to incorporate better excitation data, initial material properties, or other pertinent information. The final results of the initial demonstration are that cladding strains were not large enough to cause structural failure, but cyclic strains roughly projected for the entire route were significant in some cases. The number of cycles that the model hits certain strain "bins" are counted. These are extrapolated for a 3000 mile trip and the damage ratio is calculated which is the number of cycles the fuel rods hit certain strain bins over the 3000 mile journey divided by the number of cycles to failure. Under the final model and set of inputs, the total damage from summation of the worst shock and vibration cases is ~18% of the expected fatigue limit. Therefore the fuel rods are not expected to fail during NCT given the assumptions listed. Additionally, a number of sensitivity studies were performed. It was found that the areas of highest sensitivity were the cladding elastic modulus, the spacer grid stiffness, the spacer grid location, and gaps between the assembly and the cask; these findings can be used to guide future work.