Method To Evaluate Limits Of Lattice Expansion In Light Water Reactor Fuel From An Axial Impact Accident During Transport

Packages transporting Light Water Reactor (LWR) fuel in the public domain must comply with IAEA Transport Regulations, which state safety standards for both normal and accident conditions. Hence, an impact accident which causes deformation to the fuel may influence package safety and hence this must be analysed to demonstrate compliance with regulatory standards. For example, an impact event that causes fuel pin lattice expansion can increase the overall reactivity of the system. A limited length of lattice expansion is typically analysed in the criticality assessment of the package under impact accidents but to consider lattice expansion over the full fuel length may be pessimistic. To ensure undue pessimism is not used, analytical methods, described in this paper, have been developed to estimate bounding limits to the length over which the fuel assembly lattice can expand due to an axial impact. These methods use both static and dynamic analysis techniques, based on the results of impact tests involving both complete fuel assemblies and individual fuel rods. These tests demonstrate that virtually all fuel rod deformations induced from an axial impact are due to interactions between the end of the fuel rod and the deformed nozzles. The static analysis method estimates the response of a single fuel rod to imposed end conditions whilst taking into account the effects of lateral constraints imposed by adjacent fuel rods, spacer grids and compartment walls. This method estimates a deformation profile resulting from end loading conditions which is translated to changes in lattice geometry and subsequently applied to the criticality safety analysis on the package contents. The dynamic technique achieves a similar objective but can also take in to account pellet specifications and the influence of plenum springs when determining the resulting mode of deformation along the rod length. Results, directly comparing static and dynamic methods are good, as are comparisons with actual fuel rod drop test data.