Stress-based vs. Strain-based safety evaluations of spent nuclear fuel transport casks in energy-limited events

The stress-based criteria specified in ASME Section III (American Society of Mechanical Engineers, 2010) have long been used to evaluate the structural safety of mechanical structures, systems, and components (SSC) associated with nuclear power plants. These evaluation criteria, however, have inevitable limitations in accounting for the structural behaviors of spent nuclear fuel (SNF) transport casks, which likely undergo significant plastic deformations in extreme accidents. Hence, the stress-based evaluation criteria are considered inappropriate for energy-limited events that involve a high impact load, such as an accidental drop. This study evaluates the effectiveness of strain-based criteria, which were recently added in ASME Section III, Nonmandatory Appendix FF (American Society of Mechanical Engineers, 2015) and provides critical comparative study over traditional stress-based criteria for assessing the structural safety of SNF transport casks. Horizontal and vertical accidental drop events specified in NUREG-1536 (US Nuclear Regulatory Commission, 1997) are simulated using nonlinear dynamic finite-element analysis (FEA). Four SNF cask models with different impact-limiter designs are numerically simulated. The design of the impact limiter varies with wood types (redwood and balsa wood) and grain directions (transverse and longitudinal with respect to the loading direction). The FEA stress and strain results for the basket and canister parts are examined according to both stress-based and strain-based criteria. The strain-based criteria provide less conservative and more sensitive estimation than the stress-based criteria.