A Validation Case Study: Steel Billet Drop Tests and Simulations as Reported in NUREG/CR-6608

Before performing safety assessments of spent fuel storage casks in drop and tipover accident simulations, method validation calculations are required. The validation process is outlined by the Nuclear Regulatory Commission (NRC) [Tang, et al., undated], and specifically requires the satisfactory replication of the steel billet drop tests, reported in NUREG/CR-6608 (UCRL-ID-129211) [Witte, et al., 1998]. In addition to reporting the test results in NUREG/CR-6608, Witte, et al. also provide simulations of the tests using the Lawrence Livermore National Laboratory explicit finite element code DYNA3D [Whirley, 1993]; several other organizations have used the Livermore Software and Technology Corporation code LS-DYNA [Hallquist, 1999]. Although other explicit finite element codes would also be applicable, the material model parameters provided for the concrete pad, upon which the billets are dropped, in NUREG/CR-6608 are specific for the Concrete/Geological Material, i.e. Material Type 16, in DYNA3D and LS-DYNA. This sole fact provides a great incentive for analysts to use DYNA3D or LS-DYNA. This manuscript briefly reviews the test configurations and results with recommendations on which configurations and results should be emphasized in comparisons with simulations. Next a brief review of the simulations presented in NUREG/CR-6608 with comments on the modeling and results and suggested improvements is provided. Then comments are provided on the utility of these results, both experimental and numerical, as a validation of the methodology with a particular emphasis on how they extrapolate to the cases of interest for spent fuel storage casks. Finally, a series of recommendations are included that should be considered, and discussed, by analysts providing simulations for spent fuel storage casks and the authorities requiring the safety assessment of these casks.