Analytical Determination of Package Response to Severe Impacts*

An important aspect of radioactive material transportation risk assessments is the amount of release from packages subjected to accidents more severe than the design basis accident (U.S. NRC 10CFR71 1995) defined as a free fall from 9 m or 30ft onto an essentially unyielding target. Current risk assessments generally use very conservative estimates of release rates for extra-regulatory accidents. To remove some of this excessive conservatism and more realistically predict transportation risks, the response of a large number of packages to extra-regulatory impacts must be determined. Cost considerations preclude testing as the means for this determination. Many tests at many different velocities or orientations would be required to obtain a sufficient amount of experience at predicting leakage. Therefore, an analytical tool, such as the fmite element method must be used. For a finite element code to be relied upon it must first be qualified for performing analyses resulting in large plastic deformations of the containment boundary of radioactive material transportation packages. An effort to qualify the finite element method as an accurate and reliable method to predict cask performance has been ongoing at Sandia National Laboratories by comparing analytical results to test measurements of the Structural Evaluation Test Unit (SETU) cask. Comparisons of deformed shapes, strains and accelerations have been made for impact velocities of 13.4, 20.1 and 26.8 rn/s (30, 45 and 60 mph). The SETU cask was designed following the method and guidance of U.S. NRC Regulatory Guide 7.6 (U.S. NRC 1978). The unit met the requirements of this guide with as little margin as practical, in order to make the package have a high probability of plastic deformation. The 13.4 rnfs (30 mph) impact corresponds to the regulatory 9 m (30ft) free fall, and the others correspond to impacts with 2.25 and 4 times the kinetic energy of the regulatory impact. One other analysis at an impact velocity of 38.0 rn/s (85 mph) or eight times the kinetic energy of the regulatory impact will be included to extend the predictions to even higher energies.