The Effect of Gaps on the Impact Response of a Cask Closure Lid

During an impact event, gaps between the various components of a spent fuel transportation cask may create secondary impacts that result in higher dynamic loads than would have occurred if the gaps had not been present. A condition of particular interest is the gap that may exist between the cask internal contents (fuel assemblies) and cask closure lid, and the effect this gap may have on amplifying the response of the closure lid during an impact. Through the use of a simple dynamic model this paper investigates the effect of a secondary impact due to a gap between the cask internals and the cask closure lid on the response of the closure lid during a 30 foot end drop. The dynamic model consists of five components: (1) The equivalent mass of the internal contents, (2) the gap between the contents and cask lid, (3) the stiffness of the cask lid, assumed to be a simply supported circular plate, (4) the equivalent mass of the lid and finally, (5) an impact limiter that applies a constant deceleration force to the cask overpack during impact. In addition, the dynamic model assumes elastic behavior. This is consistent with the Standard Review Plan (NUREG-1617), which recommends that the closure lid bolts and closure lid system within the region of the lid bolts remain elastic in order to demonstrate leak-tightness by finite element analysis. The response results are presented in terms of the Dynamic Load Factor (DLF) for the closure lid. Response is shown to be a nonlinear function of the impact limiter deceleration, gap size and closure lid diameter, thickness and inertial properties. These results provide valuable insights into the parameters that affect response and show the conditions under which gaps of sufficient size may significantly influence response.