Closure Bolt Modeling for Seal Evaluation Under Extreme Thermal Loads

The US Nuclear Regulatory Commission (NRC) recently completed an evaluation of a used nuclear fuel transportation package subjected to loads associated with the MacArthur Maze fire and overpass collapse of 2007. This historical event is used as the basis of an extreme accident scenario to investigate the performance of the package system under loads that are beyond regulatory limits and to assess the potential risk to the public. The accident scenario was modeled in a number of physics regimes, including fire dynamics models, thermal models, structural impact models, and structural thermal expansion models. In this case the thermal expansion behavior of the bolted closure was a key component of the leak rate calculations and offered a tough analytical challenge, which was ultimately resolved using a detailed nonlinear finite element model of the bolt threads with thread inserts discretely modeled. This paper describes the analytical steps that were taken, starting with classic bolt stress calculations and ending with sophisticated finite element analysis, while also putting the analysis into context of the larger analytical effort and the assessment of used nuclear fuel transportation package safety that is critical to the mission of the US NRC.