Preparation of a Guide for Design and Analysis of Bolted Closures

Radioactive material package designers are becoming more and more reliant on finite element analyses to determine the response of their packages to the normal, hypothetical accident, and other conditions. In the United States, closure designs have traditionally followed the methods of NUREG/CR-6007 [1], Stress Analysis of Closure Bolts for Shipping Casks. This document was prepared in 1992, when the state-of-the-art for finite element analyses was much less developed than it is today. For this reason, the National Nuclear Security Administration’s Office of Packaging and Transportation commissioned a working group to develop an updated guidance document detailing today’s best practice for design and analysis of bolted closures. This document is primarily concerned with fissile material packages that have relatively thin-wall containment vessels, whereas NUREG/CR-6007 was primarily concerned with cask-type packages with relatively thick-wall containment vessels. For some load conditions, this difference can have a dramatic effect on the stresses in the closure. The guidance document discusses the loadings that must be considered during the design, including preload (both tension and torsion), thermal loads, internal and external pressure loads, loads from normal conditions of transportation drops, shock and vibration loads, and the loads from the hypothetical accident sequence. Once the loading conditions have been determined, the transfer of the package loads to the closure bolts must be calculated. The guidance document discusses hand calculations for calculating bolt loads as well as finite element means of determining bolt loads and bolt stresses. Guidance is provided on comparison of these calculated loads/stresses with allowable stress limits. Finally, the document provides insight on interpretation of results, discussing the difference between a requirement for leak tightness with one for assuring pressure safety and the difference between peak stresses and average stresses.