Effect of Lead Shielding on Cask Pin Puncture

The overall weight of a spent-fuel transport cask is minimized when its gamma shield consists of depleted uranium only; however, because of its high modulus of elasticity, a gamma shield of depleted uranium only, may not meet the 10 CFR 71 puncture requirements. The hypothetical accident puncture load is that load which is sustained by the cask when it drops from a 40-inch height onto a 6-inch diameter mild steel bar. This concern prompted investigation of hybrid gamma shields consisting of an outer layer of lead with an inner layer of depleted uranium ; these gamma shield were assumed to be sandwiched between two stainless steel shells. The outer steel shell is permitted to plastically deform during a puncture event; however, if only depleted uranium is present, the puncture forces are transmitted through the depleted uranium and a permanent dimple in the inner containment shell may result. A shell of lead outside the depleted uranium shell acts as a shock absorber, diffusing the concentrated load of the pin . The purpose of this study was to determine the optimum thickness combination of a multiwall, depleted uranium/lead gamma shield for a spent-fuel transportation cask. Since depleted uranium is a more effective gamma shield than lead, the optimum gamma shield design will be composed primarily of depleted uranium with a minimum thickness of lead backing the outer shell. The lead thickness must be sufficient to alleviate regulatory concerns associated with the assumed strength of the depleted uranium shell .