[en] A cask containment system can generally be designed from two perspectives -- either the cask and its associated hardware alone are assumed to provide containment or the contents of the cask, in this case the spent fuel, are also considered part of the containment system. In the latter approach, known as a source-term methodology, some credit is derived for those contents based on their material, physical, or chemical properties that tend to limit or inhibit radionuclide ''release.'' The former approach, in which no such credit is taken, is generally called a ''leaktight'' design basis. While previous containment analyses for transport casks in the US have used both leaktight and source term approaches, recent practice on the part of both designers and regulators has been to rely totally on the cask for containment. This evolution to a ''leaktight'' criterion primarily resulted from the lack of a standardized source term characterization methodology that might have addressed specific properties of the cask contents. 3 refs., 6 figs