[en] There are three sources of releasable radioactive material in a loaded, spent fuel transport cask: (1) the residual contamination activity on the cask surfaces as a result of loading operations and previous shipments, (2) fission and activation product activity associated with deposited material on the fuel assembly surfaces (''crud''), and (3) the radionuclides contained within the individual fuel rods comprising the fuel assemblies. The US regulations for the containment requirements of a spent fuel transport cask limit radionuclide release rates to 10/sup /minus/6/ A₂ per hour and A₂ per week for normal and accident conditions of transport. Compliance with these release limits, R/sub i/, may be demonstrated by directly measurable leak rates, L/sub i/, through the mathematical expression L/sub i/ = R/sub i//C/sub i/, (i = N for normal or A for accident conditions), where C/sub i/ represents the concentration of airborne radioactive material in the cask that could escape from the containment system for the appropriate conditions of transport. This paper describes a method for estimating the contribution to C/sub N/ and C/sub A/ due to crud and residual contaminant activity. 7 refs., 3 figs., 1 tab