[en] A new nondestructive method for direct assay of ²³⁵U mass contained in Model 5A uranium hexafluoride (UF₆) product storage cylinders has been successfully tested in the laboratory and under field conditions. The technique employs passive neutron self-interrogation and uses the ratio of coincidences-to-totals counts as a measure of bulk fissile mass. The accuracy of the method is 6.8% (1 sigma) based on field measurements of 44 Model 5A cylinders, 11 of which were either only partially filled or contained reactor return material. The cylinders contained UF₆ with enrichments from 5.96% to 97.6%. Count times were 3 to 6 min depending on ²³⁵U mass. Samples ranged from below 1 kg to over 16 kg of ²³⁵U. Because the method relies primarily on fast neutron self-interrogation, complete sampling of the UF₆ takes place. This feature alleviates inhomogeneity problems and offers increased assurance of the presence of stated amounts of bulk fissile material as compared with current verification methods