The maximum yield of the W54 warhead used in the Special Atomic Demolition Munition (pictured) was 1 kt (1000 tonnes of TNT equivalent). This is larger and heavier than the US W48 nuclear shell at 155 mm (6.1 inches) in diameter and 846 mm (33.3 inches) long and weighing 53.5 kg (118 lb), which represents the smallest complete, self-contained physics package to be fielded and had a yield of 72 tonnes of TNT. Nuclear weapons designer Ted Taylor has alleged that a 105 mm (4.1 inch) diameter shell with a mass of 19 kg is theoretically possible. Conversely, reduction beyond the size of the W54 means that linear implosion designs must be employed and neutron reflectors dispensed with ("bare core"), so a much larger mass of fissile material is required and explosive yield is reduced dramatically. Taylor's figures represent the minimum size and mass to sustain a prompt criticality but the duration without tamper or neutron reflection would be short. The slope of exponential growth, estimated number of fissions, and specific fissile material are not recorded. Neptunium-236 is fissile and possesses the smallest and lightest critical mass, but isolation of the specific radionuclide makes it an impractical choice. Several other novel fissile materials are known, but U-235 and Pu-239 are the only practical options although two US tests using U-233 (critical mass some 32% less than U235) have taken place.