>>22170448 lb
depends
is the material weapons grade or what would be regarded as "dirty"?
Several factors come into play in determining whether a nuclear weapon or nuclear material is detectable above the natural radiation background. These include the configuration of the weapon or material, the amount of shielding, the type of detector, the level of the radiation background, the distance from the source and the counting time. In general, the instrumental noise in the detector is negligible compared to the random statistical noise due to variations in the counts due to the background and the source.
“Detection of illicit materials is a really difficult problem, especially for the most dangerous nuclear material, which is highly enriched uranium,” Matthew McKinzie, a nuclear physicist for the Natural Resources Defense Council, told The Journal Times. “The radiation signal can be concealed very effectively. And an improvised nuclear device, a terrorist’s nuclear weapon, can be made most simply with highly enriched uranium. In my mind that’s the toughest case. I think that’s the greatest challenge for that system: can it address that threat?”