This is counter to what I thought to be true, but Miniter has carefully researched the origins of the stories of "suitcase nukes" and finds them unbelievable. One technical factor is a bomb that size could only contain a small amount of radioactive materials:
"Gen. Valynkin is referring to the fact that radioactive weapons require a lot of shielding. To fit the radioactive material and the appropriate shielding into a suitcase would mean that a very small amount of material would have to be used. Radioactive material decays at a steady, certain rate, expressed as 'half-life,' or the length of time it takes for half of the material to decay into harmless elements. The half-life of the most likely materials in the infinitesimal weights necessary to fit in a suitcase is a few months. So as a matter of physics and engineering, the nuclear suitcase is an impractical weapon. It would have to be rebuilt with new radioactive elements every few months."Once the amount of fissile material drops below "critical mass" there is no longer enough to sustain the chain reaction and cause an explosion.