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.
This is complete and utter nonsense. Popular fissionable elements are long-lived alpha emitters, which require little shielding. Properly moderated, criticality can be achieved with a few kilograms of material. Some fission triggers for thermonuclear weapons are small enough to fit in a suitcase, and could be employed as standalone small nukes. Does anyone remember DOD's nuclear grenade? While the stories of suitcase nukes may be nonsense, they are not nonsense because such devices are impossible to construct, or last only a few months. -- Eric Michael Cordian 0+ O:.T:.O:. Mathematical Munitions Division "Do What Thou Wilt Shall Be The Whole Of The Law"