[bond7]

What kills the "suitcase" models fast is the tritium needed to augment such a small mass of fissionable material. Tritium decays into helium-3 with a 12-year half-life, which means that after a few months there is a significant helium-3 contamination. Helium-3 eats up the neutrons needed to start a chain reaction. No periodic replenishment with new tritium, no big boom.

Great stuff. Thanks.

[P8riot]

I wasn't aware that He-3 was a neutron poison, as in the case of Boron, Xenon, or Halfnium.