Since critical mass is what creates a nuclear explosion, it is not how much original fissile material is involved (as long as it above some microscopic minimum), but the ability to make it compress to that critical mass, correct? So if this is true, it would seem a sand-grain sized particle of fissile material built into the correct "implosion" design, could create a nuclear explosion, albeit small compared to what most people think of; say, enough explosive power to take out a few city blocks. If this is true, it would seem this discussion of nuclear decay, etc., is irrelevant. The question is: How big of an "implosion" device would it take to compress a sand grain size piece of fissile material enough to cause a nuclear explosion? In my mind, it seems like it wouldn't take a very large device, say, about the size of a suitcase.