Could a nuclear weapon be built and carried in a suitcase?
I've heard talk about "suitcase" nuclear weapons, which someone could carry around and detonate anywhere. Is this possible? I'm not talking about whether someone could get hold of the proper components or be mad enough to pull it off. Rather, I always thought uranium and plutonium were really heavy and the amount needed for a bomb would be too much for one person to tote around. Gold, for example, is much heavier than most people think, certainly heavier than movies typically suggest, and plutonium has a much greater atomic weight and thus should be even heavier. In the end a suitcase-sized nuclear device in the back of a truck is just as awful as a totable one, but the image put forth in the media seems highly inaccurate to me.
As so often, we need to define our terms. If you're asking whether it's possible to make a practical nuclear bomb small and light enough to carry around one-handed in a Teletubbies lunch box, the answer is probably not. However, if we expand the menu of mininuke delivery systems to include, say, a bowling-ball bag or, better yet, a garden-variety wheeled suitcase, I wouldn't rule anything out. And if we conjure up what in my opinion is an entirely plausible scenario with a guy in a parking meter service uniform pulling an ashcan-sized two-wheeled coin vault through busy downtown streets at rush hour--well, I'll make the usual disclaimer about the proper components not being easy to come by, etc. Strictly from the standpoint of design feasibility, though, piece of cake.
While the active ingredients in a nuclear bomb are plenty heavy, they're not in the neutron-star range, as you seem to think. Gold, uranium, and plutonium all weigh around 19 to 20 grams per cubic centimeter (10 to 11 ounces per cubic inch), compared to about 8 g/cc for iron. It doesn't take much fissile material to make a bomb--on the order of 10 kilograms of plutonium, a roughly grapefruit-sized sphere. You'll also want a "shaped charge" of conventional explosives to compress the plutonium to critical mass, plus a few other precisely engineered but not especially bulky items. (A gun-type weapon that smashes two hunks of uranium together to trigger the nuclear blast is simpler to make but requires more material.) One thing you won't need is massive lead shielding to ensure the delivery person lives long enough to reach ground zero--prior to detonation, plutonium and uranium don't emit significant amounts of ionizing radiation, one reason a suitcase nuke wouldn't be easy to detect. What would the thing weigh, all told? Possibly as little as 30 kilograms, or 66 pounds. It'd be a bit unwieldy to slide under the seat as a carry-on, maybe, but still pretty small.
Guesswork, you say. True, but fairly educated guesswork. Our most recent glimpse at the state of the art in portable A-bomb design was furnished by the late Russian general Alexander Lebed, who in 1997 claimed that 100 or so Russian tactical nuclear bombinos couldn't be accounted for. Lebed said each device measured about 60 by 40 by 20 centimeters (24 by 16 by 8 inches, suitcase-sized in my book) and would explode with a force roughly equal to 1,000 tons of TNT--supposedly they were to be deployed by special forces behind enemy lines. Kremlin spokesmen roundly denied all, including the little bombs' existence. The question remains unsettled (and unsettling), but there seems small doubt that suitcase nukes are buildable, since plenty of portable if not exactly Samsonite-sized A-weapons were in fact built during the cold war. One oft-cited example is a U.S. device called the special atomic demolition munition (SADM), reportedly deployed in various configurations during the 1960s. The SADM, or anyway some SADMs, supposedly had a shipping weight of about 160 pounds, which is more like a dishwasher than a suitcase, but when assembled and ready for use may have been in the 50-to-60-pound range. In declassified photos one version looks to be about the size of a small shop vac--and remember, that's 40-year-old technology. Imagine what some nuclear nerd could come up with today.
Not to worry, the experts say: The suitcase nuke threat is exaggerated--if any were actually out there, given the global surplus of fanatics, by now they'd surely have been used. Producing weapons-grade uranium and plutonium is a huge industrial operation requiring skills and equipment not easily concealed; even the craziest terrorist knows there are easier ways to make things go boom. Despite what alarmists would have you believe, you can't just buy ten kilos of P-239 on the Tashkent black market and get a recipe from alt.nukes.made.simple. To which the pessimist, knowing that we're inevitably headed toward a more nuke-dependent world as other energy sources dry up, can only reply: Not yet.