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Russian nuclear engineers are a problem. They're good enough to be dangerous, but not good enough to build decent containment.

The 15.7 kg of Ur never quite made sense anyway. That's alot of radioactivity to contain the transportation device would need alot of lead to protect the courier (making the package even more massive). I was more concerned with the story that this material could have come from the former Soviet Union and that it was consistent with material lost by FSU in the construction of low-yield weapons. I'm no expert in nuclear weapons, but the finding of Ur (any quantity) substantiates the stories published here about traffic in these materials in East Europe.

Lead is needed to shield against the gamma radiation (high energy light, like hard x-rays). The heavy alpha particles (much more damaging if exposed) are readily stopped by even modest amounts of just about any solid material (as are the light betas particles, which are just fast moving electrons). U-238 is radioactive yes, but not that much, certainly not compared e.g. to plutonium. You wouldn't want to be exposed to a lot of it completely unshielded, certainly. But you block ~95% of the danger with even thin shielding.

15.7 kg of U-238 would have been 66 mols or 4x10^25 atoms. (238 grams per mol, 6x10^23 atoms in a mol). The half life of U-238 is 1.4x10^17 seconds (4.5 billion years), so you get roughly 3x10^8 Bq for the triggering top of the decay series (300 million atoms per second). That is less than the radioactivity from 1 gram of plutonium (2 billion atoms per second). Throw in the subsequent decays down the chain, and it would probably be comparably to a gram or so of plutonium. If enriched (some U-235, lower half-life, faster reaction), naturally the rate would be somewhat higher. The small amount of U-235 in natural uranium contributes around 10% more; purified U-235 would be an order of magnitude higher than U-238.

In short, uranium is not like plutonium, or like half or a quarter as much plutonium. It is thousands of times less radioactive, so kilos of it are like very small amounts of plutonium. Most nuclear weapons use plutonium triggers; it is much more efficient. They require some shielding of course, but are readily handled with such shielding, when building, transporting, or servicing the weapons.

So it was perfectly plausible that there might be 15.7 kg of uranium - though probably not truely weapons grade uranium (meaning 90% U-235). It would hardly be impossible to transport, it would not blow up or act like the fantasy cargo in a Repo Man car trunk. When I saw the report, I was skeptical they had 15.7 kg of 90% U-235, but considered it about equally likely they had 15.7 kg of uranium (just something like 3% U-235, 97% U-238) or that they had 700 grams of U-235 in 15 kg of package and shielding. It now appears from the corrected reports that something like the latter is the case.