by CKR
Last week Phila and I pondered a statement attributed to Frank Von Hippel by the POGO blog:
a 100-pound mass of uranium dropped on a second 100-pound mass, from a height of about 6 feet, could produce a blast of 5 to 10 kilotons.I told Phila I'd check that out with someone who knows more neutronics than I do.
As scientists tend to do, and as I expected, he said "It depends." That seems to be particularly true for criticality incidents, of which a nuclear explosion is a special case.
A criticality incident is when a runaway nuclear chain reaction occurs in a critical mass of fissionable material. A runaway chain reaction is not necessarily a nuclear explosion; in fact, a number of criticality accidents have occurred, the most recent reported in Japan in 1999. Typically, they release a lot of neutrons and gamma rays, frying anyone who is close and killing them after a few days to a week of agony.
The "It depends" answer is accurate, because the ability of a runaway chain reaction to produce a nuclear explosion depends on how quickly the masses of material are brought together, the surrounding materials and their ability to reflect neutrons back into the critical mass, and other materials. I'm being a bit lazy about googling and the BBC story I've linked doesn't say, but I think I recall that the Japanese criticality was in a solution. Water is a good moderator for neutrons and helps the nuclear reaction along.
In the scenario quoted by POGO, the dropping from six feet tells us how quickly the masses are brought together, but there's also the question of how long they stay together. The recent North Korean test may have been a fizzle because the material blasted itself apart before it could achieve the fullest chain reaction possible, and they were trying to do that quite precisely.
As the chain reaction builds, it releases heat and energy. Even a little blast, like a stick of dynamite or even a small firecracker, could separate the pieces and end the chain reaction before it could yield "five to ten kilotons."
So the bottom line is that such an "improvised nuclear device" seems unlikely.
I'm also extremely skeptical of these "terrorist scenarios." It's easy to imagine a well-trained military force attacking Y-12 (or wherever) or extremely persuasive folks with well-counterfeited IDs, large chunks of explosives in their pockets, and a detailed knowledge of bomb design, shooting or talking their way (all of them at once) past the multiple checks of the areas where these materials are stored. Actually finding such a group, and finding the day when all the guards and others at the facility are snoozing or playing Pakman on their handhelds, is a bit more difficult.
It's not possible to critique the scenarios, though, because they themselves are classified, probably for good reason.
But I can't help wondering whether the security experts that dream up these scenarios aren't the same ones that came up with the idea that putting lip gloss in a baggie somehow makes it safer than lip gloss in a purse or the ones who believe that two magic liquids can be mixed to instantly destroy an airliner. If they are, we don't know any more about Y-12's security now than we did before the "security exercise."