From this week’s Economist, describing the recent nuclear accident in Japan:
After an initial wave of nausea of my own upon visualizing this scene, my initial reaction is to ask: since when has it been so easy to initiate a nuclear reaction, and indeed, a nuclear explosion? Why are all the implosive devices and kryton switches needed when all you need to do is pour a big enough amount of the stuff into a steel bucket? (Of course, being able to get away afterward is another consideration.)
I tend to suspect (WAG?) that it has to do with the purity of the isotopes in the solution, and so the real challenge in making a bomb is getting a large enough concentration of U235 and not U238. All the same, though, why all the billions for the Manhattan Project?
Nuclear chain reactions are easy: just pile up enough fissionable material.
U-235 spontaneously decays, releasing lots of energy, but it’s unlikely that a particular nucleus will break down soon. When it eventually does so, it spits out free neutrons that can strike other U-235 nuclei and make them split immediately.
In a small sample, most of the neutrons just escape (lots of surface for little volume). In a larger sample, many neutrons cause further fission, releasing more neutrons, and so on. There’s the chain reaction.
As the uranium reacts and heats up, it tends to scatter itself around and the reaction usually stops before very much energy is released. (Note that “before very much” in this context can still do lots of damage, but the bomb-makers are after whole cities. That’s where the design problem for bombs comes from.)
The tricky part is to get the critical mass all in one place in a very short time. They use conventional high explosives to get a satisfying boom instead of just a fizzle. You could never make a city-destroying bomb just by pouring buckets of fissile uranium together, but you could generate a lot of lethal radiation and spread a lot of less-lethal-but-still-dangerous material around.
You, of course, would be toast. A similar incident occurred at Los Alamos: one physicist had the job of moving subcritical masses of fissile material closer and closer together and measuring the change in the radiation level. They called it “tickling the dragon’s tail”. IIRC, there was an accident, the pieces got too close and, although he quickly separated them, he had already received a lethal dose.
Incidentally, you need a fission “device” to set off a fusion “device”. So it’s one long chain of destruction – HE, fission, fusion. So without the Chinese and their black powder we would never have built the bomb. It’s only fair that we give the technology back to them!
Pluto, I think I originally read the “ticking the dragon’s tail” story when I was in the 8th grade (~1964). I’m not positive, but I think it’s been laid to urban legendry by now. In the version I recall, which was written by a professional whose name I recognized at the time but can’t remember now, the ‘tickling’ was done by moving two hollow hemispheres of fissionable material together with a long-handled screwdriver (!?!–honest to God, that’s what I remember reading) and the ‘heroic’ young technician supposedly saved everyone standing around by leaping in and separating the hemispheres with his hands, only of course to die of radiation poisoning within days.
Offered for what it’s worth.
Also, I think it was Robert Heinlein who mentioned in one of his stories that, if the allies had wanted to end the war in Europe quickly, they could have had someone take a paper lunchbag of plutonium dust on a plane ride over Berlin and simply dump it into the air. While I know it sounds bizarre to think a paper bag would provide sufficient shielding, he provided some rationale that the type of radiaton emitted by the plutonium could be stopped by the paper, but anyone ingesting the dust would die within days.
mjollnir
I think that would be alpha radiation, which isn’t really considered radiation any more. It’s a stream of helium nuclei, and you’re right, it can be stopped by ordinary paper. It’s very deadly if ingested. I don’t know if the amount of gamma radiation emitted by the paper bag would have been enough to really injure someone.
I’m willing to accept the UL status for the tickling the dragon’s tail story, but I’ll take a look around to see if I can find something more concrete about it.
I found several accounts, with several inconsistencies (e.g., the date, the number of people in the room, etc.) It seems there was another similar accident which killed another man, Henry Daglich(? – I can’t read my notes!), and the stories get confused.
But the URL above is from a memorial to the guy in Winnipeg, his hometown. It has links to most of the other accounts I found so you can judge for yourself.
Well, if Hollywood is to be believed: the scientist who died from “tickling the dragon’s tail” was Michael Merriman, played by John Cusack in “Fat Man and Little Boy”.
DIF and Mjollnir…the story was “Situation Unsatisfactory” which is included in Heinlein’s Expanded Universe if you want to find it.
It was written during the early days of WWII when we were still neutral, and assuming we’d stay that way until the events of the story. Basically it assumes a Manhattan Project, not for the bomb, but for any radioactive weapon, with a high-mortality radioactive isotope (“the dust”) as the result.
If you want to read the low-down about how easy critical mass can be reached without even trying, read “Surely You Must Be Joking, Mr Feynmann!” This is the amazing autobiography of a nuclear and theoretical physicist.
I’m pretty sure Plutonium is very deadly. Plutonium 239 is as close to a guaranteed carcinogen as you can get.
“‘Named after Pluto, god of the underworld, it is so toxic that less than one-millionth of a gram, an invisible particle, is a carcinogenic dose. One pound, if uniformly distributed, could hypothetically induce lung cancer in every person on Earth,’ said Dr. Helen Caldicott, founder of Physicians for Social Responsibility.”
Ordinarily, when you reach critical mass, you just get a ton of radiation. This is easy. Under special circumstances, you get even more radiation, and a really loud bang. This is hard.
Plutonium is actually a pretty nasy poison chemically, but it’s completely irrelevant. Even with the most stable isotope, the radiation danger is much higher than the chemical.
As part of the Manhattan Project they took a subcritical slug and dropped it through a hole in another piece of subcritical uranium. The mass of both pieces together was above critical mass. This was called the dragon experiment. If the slug had become stuck in the hole…
The results of this experiment led them to the conclusion that the gun-type (little boy) type bomb simply had to work. The one dropped on Hiroshima was the first time it had been tested.
There were many more doubts about the implosion weapon. (The plutonium based fat-man.) That it is why it had to be tested at trinity.
If you are interested in this, drop everything and get “The making of the Atomic Bomb” by Richard Rhodes. You will find out more than you wanted to know.
It’s absorbed by bone marrow because it substitutes for iron. WAG - if it weren’t radioactive, this might cause problems anyway, but it undoubtedly wouldn’t be anywhere near as toxic as it is.
BTW, I consider the webelements site to be one of the best reference sites on the net.