Hand-creating a Critical Mass of Uranium

My wife is reading a novel that makes reference to the following scenario:

Back during the earlydays of atomic research, an atomic physicist has two sub-critical masses of uranium. To study the effects of obtaining critical mass, he places them near each other on the table and moves them closer together until something interesting happens, then separates them. He does this repeatedly, until one day he becomes too cocky and shoves them too close. In a split-second decision, he wrenches them apart, and a major catastrophe is averted. The physicist, however, is doomed to a lingering, week-long death from radiation poisoning.

After reading this, she asked me if this anecdote has any basis in fact. Now, I vaguely remember reading about this before, with the added information that 1) it occured at the old Fermilab, underneath the stadium at the University of Chicago, and 2) the physicist in question had done this often enough that he had invented a jargon term for it (which now escapes my memory).

However, in mulling the scenario over, it seems too close to an urban legend. “Atomic physicist, full of hubris, nearly irradiates half of Chicago, but instead dies to save the masses from his own foolishness.”

So, the question is: Does this anecdote have any basis in fact? How much of it is true? Who/what/when/where/why? Links would be appreciated; I don’t seem to (currently) have enough information to do an effective search.

Yes, such incidents (note plural) happened. You’re probably thinking of the incident where Louis Slotin was killed; it was dramatized in the movie Fat Man and Little Boy. Search for the term “tickling the dragon’s tail” (which refers to the experiment you described) and the guy’s name and you will get many hits.

Such incidents still happen, believe it or not. A couple of workers died last year in Japan when they poured uranium solution into a tank with a bucket by hand, creating a critical mass. There are numerous fail-safe machinery and safety protocols, but it won’t help if they choose not to use them.

There’s a scene in “Fat Man and Little Boy” where John Cusack’s character, Michael Merriman, was doing legitimate research at the New Mexico cite of the Manhattan Project with two halves of a mass of plutonium, IIRC.

He was using a screwdriver to keep the two halves apart, then turning it to bring them a little closer. Somehow, the top mass slips onto the bottom mass fully, starting a chain reaction. To stop the reaction, he reaches over a partition and removes the top mass. He then directs everyone in the room to stay put and mark their footprints with chalk so it can be determined who got exposed.

It turns out that only he did, and for the rest of the movie Merriman is in the hospital dying a most gruesome death.

I don’t know if it was based on a real incident, but it certainly parallels the story in your OP.

It’s important to note that merely bringing two masses of Uranium or Plutonium together that combined will make a critical mass will almost certainly NOT explode and ‘irradiate half of Chicago’. IIRC it IS possible that merely having a critical mass will explode but unlikely which is why nuclear weapons contain a neutron source to kick things into gear (by shooting neutrons into the mass they cause many more atoms to fission quickly thus engaging the dreaded/sought for chain reaction). The proof (in case you needed any) is that as mentioned this has happened several times and never once did anyone nuke the facility and surrounding area. Of course, it’s still an excellent idea to separate the two masses ASAP…tickling the dragon’s tail is an apt metaphor for playing with this stuff.

This is, of course, small consolation to the poor sap(s) standing next to the critical mass. As has been pointed out chances are very good that he (or she) is in for a gruesome, lingering death.

Ah, yes; thanks for the info. The incident I vaguely remembered was indeed from Louis Slotin’s “tickling the dragon’s tail.” A search as suggested by scr4 yielded a few hits that explained the incident in more detail, this one, for example.

The real incident pretty much parallels my recollection as well as the incident in the book, except for the fact that it occured at Los Alamos rather than in Chicago. I believe that my initial confusion in this particular stems from the fact that Slotin was a researcher at U of Chicago before being recruited to the Manhattan project. I think he also experimented with “tickling the dragon” in Chicago, but that’s not where the accident took place.

Just a couple of details:

  1. It couldn’t have happened in Chicago because they didn’t use enriched uranium when they built the reactor under the stadium (I thought it was the squash court) there. The critical mass in that case was very large, not something you could push around on a table with a screwdriver.

  2. As noted above, if you have the fissile material it’s easy to make a chain reaction but it’s difficult to make an explosion and it’s very difficult to make a big explosion. If the critical mass isn’t assembled very quickly the reaction proceeds relatively slowly and you get (maybe) a pop and a flash but no boom. As with any explosive device the tricky part is A) to get the reaction to proceed in a very short time and B) to contain the reaction long enough for it to reach completion. Conventional high explosives generally use something like a steel jacket. Fission weapons use the aforementioned neutron source (to get it started quickly), neutron reflectors (to raise the efficiency of the reaction) and high explosives (to assemble the compenents quickly and to contain the reaction).

Building an atom bomb requires solving two very hard problems – 1) getting the fissile material and 2) making it explode. A large portion of the Manhattan project was spent in solving the second problem. I doubt this is news to anyone reading this, but “Little Boy” operated by firing a smallish plutonium “bullet” into a subcritcal mass of plutonium, suddenly creating a critical mass. It was relatively long and thin, like a rifle. “Fat Man” used an implosion technique with high explosives wrapped around a hollow uranium sphere which was subcritical until it was driven into a small sphere by the explosives. The spherical layers of uranium, reflectors, explosives and containment made a large “fat” package.

  1. Of course, Slotin wasn’t just screwing around or tempting fate – he was measuring the reaction as the pieces were placed close together. He was determining the critical mass so they could build the bomb. If there had been less urgency the scientists would presumably made those measurements in a safer way.

“Genius”, a biography of Richard Feynman, refers to this incident. It’s said to have taken place at Los Alamos. The victim was working alone, late at night, assembling a near-critical mass from 1" cubes. He dropped an extra cube on the stack by mistake; saw a blue glow from ionization, and died two weeks later. That’s how it’s presented in “Genius”.

Jeff_42 said:

"It’s important to note that merely bringing two masses of Uranium or Plutonium together that combined will make a critical mass will almost certainly NOT explode and ‘irradiate half of Chicago’. IIRC it IS possible that merely having a critical mass will explode but unlikely which is why nuclear weapons contain a neutron source to kick things into gear (by shooting neutrons into the mass they cause many more atoms to fission quickly thus engaging the dreaded/sought for chain reaction)."

A minor nitpick - nuclear weapons contain a neutron source because they create a very short lived supercritical mass by imploding the nuclear material with conventional explosives. It is necessary to have neutrons present to trigger the chain reaction in mid-implosion. If you could assemble a highly supercritical mass just by piling up bricks of uranium (you can’t) it WOULD go off on its own, triggered by spontaneous fission.

Strictly speaking, an exact critical mass won’t give a runaway chain reaction at all. On average, neutrons are lost at exactly the same rate as they are created by fission. For proper neutron multiplication, you need a highly supercritical mass, say, three critical masses for a bomb.
“Tickling the dragons tail” by moving two pieces of uranium together will create a barely supercritical mass. They don’t actually have to be touching - just close enough that sufficient neutrons from one are hitting the other. When you get them that close, spontaneous fission will start a runaway chain reaction and a lethal neutron flux. This is called a “blue flash” incident, because the neutrons streaming through your eyeballs emit blue Cerenkov radiation right inside your eye.

What you DON’T get is a nuclear explosion, at least, not a very big one. The uranium pieces heat up very rapidly and expand, becoming subcritical again due to their reduced density and increased surface area. This is a good time to pull them further apart, since you’re walking dead anyway and they’ll become critical again as they cool down. You’ll burn your hands, but you deserve it!

If you’re being really reckless and clap those pieces of uranium together as fast as you can, you might get a bang. As soon as they are close enough to be supercritical, they start to heat up but they’re still moving towards each other. However, they will release energy fast enough to blow each other apart long before a big supercriticality is achieved. The yield of your nuclear “explosion” is likely to be slightly less than a firecracker. (Although you’re still walking dead from the neutron flash.)

To get them together fast enough to take out Hiroshima, you need to fire one piece at the other with a gun. That is literally what was inside Little Boy, a gun barrel with a uranium shell and uranium target. (There was a neutron generator as well, but it wasn’t strictly necessary.)

With plutonium, not even gun assembly is fast enough to achieve a large supercriticality. That’s why implosion and neutron generators are required, which are thankfully technically difficult to construct!

So the story could be true, but the worst case scenario is an irradiated lab rather than a mushroom cloud.

Wow, so many people posting on this at once!

Pluto - I think the gun assembly used uranium and the implosion device used plutonium, but I’m willing to be corrected on this!


… is a list of criticality accidents



… is the most comprehensive nuclear weapons site I’ve ever seen. Answers just about anything you might want to know.

p.s. U’s relatively low spontaneous fission rate makes it suitable for gun assembly, while PU’s relatively high one makes it unsuitable.

I stand corrected.

Thanks for the links!

I find the illustration and description of the Slotin incident quite chilling. Slotin removed the protective checkblocks and used a screwdriver in their place, and everybody just stood around and watched?

I was also fascinated to learn that in “gadget” and “fat man”, “A great deal of tissue paper and scotch tape was also used to make everything fit snugly together”.

I don’t think this was incident that I was remembering (the Louis Slotin incident referred to by scr4). Actually, it sounds a little more like the accident that happened to Harry Daghlian, which occured almost a year earlier. This info can be found at the first site posted by bump.

It’s also interesting to know that the worst-case scenario was not quite as bad as I had naively assumed it was.

Two questions:
(1) The first atomic reactor was built in a squash court, at the Univ. of Chicago, by a team headed by Enrico Fermi. I’ve seen pictures of this pile-with people walking on top of it! I understand that Fermi died young-was his death caused by the radiation he was exposed to?
(2) the Germans attempted to build a fission reactor-I once saw a picture of it (it was an array of uranium blocks hung on chains, which could be lowered into a bath of heavy water. Could this scheme ever have worked?

Well all the sites that I saw said Fermi died of cancer in 1954. None of them specifically attributed it to the radiation but you have to wonder.

Yeah, that’s about all I could find too. As far as those pix of people walking around on top of Fermi’s reactor go, I’m sure they were taken before the uranium fuel rods were inserted, so they were just standing on a whole lot of graphite.

As for heavy water reactors, they work just fine. I believe there are some heavy water commercial power reactors still operating in Canada.

Could the German fission story be part of Otto Hahn and Lise Meitner’s experiments, perhaps after Meitner decided to get her butt out of harm’s way ?