View Full Version : What does one see in a fissile reaction?

2003-May-10, 01:53 AM
If one brings two 1lb pieces of uranium 235 slowly together, as they reach criticality, do they start to visibly glow? At what distance? As they get closer, how bright do they get? Like glowing coals? Or too bright look at? If they were slammed together with hands, how big would the explosion be? If the first reaction in Chicago in 1942 had run away, what was the worst damage that might have resulted? The whole campus destroyed, or just the lab? If one could look into the heart of a nuclear reactor, what would one see?

2003-May-10, 02:03 AM
Don't have much time, but perhaps to start you guys on the right track:



2003-May-10, 11:02 AM
If they were slammed together with hands, how big would the explosion be?

That does beg the question, if two chunks of fissile material are slammed together and the idiot who slammed them together gets incinerated, does it make sound?

2003-May-10, 06:16 PM
If they were slammed together with hands, how big would the explosion be?

Not very. It'd be nice and dirty, though.

Donnie B.
2003-May-10, 08:18 PM
You wouldn't get an explosion (of any significant size) by swatting two subcritical chunks of 235U together by hand. You'd get what's called a "fizzle". Basically, the uranium would melt or vaporize without a true nuclear explosion.

It's pretty hard to get uranium to go supercritical. In the "Little Boy" bomb used on Hiroshima, the 235U projectile was shot into the target by a cordite charge; it was, in fact, a converted naval cannon.

The two subcritical masses must be brought together so fast that you can't count on getting a neutron from a naturally-occurring decay to occur during the very short time they are "assembled" into a supercritical configuration. One of the big secrets of the Manhattan Product was a device called an "initiator" that produced a flood of neutrons at just the right instant to assure the chain reaction started at the right time.

Donnie B.
2003-May-10, 08:32 PM
Concerning the U of Chicago "pile", I don't believe it could have exploded in any sense. It was barely big enough to produce a sustained chain reaction, and nowhere near capable of supercriticality. It was unpressurized and uncooled so there was no water to create a steam explosion. The worst effect of a runaway reaction would have been the death of the experimenters by acute radiation poisoning.

I suppose it might have gotten hot enough to melt the fuel, but the natural uranium it used was contained in voids within the graphite structure and wouldn't have gone anywhere, unless the graphite caught fire. That would have been fairly nasty, something like a mini-Chernobyl (without the explosion) -- radioisotopes spread all over the place. It would have spread easily, too, since there was no containment structure except the stadium itself.

However, it wouldn't have been anywhere near as bad as an accident with later reactors, because it wasn't run very much -- it didn't have time to build up the very dirty daughter products (like Plutonium, Cesium, and so on) that make commercial spent fuel so troublesome.

Donnie B.
2003-May-10, 08:44 PM
Oh, one more thing: the critical mass of pure 235U metal is well over 2 pounds. I doubt that your mini-chunks would do much more than warm up a little. 8)

2003-May-13, 05:14 AM
In a uranium nuclear reactor, the neutrons released are very high energy, and they will hardly react with the nearby uranium. Usually some other material is included to slow down the neutrons. In addition, I think they line the walls of the chamber with some material that can keep the neutrons inside.
If you somehow initiated a reaction in your pieces of uranium by slapping them together (I doubt you could do it by hand - can you make C4 explode by doing the same?), then the starting neutrons would almost all fly out into the air, failing to sustain the reaction. It would be over as soon as it starts.

Donnie B.
2003-May-13, 09:28 PM
xriso, you are correct that a sustained chain reaction (as in a nuclear power plant) requires a moderator -- some substance that reduces the emitted neutrons' energy to better match the fission cross section of 235U. Good thing, too -- the moderator is a major contributor to allowing a (relatively) safe and controlled reaction.

However, this is not true in a bomb core. Highly enriched 235U will fiss without a moderator, if there's enough of it. The fission cross-section is lower for "fast neutrons", but large enough to sustain a chain reaction in the pure isotope. Indeed, that's exactly what the core of the Little Boy bomb was: a "bullet" and "target" of virtually pure uranium 235, each piece subcritical by itself but above the critical mass when assembled.

You make a good point about the reflector, however. The calculations for critical mass assume that all the emitted neutrons will eventually encounter a nucleus to absorb them and (maybe) fiss. That is, they ignore boundary conditions, or "leakage" of neutrons out of the core. There are several ways to overcome this, including a "tamper" (neutron reflector) around the core, an oversize core so supercriticality can be attained even with the leakage, or "implosion" methods that increase the core density and reduce the critical mass. Real weapons use a combination of these techniques.

At Los Alamos, during the Manhattan Project, experiments were performed that were not unlike the "whack two pieces together" method. Experimenters used a test rig to drop slugs of 235U through rings of 235U, slowly increasing the size of the pieces as the material became available. Eventually they achieved pulses of neutron production that proved that the bomb would work (and gave the experimenters radiation sickness, in some cases). This is why the Hiroshima-style bomb was never tested before it was used -- there was experimental data that proved it would work when scaled up to weapon size. That was not true of the Nagasaki implosion gadget, which had to be tested "all-or-nothing" at Alamogordo.

2003-May-14, 02:00 PM
Isn't part of the reason that nukes are detonated by explosive that the reaction mass will vaporise if the masses aren't brought together quickly enough? I think what will happen is you'll start bringing the masses together, they'll get real hot, you'll drop them and hurt your toes. That won't matter though, because after you drop em', you'll fall to your knees and vomit, pass out, and eventually die. Probably in that order. But if you pass out and then vomit, you'll die a lot quicker.