Before the Fat Man device was developed, Manhattan Project designers looked at a gun-type device using plutonium instead of uranium. The problem was that the fission would start to happen prematurely and the device would fizzle. My question is how big a fizzle are we talking about?
Thanks,
Rob
I don’t know that anyone has published a non-secret answer. Everything I’ve read suggests that the explosion would have not been substantially bigger than an equivalent weight of chemical explosives.
The gun method didn’t have any mysterious properties. They of course didn’t want to pack the whole critical mass together, so they needed to split it into parts, separate them enough so that their neutron emissions wouldn’t start things up, and then bring them together for the explosion.
If it had been possible, I’m sure they would have used springs! However, they soon figured out that the stuff starts “exploding” as you bring the parts together, so you have to get them together quick before the buildup of energy turns everything into plasma, which has an annoying habit of wanting to not be in close quarters.:smack:
So you bring the parts together real fast. That means either a gun or implosion.
At first, they were very worried. It seemed that a gun that would prevent a fizzle even for U235 would be on the order of 15 feet long and weigh like 20 tons. They got over the weight problem easily enough; they’d forgotten that gun only has to be fired once, so you don’t need the same strength as an ordinary artillery piece.
Further testing showed that U235 wasn’t as excitable as they first thought. The gun length was reduced; that’s when Thin Man became Little Boy. At the same time, they figured out that no muzzle velocity would be fast enough for Pu. It was just too reactive. Whether its emitted neutrons were faster, or its atoms were more likely to split when they absorbed a neutron, I don’t know.
In any event, by the spring of 1945 they were in deep doodoo. They had a straightforward mechanism for setting off U235, but they didn’t have much of it, the stuff proving incredibly difficult to isolate. They had pounds of Pu coming out of the Hanford reactors, but just the first beginnings of an implosion bomb. Los Alamos worked for over 2 years to perfect implosion.
I believe you’ve read the Richard Rhodes book about the making of the atomic bombs. That story about the big gun always stuck with me.
The other part is that supposedly it was a weapons guy attached to the Manhattan Project who realized what to do about it. Fermi, Teller, Feynman and the other geniuses they had didn’t come up with that one. They were scientists solving a scientific problem, while it took a soldier to make a usable weapon.
Actually, faster neutrons are bad. You need a certain minimum energy to trigger the fission, but above that, the faster neutrons have a lower probability of interacting with the nuclei. That’s why reactors have moderators, to slow down the neutrons so they’ll react better.
Don’t remember where I first read about the big gun, but I have read Rhodes and it is in there.
It was a PhD chemist, Kistiakowsky, who solved the implosion problem. If you mean the big gun problem, I’m not sure whether or not it was a weapons person or just something seeing the fallacy in their assumptions.
From here there is some info on the gun type device using plutonium. (There are some cool pics of the casings in the article) Seems that even if they could get it to work, the length of the bomb would be too great to be practical.
[quote=“Mach Tuck, post:3, topic:529524”]
I believe you’ve read the Richard Rhodes book about the making of the atomic bombs. That story about the big gun always stuck with me.
[QUOTE]
Funny, I thought the same exact thing. It is an outstanding book. I just bought *Arsenals of Folly *by Rhodes.
nevermind
Bombs based on uranium hydride and plutonium hydride, which would self-moderate to produce slow neutrons, were tested. But it turned out that what was gained by increasing the neutron cross-section was lost by the reduced number of generations the reaction had time for before it blew apart. The tests fizzled badly and all nuclear weapons today are fast-neutron.
I was curious about a ground- or ship-based terrorist weapon. If you and your yahoo buddies managed to steal sufficient plutonium, even plutonium poisoned with Pu-240, would the fizzle be significantly larger that the charge needed to shoot it down the gun barrel? A .02 kt fizzle from 100lb of plutonium would still be a very effective terror weapon.
Rob
For a minute there, I thought this was going to be about an embarrassing incident in Nick and Nora’s sex life…
Who are Nick and Nora?
Rob
Nick and Nora Charles, main characters of the Dashiel Hammett detective novel The Thin Man and the film based on it.
FWIW, Wikipedia says the bomb was named in homage to the films.
I thought it was going to be about the films as well.
Large enough to ruin someone’s morning.
In section 17 of the Los Alamos Primer (a pdf), Serber did a back-of-the-envelope estimate of the yield of a uranium-gun predetonation fizzle and came up with 60 tonnes. Still huge by non-nuclear standards.
Of course, the numbers for a Thin Man plutonium gun would be somewhat different. However, while you might think that the higher spontaneous fission rate would wreck the assembly earlier and hence significantly lower the yield, Serber’s argument implies that that intuition is wrong, at least to this level of exactness. Crudely, fissions before the assembly is critical are irrelevant. Once it’s just critical, a chain reaction will propagate, but only barely. What’s the more important process at this stage is that the core is still becoming more critical as the pieces move together. Even with many spontaneous fissions at this point, the core still manages to become more than barely critical. Hence the overall bang is still non-trivial.