Hacker News Reader

Comment by alkonaut | Hacker News Reader

If you just want a bad atomic bomb, isn't it enough to put 2 chunks of fissile (enough) material at either end of a very thick steel tube and shoot them together using explosives? The question is how you use $2k to buy or produce the fissile material, because if the BOM was really $2k including "weapons grade" fissile material then the interesting question is how to get the fissile material that cheaply.

If the BOM is $2k excluding the fissile material then... is that really surprising? I thought crude bomb designs were pretty easy to create if you have no interest in maximizing yield or minimizing how dirty the bomb gets?

View on HN

This would not work with plutonium, which is what Philip’s “Nagasaki-like” (Wikipedia) design apparently used. The gun design does not create the critical configuration fast enough to prevent it ‘fizzling’ - the assembly blowing itself apart before the chain reaction has progressed far enough for a nuclear-scale explosion. Instead, you need an implosion device. This is much more challenging, requiring explosive lenses (precisely-shaped charges), multiple precisely-timed detonators, and an initiator that will produce a burst of neutrons just as the explosive’s shock waves reach the center of the fissile assembly.

I am sure you are right, though, that the bill of materials did not include the fissile material.

Anyone surprised by Phillip’s result was perhaps assuming that it would take something like the Manhattan Project to independently design a bomb.

>> perhaps assuming that it would take something like the Manhattan Project to independently design a bomb.

It's always easier to replicate someone else's work, especially when no one knew if it could be done in the first place.

Yeah, probably the most difficult part of the entire process of making a bomb is enrichment. The uranium in uranium ore is 99.3% (useless) U-238 and only 0.7% (useful) U-235. These are chemically identical, so the only thing you have to go on is the slight difference in weight, thus the _extremely_ difficult methods like centrifuges. (You need to build a giant "cascade" of them refining at different levels.) I'd say it's basically impossible that he figured out a way to refine uranium for $2000.

> I'd say it's basically impossible that he figured out a way to refine uranium for $2000.

I'm sure you could get the cost way down if you traded throughput/time. Tub grinder vs homeowner wood chipper.

I've noticed that HN is very critical of young prodigies. It's hilarious that we are doing something similar here but this time with nuclear weapons insteasd of regular science fair fare.

According to Phillips' book, his main innovation was designing the shaped charges that brought the halves of uranium/plutonium together at the right rate.

This he achieved by calling around at Dupont (IIRC) until he found someone with the knowledge, which was willingly shared. (Presumably this critical info was made inaccessible after the paper was graded.)

Freeman Dyson (who worked on the Manhattan Project and later at at IAS Princeton) reviewed Phillips' design, confiding to the course's professor, "Yes, this should work." Phillips' term paper was promptly classified. His only other feedback was to receive an A in the course.

No expert here; apparently this requires more / more enriched uranium. Since enriching uranium seems to be the bottleneck, this is suboptimal. This is why the Hiroshima bomb worked like this, but the second one, in Nagasaki, worked by compressing plutonium with explosives instead.

There's also the question of "compactness". If you want to carry the bomb by plane, say, it has to be light-ish and compact-ish, which this design lends itself less to. If you want to shoot it up in a rocket, this is even more important.

The article states it was the size of a U-Haul. Nothing flight-worthy

Fat Man was 20k pounds, and about the size of a large automobile.

No it mentions "the actual bomb would fit in the size of a U-haul trailer".

Depends on the airplane you drive it into.

> If you just want a bad atomic bomb, isn't it enough to put 2 chunks of fissile (enough) material at either end of a very thick steel tube and shoot them together using explosives?

Linear (gun-type) fission design was tried at Los Alamos in early days of Manhattan project and was abandoned in favor of spherical implosion device [0].

[0] https://en.wikipedia.org/wiki/Nuclear_weapon#Fission_weapons

Note that the paper is from 1976. $2000 at 1976 is totally different in value than it's today.

$10,414 in today's dollars.

Reply on news.ycombinator.com