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Comment by danbruc | Hacker News Reader

... if you happen to have a critical mass of enriched Uranium [1] laying around in your garage.

[1] Preferably not in a single lump.

Apparently it's a plutonium design, which is interesting because if it's using implosion then the detonation timing and quality of the explosives is commonly thought to be critical (notably the "Fat Man" plutonium bomb was tested before being dropped on Hiroshima, whereas the uranium bomb design wasn't all-up tested before Nagasaki as it was held to be obviously reliable)

I imagine that between the '40s and '70s many of the really hard problems became a lot easier. E.g. using solid-state electronics for the timing, modelling shaped charges as a textbook exercise rather than a research problem, etc. What surprises me is that he managed to get a dirt-cheap initiator.

Maybe he designed a geometry that can achieve the required implosion speeds with slow, easily available explosives ex. AN, KN, AP etc. The size of the device (a container) seems to suggest this.

The actual initiation is the easy part and would have been easy in the 70s, exploding wires that are subjected to a number of kilo Ameperes by some kind of avalanche, solid state switch.

I meant a modulated neutron initiator, not the explosive detonator. India was still using vapour deposition of polonium to make theirs, around the same time. I'm no engineer but that sounds like it costs a lot more than $2000.

HN never ceases to surprise! I am really curious how you happen to know such details about India's nuclear bomb.

The internet probably. [1]

[1] https://nuclearweaponarchive.org/India/IndiaSmiling.html

The cheap initiator is a solved problem since 1989.

Yeah, we've got better at making neutrons since 1970. Mostly this is a good thing!

Since his design dates from 1976, you reckon it would be cheaper now?

> using solid-state electronics for the timing

Curious... is timing actually that big of a deal when you've already got critical mass? What would happen if you just dropped one half critical mass from 1m above to the other half?

With an implosion type bomb you need to make sure that the shockwave from the explosives hits the plutonium pit all around it almost simultaneously. If there was a portion where the explosive shockwave was delayed then the forces on the rest of the pit are going to tend to "squish" it out towards the portion where the explosion hasn't hit it yet. This asymmetrical compression means now you don't get the ideal compressed sphere so now while it's still going to reach criticality, it's going to be blasted apart with much less of the fissile material being fissioned. A great real world example of why timing is so critical is the first North Korean nuclear bomb test.

https://en.wikipedia.org/wiki/2006_North_Korean_nuclear_test...

They had a yield of less than a kiloton. Obviously way greater than conventional explosives but it should have been an order of magnitude more powerful.

In order for the explosives to form that crucial explosive lense you need detonation across numerous points on the outside simultaneously so you need a detonator that's extremely consistent on timing so a simple resistive detonator with some primary explosives packed around a filament is not going to be effective. The method of choice is an exploding bridgewire design. Basically you have a thin wire and you pass enough current through it to vaporize the wire so quickly that the wire itself generates the initial shockwave as it gets turned into plasma. The rest of the explosive lense is just two different types of explosive with different propagation velocities. The two are combined with a parabolic (or hyperbolic, I forget off the top of my head) face between the two layers so that the explosion that started from a single point goes from convex to concave so it hits the outside of the pit all at the same time.

If you simply dropped a half-critical mass onto another and it was prompt critical then yes, it would explode, but with not nearly as much energy being released as a proper nuclear weapon.

Excellent, thanks for that detailed explanation!

It would preignite and fizzle. No nuclear yield, but a pile of radioactive stuff.

ah, ok thanks for that.

Even though OP had their research taken away, I wonder how long it will be before you could simply ask GPT-3 how to make a nuke for under $2k

I mean if you are starting from plutonium and assuming it's gonna cost $2k you are lying to yourself. I can only imagine... "Step 1. buy 5 kg of plutonium for $1k".... I think plutonium is 4 mil per kilogram according to google.

which means that's even harder to come by bc unlike uranium there's no naturally occurring plutonium on Earth

It seems you have the names and cities reversed.

Hiroshima / 1945-08-06 / Little Boy / gun type Uranium / untested

Nagasaki / 1945-08-09 / Fat Man / implosion type Plutonium / tested with Trinity 1945-07-16

isn't getting the weapons grade Uranium (or Plutonium) what is most difficult about making a bomb anyways? My assumption is, once a terrorist group has achieved that, and has transported / smuggled it to their target, it's game over anyways. Harbor towns must be particularly vulnerable to this, considering the amount of shipping containers going through there...

For a simple bomb you want Uranium, Plutonium emits too many neutrons which are likely to cause a small chain reaction while your critical mass is in the process of forming and this predetonation can then blow everything apart before a proper critical mass has been formed. A gun type design where you just smash two subcritical lumps of metal into each other is, as I remember, nearly impossible to achieve with Plutonium you can realistically produce.

EDIT: Interestingly another commenter says that this actually was a Plutonium design.

No, making a true atomic bomb is extremely difficult science and engineering.

Now, a dirty bomb, which is just a conventional explosive together with radioactive material - designed to spread radioactive material instead of releasing the fission energy into one giant explosion - is easy to create, but not that much more effective than just a conventional bomb. It would trigger a massive cleanup effort, and it would surely give some people cancer later down the line, but it wouldn't be that spectacular.

No, it is not that difficult, at least if you are fine with not having the most efficient weapon. You take a metal pipe, put a lump of Uranium and some explosive at each end and to a first approximation you are done. The USA did not even test their gun type design before dropping it onto Hiroshima because it was so simple that they were confident that it will just work.

Extracting u238 from 235 is a hard process. Using flourine gas, it requires you to turn an ore, to a metal, then to gas and then back to metal. If terrorists can get that done they should quit terrorism and just open a chemical plant

But they would not, as terrorism isn't about making money.

Terrorism isn't about solving extremely hard chemical engineering scaled process problem either.

It's not about becoming a commercial pilot or truck driver either but those are a means to an end.

I'm not aware of any terrorist who actually became a commercial pilot, merely those who took a few lessons (1). And I think that's at the upper end of the lengths that terrorists will go to: Past a certain point the low-tech approaches are just way more "bang for the buck", so to speak. And past a certain specialisation of skills, it's way more effective to get that high-paying job and donate to political causes.

1) https://www.sandiegouniontribune.com/lifestyle/people/sdut-g...

It’s not hard to get uranium. You can buy it on Amazon. It is enriched uranium that is hard to come by. I’m guessing the low cost for this project is that he found a way for low enriched uranium to go supercritical.

Enriched Uranium was of course implied but I edited the comment now. Another commenter mentioned that it was a Plutonium design, in which case the Plutonium was almost certainly not part of the bill.

....and you don't mind being lifted out of bed at 3am by the security services in your country, face a lengthy interrogation and a potential prison sentence.

I feel(*) it's not about the critical mass, but the critical density - that's why you need strong compression (either chemical explosives, or xrays from a smaller nuclear bomb) to trigger the explosion (exponential runoff chain reaction).

So this guy figured out how to create the critical density for 2k USD worth of components, which is actually the "hard" part of building a bomb.

(*) not a nuclear physicist

You need to compress not the solid chunks of metal, but the gaseous / plasmatic plutonium after a few microseconds past the critical point. The problem is that it emits neutrons too quickly, and the thermal runaway of the critical mass happens too quickly. If you do not hold the plutonium together, it will just fly in all directions before enough of it has reacted.

No static casing us string enough; a dynamic caring made of highly compressed results of a conventional explosion works.

You don't have to compress the material if you have enough but if you can compress it, then you can achieve detonation with less material.

But it's a plutonium design, which according to common wisdom is too fissile for slow assembly but needs both compression and enough inertia to stop it from exploding with a very low yield.

I meant in general, for Plutonium you realistically need an implosion type design, but I think theoretically you could even have a gun type Plutonium bomb, just making clean enough Plutonium is totally uneconomical.

And you still need a looooong barrel and a fast moving plutonium part (see the thin man design on wikipedia and then extrapolate from there) even creating near close enough pure plutonium since economically you won't have pure.

"Parsons’ first order of business was the plutonium gun. Because it needed a muzzle velocity of at least 3,000 feet per second it would have to be 17 feet long. It should weigh no more than a ton, a fifth of the usual weight of a gun that size, which meant it would have to be machined from strong high-alloy steel. It would not require rifling but needed three independently operated primers to make sure it fired. Parsons arranged for the Navy’s gun-design section to engineer it."

- Richard Rhodes - The Making of the Atomic Bomb

Can you rotate a plutonium "carousel" fast enough and then make all the pieces crash into a wall, onto each other...

Edit: Idk, something about flywheels, they can store a huge amount of energy and release it very quickly.

Probably not, we are talking really short timescales here. The nuclear explosion is over before you can even see the first crack in the housing of your bomb.

Or you can just buy a (literal) boatload of smoke detectors (americium) and medical waste (curium) and make an americium bomb - or use a neutron gun to transmute your americium into plutonium via curium.

The last time someone was going down that road it did not end too well.

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