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Comment by srean | Hacker News Reader

More qualified people will correct me, but I think there is more to it than dropping a piece of fissile material over another. I think the trick is to keep the pieces together long enough before the forces of explosion rips them apart.

As far as I know there have been several examples of accidents were scientists did actually drop a fissile blob over another, and the material did go critical, but thankfully the scientist(s) involved were fast enough to separate the pieces so that no catastrophes ensued. They themselves died as a result of exposure to radiation. As I said, more qualified people will be able to fill in the details and citations.

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> More qualified people will correct me, but I think there is more to it than dropping a piece of fissile material over another

“With modern weapons-grade uranium, the background neutron rate is so low that terrorists, if they have such material, would have a good chance of setting off a high-yield explosion simply by dropping one half of the material onto the other half. Most people seem unaware that if separated HEU [Highly Enriched Uranium] is at hand it’s a trivial job to set off a nuclear explosion … even a high school kid could make a bomb in short order.” - Luis Alvarez, Nobel Laureate in Physics, 1988

The key being, as I understand it, actually getting weapons-grade material of sufficient purity. Though the actual design of a practical, controlled and efficient, weapon would doubtless be more somewhat more complex. For which the n-th country project is well worth reading about:

http://www.theguardian.com/world/2003/jun/24/usa.science

That is correct. i work in nuclear material security and safeguards. For what I can say, which isn't much, it's the security of the material (HEU and Pu) that makes it hard to make a bomb. Its hard to enrich, and furthermore you need HEU or Pu. LEU won't get you a big boom. Without enough enrichment, no Bomb.

Theft and diversion is a big deal with Pu and HEU for this very reason. Which is why access to either has a two man rule. And because you can safely handle Pu and HEU,(its just an alpha and criticality hazard so unless you inhale or eat them, or you have bad geometry it's no big deal), that makes it vital that you carefully account for all of it. It won't kill the thief, and you don't need much Pu for a bomb.

I suspect you're referring to this:

https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_accid...

"At 3:20 p.m., the screwdriver slipped and the upper beryllium hemisphere fell, causing a "prompt critical" reaction and a burst of hard radiation. At the time, the scientists in the room observed the blue glow of air ionization and felt a heat wave. In addition Slotin experienced a sour taste in his mouth and an intense burning sensation in his left hand. Slotin jerked his left hand upward, lifting the upper beryllium hemisphere and dropping it to the floor, ending the reaction. However, he had already been exposed to a lethal dose of neutron radiation."

That wasn't even the first fatal criticality incident involving that particular plutonium core (known as the "Demon Core"). Funnily enough, I submitted an interesting article just a few hours ago on the history of this core:

https://news.ycombinator.com/item?id=6229011

I'm sure a physicist can get the details of this better than I can, but I'm pretty sure it's sufficient to create a subcritical, hollow sphere of fissile material and then slam a matching sphere of fissile material into it very fast.

The "very fast" portion could be accomplished with a steel pipe and explosives. The manufacturing would certainly be within the capability of a reasonably skilled group of insurgents/freedom fighters. The big question is where they'd get the fissile material.

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