REALLY Heavy metal work

Still the critical component is not described. It is the mechanism to trigger the conventional explosive segments all at the identical time. The key word being - identical. The failure of these devices in the last North Korea test probably caused the dud. Until Iran is able manufacture the devices and get them to work properly, all the uranium in the world will not get them the bomb.

Paul

Well, that *is* really a problem. All of the plutonium in the world won't get them a bomb, but the gun trigger for Little Man could probably have been made in a good basement shop. That's why it's a lot scarier for them to have enriched uranium than plutonium. Any goof can make a gun-trigger uranium bomb, if they have a plan that gives them useable dimensions.

The US made a few gun-trigger bombs, and the Brits made a few, and then we both scrapped them. They're dangerous as hell: anything that will set off the explosive charge, like lightening or a big spark, can, theoretically, make them go "boom."

Gun triggers won't work with plutonium because the bomb will self-destruct before the chain reaction is more than a fizzle. BTW. it takes a much larger uranium pit than a plutonium pit to make a bomb.

That should be "Little Boy," the Hiroshima bomb. We had never tested a gun-tirgger bomb before we dropped Little Boy.

Its so big because it is hollow. The chemical implosion crushes it and mixes it with the neutron trigger in the center, starting the chain reaction.

Modern devices have a solid 239 Pu sphere in the center that is compressed to about 2 X normal density to create the critical mass. The reaction is started by a pulse linac shooting protons at a target timed just right so the neutrons get there at the instant of maximum compression. The Swan device uses only two exploding wire triggers and 2 explosive lenses of varying propagation rate that form a spherical explosion front. They have a "driver" and a tamper to enhance the compression and delay the flying apart of the compressed pit.

Some of these Swan devices are only a little bigger than that "pit" in your picture!

Jon

That's the first good news I've heard in a long time.

I hope Iran does not develop or purchase a nuke. Too many targets they would love to destroy. Israel, USA, etc.

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Paul

Somewhere on the web -- I think I found it on Wikipedia -- there's a discussion about early thoughts on gun-trigger plutonium bombs, along with a picture of some ten prototypes that were scrapped. They were very long and very skinny, to try to get the velocity up enough to avoid the effect that you mention.

Apparently they originally thought that a gun-triggered Pu bomb would work, and it was one of the big names (Oppenheimer or Feynmann) in physics that re-did the calculations and figured out that it was a no-go.

Yeah, in theory you can make a gun trigger for plutonium. I've seen values of barrel length ranging from a few hundred meters to a kilometer.

In fact, when N. Korea exploded its first bomb in a tunnel, I wondered (and still do) if that's what they tried.

Right. They pursued both trigger mechanisms until they realized that gun triggers weren't going to be practical with plutonium.

In talking with people about this over the years I find that few people know that the Hiroshima bomb was an untried gun-trigger device, but that the Nagasaki bomb was an implosion device based on the Gadget used in the Trinity test.

And it disturbs me a bit that reporters often rely on stories about the extreme difficulty of building a working spherical-implosion trigger, not noticing whether it's plutonium or uranium that's being used for the pit.

I find it scary that Iran went for uranium.

Well, I think the major difference between the nuclear programs of N. Korea and Iran is than Iran wants to blow up Jews, but N. Korea just wants to extort foreign aid from the West and convince their home-boys that the leadership has really big balls.

So for N. Korea Pu is the best, because they can scare the hell out of the dimwits with money, but the real experts won't be rattled. Whereas for Iran U-235 is the best, because they may actually be able to vaporize parts of Tel Aviv.

That's an interesting take. I'd like to hear it put that way from an analyst on a TV show sometime. It would liven things up.

A nice technical document:

On 2/24/2012 1:19 PM, Tim Wescott wrote: ...

Been my feeling as well...N Korea also knows when push comes to shove China won't let them dirty their backyard. OTOH, there's nobody in the MidEast other than Israel to take oversee that area.

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Anybody that has more than a gram of 239 Pu ALREADY has enriched uranium. You need "weapons grade" enrichment, meaning about 90% 235U) to build a breeder reactor to make 239 Pu. It has to be a fast neutron reactor to transmute the 238 U at the edge of the core to 239 Pu, and that requires a high proportion of 235 U for the chain reaction to continue. (I'm not a reactor physicist, but that's my understanding.) So, if they have kilograms of 239 Pu, they MUST have tons of pretty pure 235 U.

Jon

I wonder where they got the 235U?

Any peaceable nuclear reactor needs U-235 to fire the reaction -- it's just that the common type (non-breeder) doesn't need much.

I'm not so sure about the absolutely needing a breeder reactor -- AFAIK, you get Pu from ordinary reactors, just in small quantities. In fact, (again, AFAIK), part of the reprocessing of nuclear fuel is getting the Pu out.

Yeah, that jives with what I think I remember.

But it takes a certain quantity of 235 to get rolling.

I seem to remember that it takes more material to build a reactor than a bomb...

Except that with uranium you can use a simple gun. The imploding-sphere design is necessary to work around the properties of plutonium, which has to form a critical mass much more quickly than uranium or you get a fizzle. Uranium just has to have two pieces slammed together really hard.

By the way, this timing "magic" was achieved with 1940s electronics. Shouldn't be any problem today.

If Pakistan can do it Iran can do it, don't pretend that it's beyond them.

great article, I love it!

Uranium gun was believed to be so reliable that they felt they didn't need to test it.

Well, there's no other way though: the only way to get plutonium is to refine it from spent fuel created in a conventional U235 reactor. In short, neutrons from U235 fission hit the normally inert U238 and convert it to Pu239

Yeah, but it still seems to be an amazing leap of faith. They hadn't tested uranium in a bomb, and they hadn't actually used the gun trigger in a bomb test. That's some confidence in the science, all of which was still pretty uncertain.

Right. But producing weapons-grade uranium is a vastly bigger project, in comparison with using a reactor to produce plutonium.

Again, what seems to be missing in the press accounts is these points: