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  #1  
Old 09-12-2012, 03:08 AM
kombatminipig kombatminipig is offline
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Smallest (by mass) nuclear weapon

Full disclosure: this is research for a book, so no need to call the feds.

Question is simple, what is the smallest feasible (by mass) thermonuclear warhead that could theoretically be constructed? The device itself can be of any size, but the main plot point is that the fissionable material needs to be light enough to be transported in a suit case.
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  #2  
Old 09-12-2012, 03:27 AM
Der Trihs Der Trihs is online now
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Theoretically? At one point there was talk of nuclear bullets using californium, before it became obvious each would cost billions.
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Old 09-12-2012, 03:36 AM
Ranchoth Ranchoth is offline
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Originally Posted by kombatminipig View Post
Full disclosure: this is research for a book, so no need to call the feds.

Question is simple, what is the smallest feasible (by mass) thermonuclear warhead that could theoretically be constructed? The device itself can be of any size, but the main plot point is that the fissionable material needs to be light enough to be transported in a suit case.
Just a point; does it need to be a thermonuclear warhead, or would a simple fission warhead suffice? And what kind of yield are you aiming for?

(Also, for the record, the W54 warhead, which was actually built and used in several different weapons, had a weight of about 50 lbs., and was about 11"x5".

Last edited by Ranchoth; 09-12-2012 at 03:40 AM..
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Old 09-12-2012, 01:35 PM
Blakeyrat Blakeyrat is offline
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Originally Posted by kombatminipig View Post
Full disclosure: this is research for a book, so no need to call the feds.

Question is simple, what is the smallest feasible (by mass) thermonuclear warhead that could theoretically be constructed? The device itself can be of any size, but the main plot point is that the fissionable material needs to be light enough to be transported in a suit case.
Critical mass of Uranium-233 is 15 kg, which is approx. an 11 cm sphere. From wiki.

11 cm sphere would be doable in a large suitcase, but I don't know how big all the support infrastructure needed to make it actually work would add.
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Old 09-12-2012, 01:41 PM
Qwertol Qwertol is offline
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Originally Posted by Blakeyrat View Post
Critical mass of Uranium-233 is 15 kg, which is approx. an 11 cm sphere. From wiki.

11 cm sphere would be doable in a large suitcase, but I don't know how big all the support infrastructure needed to make it actually work would add.
Isn't critical mass smaller when you are using neutron reflectors?
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Old 09-12-2012, 01:44 PM
Patch Patch is offline
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Originally Posted by Blakeyrat View Post
Critical mass of Uranium-233 is 15 kg, which is approx. an 11 cm sphere. From wiki.

11 cm sphere would be doable in a large suitcase, but I don't know how big all the support infrastructure needed to make it actually work would add.
The backpack nuke (or more accurately the Special Atomic Demolition Munition) on display at a museum in Albuquerque, New Mexico, is about the size of a large household trashcan mounted on a backpack frame. It weighed about 150 lbs.
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  #7  
Old 09-12-2012, 01:50 PM
Gray Ghost Gray Ghost is offline
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I know I've cited to it before, but Cary Sublette's Nuclear Weapon Archive, might be of interest to you.

From it, he calculates (at 4.2.3.1):
Quote:
Since volume increases with the cube of the radius, a thick layer of anything (explosive or reflector) surrounding the fissile core will add much more mass than that of the core itself.

Referring to the Reflector Savings Table 4.1.7.3.2.2-3 we can see that for beryllium thicknesses of a few centimeters, the radius of a plutonium core is reduced by 40-60% of the reflector thickness. Since the density difference between these materials is on the order of 10:1, substantial mass savings can be achieved. At some point though increasing the thickness of the reflector begins to add more mass than it saves, this marks the point of minimum total mass for the reflector/core system.

In general, minimum mass and minimum volume designs closely resemble each other. The use of a hollow core adds negligibly to the overall volume.

The absolute minimum possible mass for a bomb is determined by the smallest critical mass that will produce a significant yield. Since the critical mass for alpha-phase plutonium is 10.5 kg, and an additional 20-25% of mass is needed to make a significant explosion, this implies 13 kg or so. A thin beryllium reflector will reduce this, but the necessary high explosive and packaging will add mass, so the true absolute minimum probably lies in the range of 10-15 kg.
This is assuming you have Los Alamos to design it for you and Rocky Flats/Pantex to fabricate it for you. Parameters for a terrorist-designed weapon are covered later in the cite, and he estimates their mass at >100 kg, 200kg for a plutonium device.

EDIT: Oh, and per Ranchoth above, this is for a one stage fission weapon. Multi-stage devices aren't dealt with. The parameters for the W80 might be a good place to start.
Quote:
The W80 is physically quite small, the "physics package" itself is about the size of a conventional Mk.81 250 pounds (110 kg) bomb, 11.8 inches (30 cm) in diameter and 31.4 inches (80 cm) long, and only slightly heavier at about 290 pounds (130 kg).

Last edited by Gray Ghost; 09-12-2012 at 01:53 PM..
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  #8  
Old 09-12-2012, 03:01 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Originally Posted by Blakeyrat View Post
Critical mass of Uranium-233 is 15 kg, which is approx. an 11 cm sphere. From wiki.

11 cm sphere would be doable in a large suitcase, but I don't know how big all the support infrastructure needed to make it actually work would add.
But what would happen when you try to carry that suitcase through the metal detector in the airport?
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  #9  
Old 09-12-2012, 10:31 PM
Ranchoth Ranchoth is offline
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But what would happen when you try to carry that suitcase through the metal detector in the airport?
...attach it to a nose ring, so you can sneak it past on the little tray?
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  #10  
Old 09-13-2012, 12:09 AM
Patch Patch is offline
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But what would happen when you try to carry that suitcase through the metal detector in the airport?
Tom Sizemore got one though claiming it was a garbage disposal. You might give that a try.
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  #11  
Old 09-13-2012, 01:34 PM
MikeF MikeF is offline
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The Holy Hand Grenade of Antioch was quite small but I don't think it was nuclear.

http://www.youtube.com/watch?v=xOrgLj9lOwk
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  #12  
Old 09-13-2012, 01:54 PM
billfish678 billfish678 is offline
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As noted a decent neutron reflector does add significantly to the mass and volume required if you want a something like a functional nuclear weapon in the kilotons range rather than a fizzle in the hundreds of tons range.

But it just occured to me you could (possibly) design a "backpack" nuke that normally wouldn't work too well...unless you made a point of placing it a large body of water before setting it off.

Pool party anyone?
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  #13  
Old 09-13-2012, 02:19 PM
The Second Stone The Second Stone is online now
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Originally Posted by Der Trihs View Post
Theoretically? At one point there was talk of nuclear bullets using californium, before it became obvious each would cost billions.
I've heard Halfnium would also work similarly.
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  #14  
Old 09-13-2012, 05:26 PM
Kevbo Kevbo is offline
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King of the wild frontier
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Old 09-13-2012, 05:52 PM
Bosda Di'Chi of Tricor Bosda Di'Chi of Tricor is offline
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Originally Posted by Kevbo View Post
I knew that would come up.

How did we ever get that crazy?
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  #16  
Old 09-13-2012, 06:30 PM
Gray Ghost Gray Ghost is offline
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Originally Posted by Bosda Di'Chi of Tricor View Post
I knew that would come up.

How did we ever get that crazy?
Being outnumbered 5-1 + in tanks tends to make one come up with unusual solutions. Like the neutron bomb.

The W-54 that Ranchoth mentioned up above, is the warhead at the heart of the Davy. As well as the SADM. I think it's amazing that we haven't been able to improve on its size in 50 years. So far as you and I know. I guess the old Soviet 'nuclear briefcases' that Alexander Lebed and Stanislav Lunev went on and on about, may have been smaller., though probably not as small as that mockup that Rep. Curt Weldon was holding up.

I can't stop laughing, looking at the pictures of the Davy Crockett. It looks like something right out of the Fallout series, like it should be on the cover of a Tom Swift book. I guess they test-fired it at the end of U.S. atmospheric testing and it worked. You couldn't get me within 10 miles of that thing launching.
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Old 09-14-2012, 11:47 AM
Big G Big G is offline
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I happen to be reading "Project Orion" by George Dyson, about the development of a nuclear bomb powered spaceship in the 50's and 60's. One of the people involved was Ted Taylor, who had spent time at Los Alamos trying to see how large and small you can make fission bombs. In Chapter 6, he is quoted as saying "I was narrowing my focus, getting the quantities of plutonium that one could use to make nuclear explosions, down into less than a kilogram. Quite a bit less." He also talks about a "...full implosion bomb that you could hold in one hand that was about six inches in diameter."
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Old 09-14-2012, 12:01 PM
Big G Big G is offline
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I got ahead of myself and posted before I read the whole thread, but I missed the edit window to add this:

The beginning the the second quote from Ted Taylor that I excerpted above references the Davy that Kevbo brought up.

""I tried to find out what was the smallest bomb you could produce, and it was a lot smaller than Davy Crockett, but it was never built in those years," says Ted. "But it certainly has been since then."" ("Project Orion" by George Dyson, pg 55)
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Old 09-14-2012, 12:09 PM
billfish678 billfish678 is offline
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Originally Posted by Big G View Post
He also talks about a "...full implosion bomb that you could hold in one hand that was about six inches in diameter."
Oh, you can probably make a nuclear device pretty damn small.

It just depends on what you want to call a "nuclear weapon".

A 10 kiloton blast yeild. Nuclear Weapon for sure. 1 kiloton? A 100 ton? One ton? A hundred pounds? Ten pounds?

Note that for an Orion like project,even a crappy bomb that weighed, oh lets say 20 pounds, but had a one ton yield, would be vast improvement when it came propulsive efficiency over even the best chemical rockets. Thats roughly a hundred times more energy per mass.

Last edited by billfish678; 09-14-2012 at 12:10 PM..
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Old 09-14-2012, 01:01 PM
Gray Ghost Gray Ghost is offline
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..One of the people involved was Ted Taylor, who had spent time at Los Alamos trying to see how large and small you can make fission bombs. In Chapter 6, he is quoted as saying "I was narrowing my focus, getting the quantities of plutonium that one could use to make nuclear explosions, down into less than a kilogram. Quite a bit less." He also talks about a "...full implosion bomb that you could hold in one hand that was about six inches in diameter."...

""I tried to find out what was the smallest bomb you could produce, and it was a lot smaller than Davy Crockett, but it was never built in those years," says Ted. "But it certainly has been since then."" ("Project Orion" by George Dyson, pg 55)
The W48, used in the 152 and 155mm nuclear artillery shells, was about the diameter you cite. Interestingly, the wiki for the W48 mentions a quote from Dr. Taylor that 105 mm nuclear artillery shells were possible. He certainly had style; I don't know of any other man who lit his cigarette from a nuclear explosion.

Browsing around, I have seen figures for Pu-239 critical masses that were on the order of a kilogram. They required dissolution in water, and were the bottom of the extrapolated curves of critical mass, but that's still quite a bit smaller than the 10.5 kg Sublette cites for the critical mass of a bare alpha-phase sphere in air. More exotic configurations have been proposed and experimented with, including the layering of thin foils of Pu-239 atop thicker slabs of D2O and then dunking the whole thing in liquid helium. Evidently, the critical mass for Pu-239 can get down into the sub-ounce range in such experiments.

I wonder if, since water is such an effective moderator, and an effective moderator helps improve the likelihood of a neutron fissioning an actinide nucleus, how effective would ice be in a device design? If you dissolved Pu into water, and froze the lot, would that lower the required amount of Pu?

I'm sure I'm missing something fundamental here.
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Old 09-14-2012, 01:32 PM
Gray Ghost Gray Ghost is offline
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Originally Posted by billfish678 View Post
...Note that for an Orion like project,even a crappy bomb that weighed, oh lets say 20 pounds, but had a one ton yield, would be vast improvement when it came propulsive efficiency over even the best chemical rockets. Thats roughly a hundred times more energy per mass.
You're right, and it was amazing to me when I went through the basic numbers and saw just how much more efficient it was. The Orion wiki has the equation for calculating Isp. Plugging in their number of 0.5 for collimation, and Dyson's own number for debris velocity of 3000 km/s, yields an Isp of around 150,000 s^-1. (Admittedly, this is derived from the energy in a deuterium fusion reaction.) The Isp for large rocket engines like the retired Space Shuttle Main Engine is in the 350-450 s^-1 range. You still have to build the bombs for an Orion though, and I wonder what the cost of that is compared to liquefying hydrogen and oxygen?

There have been several plans for exotic modifications of the scheme since then. I think building a 2 km ring out of ceramic semiconductor is going to be a bit difficult.
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  #22  
Old 09-14-2012, 04:44 PM
Lumpy Lumpy is offline
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The thing about the Orion nukes is that unlike a bomb that has to be delivered, for the Orion scheme the size and weight of the nuclear charges didn't matter, at least within very broad limits, given that they were talking about ships with a gross weight of thousands or tens of thousands of tonnes. Nor was yield critical, given that the whole scheme revolved around coping with almost more yield than could be withstood. The main design focus of the Orion nukes was obtaining a useful explosion from the absolute minimum amount of fissionable material. All other components of the charges- depleted uranium, lithium deuteride, beryllium, etc.- would be orders of magnitude cheaper.

In general, your standard fission implosion design depends on two things: compressing the fissionable material to the smallest possible volume, and then keeping it from blowing itself apart before the chain reactions have had a chance to occur. The first is done by the explosive charges that compress the core- the more explosive you can pack on, the higher a density you can achieve. And then the core is surrounded by what's called a "tamper", a shell of dense material that by simple inertia gets in the way of the nascent explosion and holds it in for a few extra microseconds, allowing more chain reaction generations to occur.

For Orion, you could use both a charge of explosive and a tamper way too large and heavy to be practical for a bomb or warhead, but ideal to enable each charge to use the smallest possible amount of fissionable material. And the extra weight was actually useful, since the design goal was to convert as much of the bomb's energy as possible into kinetic energy in the form of a hot fast moving plasma, and the extra mass facilitated this.
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