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Author: Subject: Mechanics and Physicochemistry of Nuclear Weaponry
Turel
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[*] posted on 18-12-2003 at 21:11
Mechanics and Physicochemistry of Nuclear Weaponry


I just had an inkling desire to yak about nukes for a bit, so I made this post. Usually it is my style to start off with some formal introduction and then get less and less informal and more blunt as the post goes on, but fuck it. I just feel like wasting time and pointing out cool shit, so here goes:

A good place to get an introductory feel for nuke operation is http://people.howstuffworks.com/nuclear-bomb.htm which is pretty accurate in most parts. I will point out the inaccurate parts throughout the post, but most of it is mere nitpicking on my part.

Overall, the Howstuffworks article is pretty accurate, and I won't repeat that information. I'm going to assume everyone reading beyond this has read the Howstuffworks article, and understands the very basics of nuclear chemistry. My first nitpick is on page 4 where it claims that weapons grade uranium is 'at least 90% U-235'. In reality, at least in the USA, weapons grade Uranium is at least 96% U-235 and is often higher. It goes on to talk about supercriticality, and it's definition of such is correct. Criticality is the point at which an equal amount of neutrons produced are also being absorbed, which can technically sustain a nuclear reaction. Practically speaking, it barely even yields a fizzle before the energy produced explodes the mass of material to a subcritical density and the reaction halts entirely. In modern designs, the nuclear mass is brought to hypercritical densities very rapidly through methods I'll illustrate later in this post. Equally important to the level of density achieved for reaction, is the rate at which it is densified to this state. The nuclear fuels used for modern fission stages (99.5% Pu-239) spontaneously undergo fission at just under critical densities. This fission produces energy, heat, and pressure, which begins to cancel out inward imploding force, slowing the implosion and eventually reversing it. The rate constants for the implosion force and the pressure generated by fission are such that the fission pressure grows much faster than the imploding force. Faster implosions reduce the difference between the two constants and allow increased densities to be reached before the implosion reverses, and thus allows more material to react and achieves a higher overall efficiency of conversion.

The concept of critical mass is rather misunderstood. The proper term *should* be (but isn't) critical density. 'Back in the day' the physicists and the technology only allowed for a specific level of compression, which limits the rate of the reaction for a given mass. Thus the quantity of mass able to be reacted was dependent on the mass subjected to the reaction, and this gave rise to a minimum to sustain the reaction at all, called the critical mass, (critical mass of U-235 at the time was about 6kg, which amounts to a pretty sorry implosion force today). Over time, as implosion technology increased, the term's definition was updated to reflect this, but the name never changed.

The neutron generator proposed by the article surprised me in that for the first time it is correct. I have always seen Radium/Beryllium cited as the old neutron generators, which is entirely false. Although not used any longer, Polonium/Beryllium neutron generators were used for decades to initially swamp the reaction center with free neutrons just as the implosion was coming to a halt. The description of the tamper is pretty good, but it omitts the third reason for making it out of U-238 as opposed to some other dense material. Outside the tamper is a massive web of steel mesh powder-coated in a flaky material. The mesh serves to reduce the outward expansion of the uranium tamper, forcing it to expand inward. The uranium transfers heat to the steel, which transfers heat to the flake powder-coating. This powdercoting removes heat by vaporizing off of the steel, slowing the steel's expansion. The use of U-238 is smart because it will not undergo fission with low energy neutrons; if it collides with them, it has a high probability of reflection. Medium energy neutrons get absorbed, followed by transmutation, but not fission. The actual transmutation occurs very little due to the lack of time before total explosion. High energy neutrons are prevented from escaping the device by initiating fission in the U-238, which further heats it up. Uranium is extremely dense, and it's nuclear properties and abundance of the 238 isotope make it a wise choice to implement as the tamper.

Next, more nitpicking, but nothing big. To my knowledge, the uranium gun weapon did not use a uranium bullet, but a plutonium one fired into a uranium sphere. I have seen diagams which describe the device as a sphere split in half and the halves are rammed together. This is obviously incorrect. The sphere of U-235 is imploded with explosives, and a plutonium bullet is fired into the imploding mass, initiating low efficiency fission. The claim that it achieved a 1.5% efficiency is amazing to me, as I would never expect that from such a device. For many years the record efficiency for a Mark 3 implosion weapon was 4.5% efficiency; and even today, the most developed fission stages still can't clear 6%. The next thing that catches my eye is the 'plutonium daggar' design, which I have never seen before. Honestly the first thing that comes to mind is that this is complete bullshit, but I suppose it is possible. Such a device would have horrible efficiency (opinion of course), and would require extremely intelligent and regimented sequencing to get it to work properly. Overall, it is a retarded design in my opinion.

The description of the Ulam-Teller Configuration for a thermonuclear fusion weapon decent. There are a few things it omits, and the illustrated sequence is not entirely accurate. In step one it sums up the primary stage simply as, "The fission bomb exploded, giving off X-rays." This is vastly oversimplified. You know what? As I read this, I realize it will take too much type to repeat data and restate the correct versions. I'm just going to recreate the detonation sequence for a modern day 3 stage thermonuclear weapon based on this design. But first, I need to describe the physical design, because it is different than the original Ulam-Teller in several ways.

The design is similar to the diagram shown on Howstuffworks, with the following changes: That stupid daggar design first stage is not used. It is a multilayered sphere of various metals. The second stage is not so large as to take up the entire volume of the warhead diameter. It is more concentrated, and surrounded with a skirt of honeycomb metal girding to support it. Inside the honeycomb is a special hydrocarbon-impregnated foam, adding additional support, as well as chemical functionality during detonation. The tamper (on the second stage the tamper is called the blast shield) is not pure Uranium, it is pitted and polished U-238 doped with cadmium, and has a thin layer of pure cadmium over it and another layer of pure aluminum over that. Behind the second stage is a pressurized canister of a mixture of deuterium and tritium gas. On one side of the warhead is a lasing device and a series of electrodes and screens. Inside the primary stage, there is no neutron generating mass of polonium/beryllium alloy. Instead there is a tube and related support called the carriage assembly, and the pit, which is a walnut sized sphere of lithium-6 deuteride with a pea sized sphere of Pu-239 inside it. The carriage assembly extends all the way to the surface of the pit, and is about 10mm inner diameter and about 3cm thick. I don't know what it is made of.

Detonation:

1.A Sphere of layered explosive shaped charges begin their detonation sequence. The sequence, timing, and layering is engineered to take advantage of standing waves (this in turn influences the design of the thickness of each layer of material within) to create a succession of shockwaves and cancellation waves to rapidly compress and impart strong momentum to all regions of the inner spheres, not just the surface. Shock waves are the main driving forces for implosion, not sheer overpressure.

2.Meanwhile, control mechanisms have started to stream a mixture of D/T gas from the canister down the carriage assembly, up along the side of the weapon. Here the gas becomes largely ionized by passing through screen electrodes and is immediately introcuded to a pseudo-standing wave laser, in which the nodes and antinodes are dynamic points, accelerating the deuterons/tritons toward the front of the device. At the front of the device, their velocity is altered to vector the ions into the main carriage, the tube that extends to the pit surface. Along this tube they are accelerated further by short period blasts of microwaves (no longer accelerated by the pseudo-standing wave) from small versions of a device called a klystron. The timing is such that the tritons and deuterons strike the pit at as close to the optimum implosion density as achievable.

3.Fission is induced in the pit. The temperature skyrockets, as does the internal pressure and photon emission rate. The measures explained before regarding the tamper are in place to increase fission time.

4.The blast shield protects the fissile material in the secondary stage. The aluminum layer reflects many of the X-ray photons away, and absorbs what it can't reflect, heating up as it does. Neutrons pass right through the thin, not-dense layer of aluminum like it is not even there. The slow neutrons are captured by the layer of cadmium metal, and faster neutrons are both captured and reflected by the U-238 underneath the cadmium layer. The pitted and polished surface of the U-238 shield reflects neutrons at a different angle than they enter at, where they are deflected into regions of thicker cadmium skirting, increasing their absorption. These measures prevent premature fission of the hollow plutonium rod in the second stage.

5.The entire inside surface of the outer casing is covered in reflective aluminum. The massive quantity of radiation produced is vectored to the sides of the secondary stage, where it permeates the honeycomb support and foam, and exerts pressure on the tamper surrounding the secondary stage itself.

6.The foam is specially engineered to absorb the majority of shorter wavelength radiation, and vaporize (chemical decomposition) and re-emit lower energy radiation to be passed onto the tamper. The foam exerts tremendous overpressure on the surface of the tamper, causing it's expansion from heat to be largely inward, compressing the nuclear fuel inside.

7.The volatized foam has decomposed into gaseous products that are largely transparent to the radiation. The hollow plutonium rod compresses inward, with decent uniform compression. The reason it is hollow is to create a region of lesser density in the very center of the tube, where the mean free path of the produced neutrons is greater than the path outward from the center. This allows neutrons to race along the length of the fissile rod, ensuring even fission distribution along the length of the rod, reducing fizzle.

8.The heat, energy, and pressure, as well as the extreme density of the region inside the secondary stage initiates fusion reactions inside the cylinder of LiD.

9.Excess neutrons produced that escape are largely absorbed by the tamper of U-238 and induce fission inside the tamper. This final fission stage is the reason modern bombs are called 3 stage bombs. Fission primer, Fusion bomb, Fission shell. In reality, they are 4 stage bombs with a possibility of being called 5 stage.

The purpose of the pit is to introduce fusion and neutron excess into the primary stage, boosting yield and dramatically increasing radiation output, by about 500 times. The small surface area of the pit ensures massive radiation of energy and neutron escape. This process is called 'Boosting'.

Modern nuclear devices are much smaller than those deailed on howstuffworks. A modern warhead can be carried by four people, by hand. It weighs less than 600 pounds. But these warheads are also not the superbombs of yesteryear.

More to come later if members are interested.

-T

[Edited on 19-12-2003 by Turel]

[Edited on 19-12-2003 by Turel]
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[*] posted on 19-12-2003 at 14:02


Already in the sixties the US-Army had nuclear warheads to be carried in a rucksack or to be fired by a 150mm cannon. I guess those weighed much less than 600 pounds.



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[*] posted on 19-12-2003 at 17:24


Turel,

A good start at an advanced subject matter, but you pick an easy target and introduce almost as many errors as you correct. A large number of sites on the internet have in depth reliable information. Try starting with,

http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Nucle...
http://www.fas.org/nuke/intro/nuke/design.htm

In addition to information on those pages, you may want to consider completely replacing....
The concept of 'implosion reversal' with 'premature detonation', and its causes, spontaneous fission, should warrent quite a lot of space.

The 'pseudo standing wave laser' which sounds absolutley nothing like a laser, but does sound rather like a linac setup for neutron stripping.

The rant about critical density, as the notion of a critical density is just as flawed as the notion of critical mass, yet critical mass is useful and it also cuts out your rather suspect ideas about 'rate of reaction'.

Most other things should be corrected by the above 2 URLs, which contain relativly few errors, one that I could find was the oft quoted idea that the Gadget tower vapourised.
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[*] posted on 20-12-2003 at 00:04
Just FYI


The record to my knowledge for the smallest nuke belongs to the Spetz Natz(GRU agents,ussualy thoght of as commando's)at just 1ton.Though they are mainly rumors the verifed is 10 tons by the US.

Bigest I belive curently are 100 megaton russian's.
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[*] posted on 1-6-2004 at 01:58


Turel i thought the VOD of the HE gets spead up through a vacum chamber, the waves then crash into Heavy water, the netrons get reaped of,and then travel into the plutium, the uriunm is just used as the fulla.

Even though electrons from the explosive might be used instead of netrons.
"Warheads to be carried in a rucksack", i think they were dirty bombs, ie spread radation
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[*] posted on 10-7-2004 at 03:41
Atomic Weapons


I have a DVD called "The Atomic Bomb Movie" narrated by William Shatner.
One of the devices shown in this flick is "Atomic Annie", an artillery piece that
fires a 10-inch shell with a 15 kiloton yield. I was impressed with how that
much power was condensed from a 10,000 LBS. bomb to an artillery shell.
The DVD was released in 1999 and even includes 2 sets of 3-D glasses for viewing.
The movie didn't state the weight of the shell even though it may have been useful.
I'm going to rip the .VOB files and upload to KAZAA if anybody is interested.
I'm new here so if you have any other place I can upload this flick I'd be more
than happy to do so.




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[*] posted on 10-7-2004 at 07:51


Plenty of details here:
http://nuclearweaponarchive.org/

The secret to being a bore is not to leave out a single detail.;)
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[*] posted on 11-7-2004 at 15:56
.VOB


The files are available on KAZAA. They're named TRINITY_x.VOB. The 'x' is a number
from 1 to 4. They can be played in WinDVD.


[Edited on 11-7-2004 by MadHatter]




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[*] posted on 11-8-2004 at 20:57
Backpack nukes


There was an article in an old SOF magazine about the nuclear program in Vietnam. They talked about "backpack nukes". There were 2 designs, both weighed suprisingly little, I don't recall exactly how much, but I'll look. They were both aound the order of half a megaton each IIRC.
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[*] posted on 11-8-2004 at 22:22


I do know the russians have a design of a backpack nuke. IIRC it weighs under 50 pounds and had I think a yeild of 2 Kt. Ill do some searching and see what I come up with.

EDIT http://www.humanunderground.com/archive/fbi-nukes.html

[Edited on 12-8-2004 by fvcked]
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[*] posted on 12-8-2004 at 07:06


Quote:
Originally posted by MadHatter
I'm going to rip the .VOB files and upload to KAZAA if anybody is interested.
I'm new here so if you have any other place I can upload this flick I'd be more
than happy to do so.


If it's OK with axehandle, you could upload them to his FTP.




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[*] posted on 21-9-2004 at 00:01


On the orion project, a space ship powered by nukes, they were 50kg.
"The mesh serves to reduce the outward expansion of the uranium tamper, forcing it to expand inward. The uranium transfers heat to the steel, which transfers heat to the flake powder-coating. This powdercoting removes heat by vaporizing off of the steel, slowing the steel's expansion. "The white powder will proalbe be a high C,H compond, styroform works good, the uranium realese netrons which can't find there way through the different placements of C,H atoms, it has a opaque effect, producing plasma by heat, ie prtons,electron striped off, the netron then goes into the plutoum.
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