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unionised
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"The implosion system for a Pu or U core is also going to be a problem. "
Unless you do it the easy way, don't use one, and build a gun type device. OK it needs more Pu or U but since it's just an academic exercise that
doesn't matter. After all if you can get the first kilogram the second one is easy.
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akinmad
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| Quote: | Originally posted by unionised
"The implosion system for a Pu or U core is also going to be a problem. "
Unless you do it the easy way, don't use one, and build a gun type device. OK it needs more Pu or U but since it's just an academic exercise that
doesn't matter. After all if you can get the first kilogram the second one is easy. |
As far as I know gun type device cannot be made with Pu, since Pu needs faster assembly timing than U, due to neutron cross section (???).
IIRC, during Manhattan project, they determined a gun type device shall work and did not even test it. But calculations indicated that Pu cannot be
used in a gun type device and they developed, implosion system.
[Edited on 15-4-2006 by akinmad]
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12AX7
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I remember something like that. I don't happen to remember which metal goes with which. 
+/-100ns is easy to get electrically, so it's up to you explosives chemists to figure out the lens itself. 
Tim
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a_bab
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It's a matter of getting the detonator detonate at an exact moment, +/-100ns.
The problem is that usually it may take some time for a lasy detonator to perform it's function when needed (that is, the delay of let's say 200 ns is
going to ruin the bomb, and even if the lens will go off the Pu would fail to go overcritical and will become part of a 'dirty bomb')
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IrC
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Less than 20 percent U235 will go critical. http://www.sciencemadness.org/lanl1_a/lib-www/la-pubs/003394...
Gas diffusion was used in the 40's, where the lighter 235 was more likely as a gas fluoride to travel through the pores in a sintered bronze wall than
the 238 was. There was nowhere near the high tech needed then to build the modern centrifuges. The Pu device needs rapid assembly as the Pu241
impurities even in trace amounts make so many neutrons it fizzles the bomb before assembly. One way around this may be explosive driven collapsing
rings.
There seems to be much speculation going on about all this and it is easy to find some useful information.
One place you may gain some insight is: http://nuclearweaponarchive.org/Nwfaq/Nfaq0.html
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hinz
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| Quote: | Originally posted by a_bab
It's a matter of getting the detonator detonate at an exact moment, +/-100ns.
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Why not just using some detonating cord. If you would take one chuck of some kind of explosive and stick as much det. cords in it as you need
detonators. The delay you would be able to controll with the lenght of the cord, longer cord=>more time till the detonation reaches the explosive
around the bomb. The little differences in the VoD doesn't matter, I think. The only problem is that you need some distance betwen the cords to
prevent that the detonation moves from one cord to another.
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Chris The Great
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That is actually a very good idea. But I don't know if det cord has the proper tolerances... I guess you could buy a bunch and measure it's VoD 100
times and see how much error you get...
Still, exploding bridgewires on a 10kV pulse cap with a few thousand joules of energy is going to get the tolerances right if fired with a sparkgap
switch. The problem would be making sure the exploding bridgewire hits every cap equally, not whether they all explode simultaniously.
While it is true that 16% U-235 will go critical, remember the critical mass they found was "only" 692kg of it. The size of the explosive charge to
drive that and the engineering difficulties in making it... plus getting that much uranium... And then the final size of the bomb 
Gas diffusion requires even larger plants than centrifuges. 40's tech, not ultra-complex, but still wholly impractical.
The collapsing ring thing is old news I thought, at least I got the impression it has been around for a long time.
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IrC
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I am sure it is even older than we know, but I notice it does not get mentioned much in Pu assembly time conversations for some reason or other.
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Marvin
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Det cord in various forms was tried during the manhatten project. It simply isn't good enough.
I suspect something like a marx circuit would work as an electrical system.
I dont know anyone who would have the first clue how to design an implosive lens though.
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Chris The Great
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Thanks for the info on detcord Marvin. Got a source or anything on it? It could (unlikely) be that their det cord simply was 50's stuff and sucked.
The fact they said it can't work means nobody else looked into it... a lot of old stuff is like that for us "amatuers".
The concept of an explosive lense is quite simple and can be solved by simple trigonometry. The explosive lense is basically a cone, the inner
explosive is a low VoD type and then there is a layer of high VoD explosive on the outside. The high VOD explosive detonates much faster and as it
detonates it initiats the low VoD explosive- the outside detonates faster than the inside. This, when done correctly (you need to work out the
rations for whatever explosive is used) forms the detonation into a piece of a semi-circle. With many cones on the surface of a sphere, you get a
spherical implosion wave.
I have designed one, it was actually fairly small, but the design was very straightforward.
The concept is covered in Brassey's World Military Technology; Explosives, Propellants & Pyrotechnics, starting on page 36. It covers several
methods of wave shaping as well as a type of implosion device. You'll notice that the design can be streamlined to the one I describe above by simply
removing the excess high VoD explosive not needed to transmit the detonation.
The challenge comes in making your entire explosive lense assembly uniform. You can't have any varying densities in the explosives, etc. It is more
of an engineering problem than a design one. You'll need to check all your VoDs beforehand as well.
Idea: for testing your implosions system, use Xrays. A steel tamper will allow you to "see" the center of the bomb, which you could put something
like tungsten or other VERY dense material in. The steel will be visible from the sides, but not from the middle, so you can see if it collapses
evenly! Difficult, yes, but you're building a mock nuke implosion system.
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Mr. Wizard
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This web site is one of the most comprehensive you will ever find. It is full of details that will educate and amaze you. If anyone knows of a better
one, please let us know.
http://nuclearweaponarchive.org/
I love this quote:
"...And these atomic bombs which science burst upon the world that night were strange even to the men who used them."
H. G. Wells, The World Set Free, 1914
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Chris The Great
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I remember the site now! It is a great collection of information. I printed off the entire thing several years ago. It had since slipped from my
memory.
It is a great site for understanding the basic to more advanced concepts of nuclear weapons. I have never seen a site that even comes close to it.
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Marvin
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I don't have a reference for the detcord, it was a casual remark from someone else who looked into the technology more seriously and shattered the (I
thought) rather cunning design I had in my head.
The basic science behind the implosive lens is quite simple and given the refractive index of a transparent material n is the ratio of c to the speed
of light through it, I'm sure convensional lens making math would give *an* answer.
The first major problem I have is that the tabulated speed of a shockwave through the material depends not only on the density but also the degree of
confinement, steel versus copper tubes, how wide the tube of explosive is and the answers seem barely accurate to 10% under even the best
circumstances. I have no idea how to model a shockwave through bulk material. I did wonder about working out the values to a focal point and then
testing on steel plate to refine the numbers.
The second major problem I have is not only is it about focusing the expanding shockwave segments so they converge on the core, its also about turning
the convex wavefront produced by the detonator into a concave one so you have a spherical implosion, a solution I don't think is automatically
satisfied by solving problem one (whats to stop the inner and outer edges of each det wave segment reaching the same point at the center, but at
different times?). This to me seems akin to controlling the phase of an optical wavefront as well as its direction and is the sort of problem that
wants to make my brain try to escape through my ears. All using a medium with a det velocity that depends on the strength of the det wave and many
other factors.
Of course when those are all solved its all just engineering
I'm with you on the X rays, for a U-235 core the obvious option is to try ordinary uranium metal. It ought to behave almost exactly like the real
thing (aside from going critical of course). The biggest problem here is you really need very bright, very short pulses of X rays to take pictures.
Short of doing the detonation next to a disposable synchotron or a short distance from someone elses successful A bomb, I haven't solved this part
yet.
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akinmad
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| Quote: | Originally posted by Marvin
The basic science behind the implosive lens is quite simple and given the refractive index of a transparent material n is the ratio of c to the speed
of light through it, I'm sure convensional lens making math would give *an* answer.
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IIRC, there is a document in LANL set, which describes how a plastic wave shaper shapes the shockwave in a slapper type detonator.
I mean, it seems to me, the weapon designers do not use traditional explosive lenses which are initiated at a single point, but rather than slapper
detonators which are capable of initiating large areas, thereby eliminating need for complex lens designs.
Again in the Trigger List I mentioned above there was a section dedicated to slapper detonators. Regards.
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Marvin
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The plane wave capability of slappers seems to be based on large numbers of individual detonators over a surface. The documents suggest half an inch
apart, given electrical and size restrictions over implosive lens which is all solved by design, this seems a poorer solution.
If slappers could be produced in a very uniform way and initiated in large numbers, this might be satisfactory for getting a bang.
I can't find anything on waveshapers with slapper in the title.
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Chris The Great
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Yes, lots of detonators will work in place of a spiffy implosion system (they did this for the Trinity test). However, an implosion system will use
far less explosive and be much much much smaller. I'd rather desgin something that could possibility be transported to a remote location for getting
a "bang".
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Organikum
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Without getting into details about the actual construction of a nuclear weapon: Doesn´t a natural uranium reactor as this canadian design breed
plutonium which can be more easily separated then U235? Requiring just natural uranium (maybe slightly enriched which could be done with simpler
technology then centrifuge arrays) and heavy water?
The CANDU reactor is in my understanding a breeder able to breed about anything including plutonium. The production of heavy water was discussed here
on this board and should be no big obstacle when coupled for example with a large scale electrolytic seawater desalination installment - very low
yields compensated by the sheer mass of water processed.
/ORG
PS: I looked this up and it seems that India and Pakistan used heavy water moderated reactors for breeding plutonium for nuclear weapons.
Why does Iran go through the trouble of centrifuge arrays if they just want a nuclear weapon? I believe that Iran is technologically far more advanced
then Pakistan is btw.
This is a technical question, please keep politics to the other threads in Whimsy.
[Edited on 21-4-2006 by Organikum]
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a_bab
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Among the first nukes any nuclear country had were the uranium canon type ones. Plutonium is difficult to implode, so I'd go for uranium if I wanted
something simple.
On the other hand Iran declaired that they need the uranium for civilian purposes; that may answer your question.
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Marvin
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No.
First nuke detonated ever - plutonium. First nuke the UK had, plutonium. First nuke india had, plutonium.
The complexity of the implosion system is dwarfed by the problem of enriching the uranium. Not even by technical requirements, but sheer scale. Add
to that the very poor performance of gun type designs, large fuel requirements, poor yeild, the advantage of uranium vanishes.
Building a reactor just for plutonium is a fairly big event, you'd need in the region of 100 tons of natural enrichement fuel. This is probably the
smallest, easiest to hide method. Graphite is probably the way to go too, D2O is expensive and very visible (Needs a high capacity infrastructure).
In the long run D2O is far superior of course.
Power reactor on the other hand more or less needs fuel enriched to 3-5% 235 to be cost effective. Iraq had enough of this for 2 research reactors
and did nothing with it (Isreal bombed the facilities they were planning to use them in). The problem is a genuine power producing reactor could also
make bomb grade plutonium on the side, and would be very difficult to monitor. If you are alowing enrichement to 3 or 5% in the sort of quantity that
would fuel a large power plant, this could also be abused to produce weapons feasable material. Its the infrastructure, rather than the threat of the
first bomb.
The USA loses track of several bombs worth of plutonium a year. Maybe its being stored by someone for military purposes, maybe it ends up in the
buiried waste, maybe it doesnt even exist and is an artifact of essaying errors. But if the USA can't keep tabs on its own plutonium production
accuratly, monitoring another country would be impossible.
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a_bab
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Yeah, you are right. My mistake. I was reading about the Pakistan and I guess they had uranium bombs, before they switched to Pu ones.
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Marvin
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After checking a few things, I'm not as right as I thought. US, USSR, UK, India and France all had plutonium bombs first. Pakistan, South africa and
China had HEU. China used implosion though, the pakistan is a 'don't know', about the only people to consider gun type seriously were the South
Africans, and they never tested it. Isreal was rather more successful in keeping things secret, but seems to be plutonium by the time the world found
out.
Certainly the gun type design is a fringe weapon but more countries had the opertunity to use HEU than I'd have thought.
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JohnWW
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| Quote: | Originally posted by unionised
(cut) OK it needs more Pu or U but since it's just an academic exercise that doesn't matter. After all if you can get the first kilogram the second
one is easy. |
Do you know of anyone who sells plutonium, or either enriched or unenriched uranium? Importing the stuff may be difficult - several countries,
including New Zealand, ban the importing of plutonium or enriched uranium.
Also, which isotope is the plutonium? - Pu-239 is the most easily made, from spent enriched-uranium fuel rods in which neutrons convert the U-238 to
mostly Pu-239, which is that usually used in bombs and reprocessed fuel rods, but Pu-244 is by far the longest-lived and least radioactive isotope,
sufficiently to occur in trace amounts in uranium ores.
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akinmad
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| Quote: | Originally posted by JohnWW
Also, which isotope is the plutonium? - Pu-239 is the most easily made, from spent enriched-uranium fuel rods in which neutrons convert the U-238 to
mostly Pu-239, which is that usually used in bombs and reprocessed fuel rods, but Pu-244 is by far the longest-lived and least radioactive isotope,
sufficiently to occur in trace amounts in uranium ores. |
Problem with Pu extracted from spent Fuel Rods, is that it is reactor grade plutonium (i.e. it contains a lot of even numbered Pu isotopes), which
causes the deviced made with them to fizzle not to detonate.
In order to the make Weapon grade Pu, one has to cook U-238 in the reactor with slower (or more thermalized) neutrons for a period shorter than from
an ordinary commercial power reactor. IIRC, Israelis were burning their fuel rods for a period of ca 100 days in the reactor then remove the fuel rods
and extract plutonium.
For that reason, IAEA inspectors install seals on the reactor vessels to determine if the fuel rods are removed prematurely. Regards
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