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Author Topic: Why do U235 Nuclear bombs need Polonium  (Read 9076 times)

Offline syhprum

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Why do U235 Nuclear bombs need Polonium
« on: 23/10/2009 11:26:47 »
Apparently the U235 gun type nuclear bomb used by America against Japan used a Polonium initiator as a Neutron source to enhance the explosion.
After the war a great deal of basic detail was released about the design of such bombs and separation techniques for U235 extraction but no mention was made of the use of Polonium (drawings showed an Xray tube like device as an initiator).
This was supposed to be a well kept secret which the Americans kept from their UK allies.
To what extent is this initiator needed, would there only be a small yield without it ?.   


 

Offline Mr. Scientist

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Why do U235 Nuclear bombs need Polonium
« Reply #1 on: 23/10/2009 12:57:42 »
Well, essentially polonium is about 400 times more radioactive than the well-known uranium. Polonium for bombs is used because it acts as a neutron source - and in standard models of such quantum behaviour, neutrons become as a perfect particle for chain reactions - devistating chain reactions.
 

Offline syhprum

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Why do U235 Nuclear bombs need Polonium
« Reply #2 on: 23/10/2009 13:26:31 »
As the half life of Polonium is 139 days whereas that of U235 is about 700 million years I feel that the ratio of their radioactivity that you quote as 400 to 1 might well be an under statement !.
As the unfortunate result of the 'tickling the dragons tail' dragons experiment showed even chance accumulation of a critical mass results in massive radiation, I would be interested to know what yield a bomb would produce without an initiator.
Details of the design of 'Little boy' are available here

http://en.wikipedia.org/wiki/Little_Boy

I note with interest that three APS-13 radar altimeters where employed as I owned one just after the war also that the warship 'Indianapolis' that carried the bomb to the airbase was sunk as it returned.
« Last Edit: 23/10/2009 17:24:16 by syhprum »
 

Offline Mr. Scientist

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Why do U235 Nuclear bombs need Polonium
« Reply #3 on: 23/10/2009 13:49:20 »
Well, you could attempt to investigate hisory's bombs and compare the resultant energy from implosion or explosion. Usually Bombs which implode are classed under fission nuclear bombs.
 

Offline LeeE

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Why do U235 Nuclear bombs need Polonium
« Reply #4 on: 23/10/2009 20:17:42 »
To what extent is this initiator needed, would there only be a small yield without it?

From:

http://en.wikipedia.org/wiki/Nuclear_weapon_design#Pure_fission_weapons

"Once the critical mass is assembled, at maximum density, a burst of neutrons is supplied to start as many chain reactions as possible."

...which implies that less chain reactions would start without the initiator.  This being so, I suspect that the reduced number of chain reactions would result in a lower yield because less of the fissionable material will have fissioned before the critical mass of material blew itself apart, and below super-criticality, from the chain reactions that did start spontaneously.  Lol - it's also theoretically possible, although incredibly improbable, that no spontaneous chain reactions start, or at least, not within the required time frame.  Another factor may be that without an initiator to start the chain reactions in a designated region, the chain reactions that do start spontaneously will do so at random points within the critical mass, once again leading to the critical mass blowing itself apart before the required number of reactions have occurred.  Having the initiator start the reactions in a known and designated region would also allow some degree of control of the progress of the reactions through the material by design.

Re the design of the initiator...

"In modern weapons, the neutron generator is a high-voltage vacuum tube containing a particle accelerator which bombards a deuterium/tritium-metal hydride target with deuterium and tritium ions."

Well, you could attempt to investigate hisory's bombs and compare the resultant energy from implosion or explosion. Usually Bombs which implode are classed under fission nuclear bombs.

All bombs work by exploding and the term 'implosion' only refers to the method by which super-criticality of the fissionable material is achieved.  In nuclear weapons, super-criticality is not just dependent upon mass but also the surface area of the mass.  With the Gun type of device we're discussing here, two sub-critical (for their surface area) masses are brought together such that their combined surface area is low enough to achieve super-criticality.  With an 'implosion' type device a super-critical mass, which originally occupies a higher volume, and which therefore has a higher surface area than is required for super-criticality, is compressed to occupy a smaller volume, with a smaller surface area, thereby making it super-critical.  The fissionable material does not implode by itself, but is imploded to compress it by conventional explosives.
 

Offline syhprum

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Why do U235 Nuclear bombs need Polonium
« Reply #5 on: 23/10/2009 21:11:36 »
LeeE

It is interesting to read that a vacuum tube initiator is used in modern designs as something similar was shown in the early drawings of nuclear bomb designs long before the development of British bombs.
I presume someone asked British researchers about initiators and they either did not know about 'urchins' or that they were highly classified so they gave the answer of the classical neutron generating devices
I wonder which the British designs used Polonium or vacuum tubes ?.

http://nuclearweaponarchive.org/Library/DocumentArchive/Resources/PenneyPuWeapon.html

This report written in 1947 recommends the use of 'urchins'.
« Last Edit: 23/10/2009 21:35:25 by syhprum »
 

Offline LeeE

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Why do U235 Nuclear bombs need Polonium
« Reply #6 on: 24/10/2009 22:43:04 »
Hmm... it does seem a bit confused.  I've never really looked into the different types of initiator, and when they were introduced.
 

Offline litespeed

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Why do U235 Nuclear bombs need Polonium
« Reply #7 on: 01/11/2009 17:59:57 »
I do not believe U-235 bombs REQUIRE polonium or any other augmentation design to work.  Create a supercritical mass and that mass will forthwith dissasemble itself in very short order. 

Weaponization means you want to maintain that critical mass chain reaction for as long as possible.  The manhattan project was so confident their Uranium device would work, they did not even test it.  However, they did add augmentation neutron sources to increase yeild.

However, believe me you never ever want to create a supercritical mass of highly enriched uranium on your project counter ever.  It has happened a few times accidentally and resulted in some pretty deadly effects. I think a small research reactor at Hanford(?) went supercritical and dissasembled the entire reactor.
 

Offline syhprum

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Why do U235 Nuclear bombs need Polonium
« Reply #8 on: 01/11/2009 18:07:11 »
If merely assembling a supercritical mass was sufficient it would have destroyed all of Hanford not only the reactor!
 

Offline litespeed

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Why do U235 Nuclear bombs need Polonium
« Reply #9 on: 01/11/2009 18:16:01 »
An unconstrained critcal mass of Uranium is so sudden as to end the reaction almost immediately. It has actually happenened in Uranium Piles.  In the one case I sort of remember, there was not actually much in the way of physical damage.  However, the person who 'dropped' the extra brick onto the pile did not survive the momentary flash of radiation.

This is one reason that Uranium is so unsafe to the bomb maker.  For instance, I do not believe any quantity of plutonium will go suppercrittical without mechanical compression. If you are making a Uranium bomb, however, you will inevitably be required to bring to sub-critcal masses in close proximity.  In addition, you will also need to add a device to assemble the two into a supercritical mass. Simply drop one of the pieces accidentally into the other, and well, you get rapid dissasembly
 

Offline litespeed

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Why do U235 Nuclear bombs need Polonium
« Reply #10 on: 01/11/2009 18:32:14 »
Incidentally, I am not certain the 'little boy' bomb actually used additional neutron sources.  My memory on this matter is sketchy, but I seem to recall the augmentation materials were neutron reflecors placed around the area where critical mass was to take place. The term Boron comes to mind, but I do not stand by this without additional research, which I am uninclined to do just now.
 

Offline syhprum

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Why do U235 Nuclear bombs need Polonium
« Reply #11 on: 01/11/2009 19:03:36 »
http://en.wikipedia.org/wiki/Little_Boy

The reason the the little boy was not tested was because only a very limited supply of weapons grade U235 was available and there was an urgent need to demonstrate to the USSR that they had the means to destroy whole cities before the Japanese surrended.
The only means to produce U235 before centrifuges were developed was by very inefficient gaseous diffusion plants and cyclotron like devices an extremely expensive process.
The USA built 5 Little boy bombs but never produced enough U235 to make them operational.
The USSR was unimpressed they were well informed by their agents as to what resources the USA had available.
 

Offline LeeE

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Why do U235 Nuclear bombs need Polonium
« Reply #12 on: 01/11/2009 20:33:42 »
...For instance, I do not believe any quantity of plutonium will go suppercrittical without mechanical compression.

Sorry litespeed, but the Hiroshima bomb was an 'uncompressed' gun-type weapon.  Several other gun-type weapons were also developed and successfully tested, although I believe that most of them were cannon shell type weapons.  Gun-type weapons were inherently unsafe though, relying upon a single triggering charge, and were soon dropped from inventories; they were almost as much of a risk to the countries holding them as they were to the countries they might have been used against.
 

Offline litespeed

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Why do U235 Nuclear bombs need Polonium
« Reply #13 on: 01/11/2009 22:33:02 »
Lee,

I think we agree on this point. The Uranium bombs did not need to be tested because critical mass was a given. Projecting one subcrital mass into another could have been accomplished by a simple spring system.  Using high explosives to assemble the critical mass was simply obvious way to increase duration.

Further, I have never explored whether the high explosives used to assemble the critical mass also compressed said mass.  It seems reasonable that such was the case, but I don't know.

Its all moot. The success of the plutonium device was complete. Easy to produce plutonium. Safe to work around machining technolgy without danger of inadvertent critical mass. Essentially immune to accidental discharge etc etc. An obvious winner.

However, the uranium device seems to be the method of choice for Iran.  For all the same reason listed above. Guarnteed critical mass with nothing more then, well, a nice charge of black powder. Incidentally, didn't the N. Koreans produce a Plutonium Dud not that long ago?
 

Offline litespeed

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Why do U235 Nuclear bombs need Polonium
« Reply #14 on: 01/11/2009 22:38:08 »
syhprum

The Uranium bomb did not need to be tested. As expalained above, the problem with such weapons is not that they will or will not detonate, but that they detonate where required. The implosion plutonim device requred testing because of the critical nature of the conventional explosion  sequence. No such sequence existed with Little Boy.  Light the fuse and it WILL assemble a critical mass.
 

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Why do U235 Nuclear bombs need Polonium
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