What exactly is a nuclear bomb fireball?

Hi all, this might be a very stupid question, but i have searched and cant QUITE get my head round what exactly is happening at the moment, say, the 2 Pu hemispheres in a bomb get pushed together and explode. Is it just a question of the tremendous energy is being released mostly as heat? if so, what exactly is catching on fire? is the fireball actually...fire? is it in essence no different to a regular bomb that uses conventional explosives? But in those, the chemicals inside are igniting and creating the flames. What is creating the flame in a nuclear bomb

By fireball, i mean this:

a reply to: 3danimator2014

The fireball, an extremely hot and highly luminous spherical mass of air and gaseous weapon residues, occurs within less than one millionth of one second of the weapon's detonation. Immediately after its formation, the fireball begins to grow in size, engulfing the surrounding air. This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time the fireball rises, like a hot-air balloon. Within seven-tenths of one millisecond from the detonation, the fireball from a 1-megaton weapon is about 440 feet across, and this increases to a maximum value of about 5,700 feet in 10 seconds. It is then rising at a rate of 250 to 350 feet per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 4.5 miles from the point of burst.

www.atomicarchive.com...

a reply to: Zaphod58

Ok, so its not really a flame then. Its just f*cking hot air. I guess i'm still having a hard time trying to visualise in my head the process. I mean, why does it explode in the first place? After all, high temperatures don't always equate to explosions. And the criticality accidents never resulted in explosions. Just huge bursts of radiation.

I realise it would be easier for me to google all this, which i am, but a discussion would also be nice, which is why i posted rather than just searched.

a reply to: 3danimator2014

This will give you an idea of what the core looks like.



That's a cutaway of one of the early bomb cores. The inner portion is the nuclear material, and the panels around it are explosives. The explosive has to detonate pretty precisely, but when they do they compress the nuclear material, and it goes critical and detonates.

For this reason, if you use plutonium to fuel a bomb you need to use the more sophisticated "implosion" method. With this approach the nuclear fuel is shaped into a sphere (called the "pit"). Conventional explosives are put around it. When these are detonated the force of the explosion squeezes the pit into a supercritical mass long enough for the explosion to take place. While the principle sounds easy, it is difficult to actually make it work. The pit cannot simply be surrounded by high explosives. The shock wave that compresses it must be precisely spherical, otherwise the pit material will escape out through a weak point. To create the necessary explosive force in a perfect sphere, shaped explosive charges (sometimes called explosive lens) are used. The "fatman" bomb the leveled Nagasaki in World War II used 32 charges arranged around the pit like the faces of a soccer ball. In order to create the spherical shock wave it isn't only necessary to get the charges in the right position with the right shape, but they must be detonated at exactly the right time. A charge that detonates late will create a hole in the shock wave through which the pit can escape.

Implosion designs also require a neutron trigger or "initiator" to flood the pit with neutrons during detonation. In "fatman" this was done with a small sphere with layers of beryllium and polonium separated by thin gold foil placed in the center of the pit. An implosion design may also include other layers between the explosives and the pit to create a more powerful explosion. These include a "pusher" (designed to increase the explosive shock wave hitting the pit), a "tamper" (to help the pit from blowing apart too quickly once the explosion starts), and a "reflector" composed of a material that will reflect neutrons back in the pit increasing the amount of fission. In some bomb designs these functions are integrated into a single layer of material.

www.unmuseum.org...

There's also the "gun" method where the two halves of the nuclear material were separated. One side was fixed, the other was movable. An explosive charge was detonated behind the movable portion, and it went down the tube that held it, and slammed into the fixed half, and the chain reaction event occurs.

The "gun" is the simplest way to build a nuclear weapon. The atomic bomb used on Hiroshima during World War II used this approach. The weapon consists of a tube (much like the barrel of a gun) with half the nuclear charge fixed at one end and the other half (the moving half) at the opposite end. A conventional explosive charge was placed behind the moving portion which can be thought of as the "bullet." When the conventional charge is detonated, the bullet races down the tube and slams into the fixed charge at the other end (referred to as the "spike"). Once the two halves of the nuclear fuel are brought together and held together long enough, the chain reaction starts, the fuel goes supercritical and the explosion takes place.

While the gun method is easy to engineer, it has some drawbacks. The biggest one is the need to make sure the two parts of nuclear fuel come together rapidly enough. As the two sections get about an inch apart, they will start exchanging neutrons that might start a chain reaction. If the two parts go supercritical before they get close enough, the force of the energy released will blow them apart before the main explosion gets underway. This type of failure is known as a "fizzle."

www.unmuseum.org...

a reply to: Zaphod58

Thanks for taking the time to put all that info there, very interesting even if i kind of knew all that already, BUT part of the answer i was looking for was at the end..it seems there are various stages of criticality in fission and that would explain why Slotin and the others in the room didn't explode when he made his fatal mistake. This is probably good since making a nuclear bomb would be exceedingly simple otherwise. Nuclear reactions are absolutely fascinating.

The key lies in making it go as high on the "criticality chart" (cant think of the right word) as quick as possible.

I do remember reading a very interesting article about the various things that can affect criticality of a mass of U, Pu, Np or whatever. Things like the SHAPE of the object is vital, which is why a ring or Pu like this one doesnt pose much of a risk. But spherical is bad. I also read, quite incredibly about one criticality incident where something was dropped onto the fissionable material, thus compressing it just enough to start a chain reaction for a microsecond. Crazy

upload.wikimedia.org...

a reply to: 3danimator2014

It's a bit of a pain in the ass. If it's not just right, no boom.

a reply to: 3danimator2014

Ok, so its not really a flame then. Its just f*cking hot air.

A bit more than that. The moment of fission or fusion of the mass of 'fuel;' in the pit of the device in an uncontrolled chain reaction that results in the initial 'fireball' like Zaphod linked…

…occurs within less than one millionth of one second of the weapon's detonation.

Each atom of the central pit of the device is split or fused liberating photons which we see as the 'initial' flash, so we don't really see the initial fission or fusion because of the free emission of photons that white out our vision. Like staring at the sun. You don't see the fusion going on, just the liberated light and heat.

By the way, the ensuing thunderclap is the inrushing air filling the vacuum left behind in the next moment. Like after a bolt of lightning, the noise generated is from the in rushing air to refill the ionization channel in the path of the bolt.

a reply to: 3danimator2014

It is not fire it is superheated plasma, when the neutrons smash into the unstable atom's and cause a chain reaction those atoms are broken into more than two pieces, most of the matter is released in the form of heat, light, particle radiation and kinetic energy and this has to go somewhere, of course as it leaves the area around it the atmosphere becomes just that, a fireball, the light and heat is intense enough to liquefy metal at close range and also to vaporize it, the kinetic energy send's out a shock wave that blows air outward with hundreds of miles an hour wind's.

In a vacuum such as space there would not be that wind only the small amount of plasma form the decomposited bomb itself and of course the light and heat, other than that it would simply irradiate an area around it with high intensity neutron's, indeed that is why in sci fi movies like for example Deep Impact the atom bomb has to be placed inside the asteroid because placing it along side it would have negligable effect, it would melt some of the surface and create a small amount of thrust as Neutron's then impacted and vaporised a small amount of surface material but other than that it would create only the small amount of thrust this would induce to the asteroidal body.

So that small fire ball is Superheated Radioactive Plasma.

a reply to: 3danimator2014

The key lies in making it go as high on the "criticality chart" (can't think of the right word) as quick as possible.

Right. The pressure on the central pit in the core needs to be maintained equal from all sides just long enough to begin the chain reaction which rapidly involves the whole mass of fuel, In an instant.

Like the ping pong ball mousetrap demonstration on various YouTubes. Stylized, but makes the point. Oncew fission begins it consumes the fuel in the pit in the bomb in a nano second

a reply to: LABTECH767

So that small fire ball is Superheated Radioactive Plasma.

In effect a small sun, for an instant, down here on earth. Instead of where its supposed to be , at the center of our solar system.

a reply to: intrptr

Exactly it is made up of the same type of energy though it is as you know different in one fundemental way, it is product of fission rather than fusion.

originally posted by: LABTECH767
a reply to: intrptr

Exactly it is made up of the same type of energy though it is as you know different in one fundemental way, it is product of fission rather than fusion.

Another difference is size. The containment of the bomb pit is momentary being too small to sustain the reaction. ''The sun for instance is a nuclear reaction so large gravity keeps it together, sustaining its 'sun' quality for billions of years.

a reply to: intrptr

I might pick your brains on something if you are familiar with it, I heard about these back in the late 70s early 80's.

Neutron bomb's.

I was always curious about Neutron Bomb's, I think but don't know that maybe they are something similar to controlled form of the Fizzle Zaphod58 mentions when a bomb misfire's.

Or maybe they are just based around a slightly less fissionable isotope but they supposedly theoretically have the effect of of liquefying the population of a city near there epicenter, causing massive cellular damage at medium to long range while leaving only low residual particle radiation and the majority of the structure of a city standing and undamaged, I believe this makes them the danger for the near future.

As resource wars and overpopulation loom I can envisage one side or the other foregoing the ban and fielding these at some point and I think this is a nightmarish but a definite possibility.

Basically as I recall they are essentially just a mechanism for releasing a vast amount of high energy neutrons, kind of a very low yield high neutron output nuclear device which have the effect of releasing these high energy neutrons over an area smashing into long chain molecules breaking apart there weak electron bond's such as in organic material, people, plastics and rubber at close range, people at medium to long range being the main area of damage (damage vectors) and perhaps better called depopulation bomb's as supposedly an occupying force could then just waltz in and take control of the city, replace a few parts, dispose of the dead and off they go with a whole new city/industrial site/military base for themselves and all of it's resources and infrastructure more or less intact for the taking.

a reply to: LABTECH767

I was always curious about Neutron Bomb's, I think but don't know that maybe they are something similar to controlled form of the Fizzle Zaphod58 mentions when a bomb misfire's.

Hi order, not a fizzle at all. The shielding that usually re directs the neutrons back into the chain reaction in the form of shielding is purposefully thinner in Neutron bomb casings, allowing the neutrons out 'sooner', lowering the over all explosive yield of the weapon so as to reduce the damage to populations and infrastructure.

Should open nuclear warfare have broken out in Europe between NATO and the Soviet bloc, it was thought that soviet armor could be destroyed by killing tank crew members inside their massed tanks without doing as much damage to Europe overall (insert big laugh).

The highest yield weapon is full containment with 'reflectors', the lower yield neutron bomb is still hi order nuclear detonation, the blast radius of which would be somewhat smaller, but not much. Part of an older cold war maxim that nuclear war was fightable and winnable (insert more lol).

One should consider the Hiroshima bomb, by any definition a 'lo yield' device' comparatively speaking, streamed xrays sufficient to expose all the X-ray film in the hospital basement vault several miles from the epicenter.

Wiki, neutron bomb

a reply to: Zaphod58

So does the explosion burn all the organic material within,or is it "disassembled"at the molecular level?(Need a better term)

Just curious.

a reply to: Ericthedoubter

It depends on how close you are. At the point of the fireball there's very little if any organic material left because it's so hot. As you go farther out more survives.

the nuclear bomb is an awesome, terrifying and incredible scientific achievement.
sadly i'm convinced we've not yet seen the last one used in conflict.

a reply to: intrptr

The bit where one ping pong ball goes into the pepsi syphon hole just gave me an idea: The world's longest but most completely incorruptible game of bingo!

I am not volunteering for the set up after every ball though.

originally posted by: Ericthedoubter
a reply to: Zaphod58

So does the explosion burn all the organic material within,or is it "disassembled"at the molecular level?(Need a better term)

Just curious.

Inside the initial plasma ball itself where the actual chain reaction is occurring as the fissile material is consumed, nothing survives.

Further out from that the radiant heat melts metals and sets combustibles alight. Still further out the compression or blast wave applies over pressure to building structures and hollow objects and then swats them. Like putting an empty soda can on a table, crushing it and then batting it off the table.

I keep a pic around from Hiroshima…

This bronze Buddha was melted by heat from the Hiroshima bomb. Bronze melts at around 1600 degrees F. The temperature on the ground beneath the exploding Hiroshima bomb reached about 7000 degrees. Hiroshima Peace Museum, Hiroshima, Japan. November 13, 1984.

a reply to: 3danimator2014

I believe that it has a specific physical meaning. It is the sphere of highly ionized hot plasma which is relatively opaque to electromagnetic radiation (because it has many free charges), and because it is hot, is radiating a large amount blackbody electromagnetic radiation from its surface.

Compare to the surface of the sun, the "photosphere". Why is there apparently a sharp boundary? Because there is a thermodynamic/electromagnetic transition in density such that further out the material is transparent to optical EM radiation, but closer in, it is opaque and the radiation and matter interact strongly. There happens to be a reasonably sharp boundary; I don't remember the reason.

There may be some chemical combustion in the fireball of a nuclear weapon if it is detonated on something combustible, but for the most part I think the energy was supplied by the initial nuclear release and possibly nuclear decays from fission products.