How does a nuke work?

[Aghanim]

I know how a conventional explosive work, its like this:

Detonator explodes -> Main charge starts to undergo a fast chemical reaction-> The charge changes into high pressure fireball -> It explodes. Right?

Now how does a nuke work? This is what I do know

Initiator detonates -> Nuclear fuel pile somehow starts to react -> ........??? -> Very large explosion

Now, after searching in wikipedia and stuff, I found out about fizzle and gun-type and Teller-Ulam design, but how do a pile of metal suddenly becomes a large fireball? I understand the slow nuclear reaction, the neutrons from fission causes more fission, creating heat, but why a huge amount of fission create an explosion?

[Streetwind]

The key concept you need to understand is "critical mass". If you have more than that amount of a weaponsgrade nuclear material in a single pile, then its own radiation will accelerate its own decay, which in turn releases more radiation, which in turn accelerates the decay, and so on: your pile of nuclear material just blew up violently.

Obviously you want this to happen inside your bomb, but you also want it to happen at the push of a button instead of, you know, the moment you load the material into the bomb. So the most common way to build things is to divide the pile of nuclear material into two or more subcritical parts, stored separately from each other in the bomb. You also pack some conventional explosives. When the time comes to make an ass of yourself in front of the rest of the world, you detonate the conventional explosives, which shoot the various subcritical piles of nuclear material towards each other. They collide and get compressed into one big supercritical ball, which promptly vaporizes the neighborhood (and the neighborhood's neighborhood as well).

Edited April 23, 2014 by Streetwind

[Othuyeg]

There are many different types of nuclear weapons. You can actually find the basic designs and their means of operation in Wikipedia.

Basics of fission weapons:

http://en.wikipedia.org/wiki/Nuclear_weapon#Fission_weapons

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

[sgt_flyer]

If you want to go even further, H-Bombs works by adding deuterium + tritium near one of the 'classic' fission nukes. When the nuke detonates, the heat and pressure is enough to make the deuterium and tritium undergo fusion - which releases way more energy than a fission bomb of the same weight

[TheGatesofLogic]

So you understand the basics of nuclear fission but want to understand why it causes an explosion?

You obviously understand a nuclear fuel pile, but to clarify, a nuclear fuel pile is a critical mass of nuclear material, which means that for every neutron that hits a fissile atom more than onje neutron is produced that actually goes on to trigger fission of another atom. The way this works comes in two processes actually. The first is a slow neutron reaction, which is what is used in nuclear reactors. In slow neutron fission the neutrons from split atoms are slowed down by a moderator. Although it might not be immediately intuitive, slow neutrons are actually better at splitting a nucleus in a chain reaction than fast neutrons. This means that reactors don't need a large quantity of highly enriched fuel. Fast neutron reactions, on the other hand, are what is used in nukes. Fast neutron reactions produce enormous amounts of energy in short periods of time if they can be sustained, but therin lies the problem. When the reaction occurs it increases the temperature of the material, pushing the atoms away from each other and making it less likely for neutrons to hit a nucleus, but this is not a problem with slow neutrons since they have more time to pass through the material. With fast neutrons this is a major issue, making a much higher critical-mass necessary to sustain. In a real nuclear weapon this is overcome by either assembling this highly enriched material in a very short period of time (gun type), or by compressing the material down extremely quickly, since critical mass is actually lower at higher densities (implosion type). When this happens the chain reaction occurs enormously fast, since these are incredibly fast neutrons, and the energy released is sent off in waves of gamma and x-ray radiation, as well as an enormous pressure wave generated by the expansion of the nuclear material in the bomb at incredible speeds due to the extreme temperatures caused by the reaction. All this energy adds up to both a thermal/radiation front which burns the landscape from the intensity of the rays, and a pressure/heat front which expands rapidly as a rsult of the rapid change of temperature and the expansion of materials under the thermal pressure. The radiation front is essentially what melts a city, and the pressure front is essentially what levels buildings in this case.

[Sillychris]

You get your explosion from a bunch of liberated energy, all in one spot. The heat from fission heats up the contents and surroundings drastically which increases pressure and causes rapid expansion ie an explosion. The intense heat also helps destroy things.

[xenomorph555]

NSA don't mind me...

[Aghanim]

Now I understand it. Thanks!

[Pds314]

I believe the way it works is:

Detonator-->conventional explosives generate immense pressure and heat-->neutrons generated from fission now must hit more plutonium on their way out, it starts exploding and must be held together until a large enough fraction has reacted.--> Breaches shell and loses criticality due to lower density. --> Surrounding material, e.g. air, absorbs and re-emits gamma rays--> heat generates shockwaves.

[Pds314]

Or alternatively, we sacrifice a bunch of rare metals and high explosives to the death gods and they come forth from the site of the sacrifice.

[TheGatesofLogic]

Or alternatively, we sacrifice a bunch of rare metals and high explosives to the death gods and they come forth from the site of the sacrifice.

Shhhh!!! That's a secret!!! You don't want Iran finding out now do you?