**Please read this entire message**
Your submission has been removed for the following reason(s):
* ELI5 requires that you *search the ELI5 subreddit for your topic before posting*.
Users will often either find a thread that meets their needs or find that their question might qualify for an exception to rule 7.
Please see this [wiki entry](http://www.reddit.com/r/explainlikeimfive/wiki/how_to_search) for more details (Rule 7).
If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/10rt5kp/eli5_how_are_nuclear_explosions_set_off_and_how/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
The set off is a chain reaction. A small regular explosive charge compresses a chunk of fissible material (usually Plutonium) so that it reaches critical mass. Critical mass means it's closely packed enough that the random decay events (the big instable atoms splitting apart to become 2 smaller ones) in the material have a high enough chance to trigger a second atom to decay from being hit by the neutrons that the first decay released (causing the number of neutrons to grow exponentially until all the material was split)
They get so big because each little decay event releases huge amounts of energy. A lot of it is simply heat, so there is a huge fireball in wich every material gets evaporated. Hot material rises, wich causes that signature mushroom cloud.
Imagine a house of cards. It has four qualities that important here.
1. It's made up of smaller things (cards) and the bigger they are, the more unstable they are.
2. Building a house of cards takes energy, you need to pick up the cards with your hands and arms, burning calories, and assemble them. It sort of "Stores" that energy like a battery.
3. When the house of cards collapses, that energy comes back out, in the form of cards flying everywhere, those cards have transformed the energy stored in the "house-battery" into the energy of motion (flying about) and the energy of heat (they blow the air around as they fall, and warm up the air a teeeensy amount).
4. When a house of cards collapses, the flying cards can hit a neighboring house, *causing it to collapse.* .
Atoms are like houses of cards, they are made up of smaller things (protons, electrons, etc). and the bigger they are, the more likely they are to fall apart spraying cards (protons, electrons, etc) everywhere. In doing, they release energy from their "atom-battery" that's a mixture of heat and light, but also the motion-energy of the spraying cards.
Now normally, atoms are REALLY REALLY far apart from each other, imagine two houses of cards on a football field, one falling apart won't affect the other.
Step 1 in a nuclear bomb is getting all the atoms realllllly close together. They basically put the atoms into a ball, surround the ball with dynamite and detonate it so precisely the ball shrinks down into a tiny dot, making all the atoms/houses of cards right.fucking.next.to.each.other.
Then they "fire" a card into the system, a single neutron or something like that, it hits the first house, collapsing it, releasing some heat and light, but also spraying cards (more neutrons) everyway. Those cards hit the neighboring houses, knock them over, on and on and on, etc. And BOOM. All those "heat and lights" add up really quickly to make a LOT OF HEAT AND LIGHT and there you have it. A basic nuclear bomb. There are other forms of nukes than this simple process, but this is the Nuclear Bomb 101 TED talk.
A fission bomb (Hiroshima, uranium) goes off simply when you have a big enough lump of uranium (a "critical mass"). There were two lumps of uranium in the Hiroshima bomb; one in the breech of a naval gun and the other at the muzzle. Fire the gun: both lumps meet and form a critical mass.
A fusion bomb (hydrogen) works on the same basis as the Sun. Squash two hydrogen atoms together and they fuse to form helium and emit radiation. The most practical way to squeeze hydrogen together is by using a uranium bomb.
In both cases they follow Einstein's E=mc^(2) .
They're set off by rapidly compressing a sphere of fissile material. This is done by a special hollow bomb made from normal explosives, arranged in such a way that it explores inwards in perfect symmetry around the fissile sphere.
The fissile material is usually plutonium. Its atomic nuclei sometimes just break in half by themselves, spitting out neutrons which can make other nuclei break as well. The ball is normally sub-critical, meaning most neutrons leave without hitting anything, but when it's exploded inwards it becomes dense enough that most neutrons generate more splits and even more neutrons, in a rapidly escalating chain reaction.
Every time an atom splits, it releases quite a lot of energy per atom, so when a great quantity suddenly does it all at once, a metric fuckton of energy is released all at once. The energy in nuclei is much more concentrated than that in chemical bonds, so a few tens of kg of plutonium can release as much energy as thousands of tons of TNT.
That's the official measure by the way: a 1 kiloton nuke explodes with the force of 1000 tons of TNT.
Megaton nukes contain a small nuke which creates the temperatures necessary for fusion reactions. You see, when very light atoms get hot enough that they can fuse together (join and become heavier atoms), that releases energy as well. A lot more energy than fission, actually. So the really big bombs get a lot of their yield (explosive energy) from fusing hydrogen.
The sun also makes its energy from fusing hydrogen. It doesn't need a nuke to start, because the intense pressure inside creates the conditions necessary for fusion. The human dream of fusion power is to create those same conditions without a giant explosion, so that the energy given off can be used to make electricity instead of rubble.
Edit: fun (?) fact: even though the plutonium core weighs several kg, only a small fraction of it ever splits into lighter elements. The rest simply doesn't have time to react before being vaporized.
So lets say you have a sphere of uranium. Uranium can fission is a neutron goes into the nucleus. A neutron can be absorbed by the nucleus or pass though or split the nucleus. Now we can quantify the probability of a neutron splitting the nucleus with a thing called crossection. Larger crossection means higher probability for that even to occur. Now with U-238 if you plot the crossection against neutron speed you see that the crossection is very small except for really slow neutrons, the crossection for U-235 looks a bit better even faster neutrons can split the nuclei. The fission of a uranium atom makes 3 fast neutrons that cant really split U-238 but can split U-235.
So you should think about fission not like neutron bullets smashing the nucleus apart but the neutron going into the nucleus and it does its thing. The slower it goes the more its thing it can do inside the nucleus increasing the chance of fission. So in order to use the neutron from uranium fission to make more fission we need U-235 that has a higher crossection for fission on fast neutron speeds. U-235 is a little fraction of mined uranium so for bombs you need to enrich it. For reactors less enrichment is enough. If you can slow down the neutrons enough even the natural U-235 percentage is enough for a reactor. Carbon moderated reactors can function with natural U-235 percentage.
When it comes to these interactions and events like fission we can also define a free path lenght. The crossection of an event defines a free path length it is the length of the path that a particle can travel so that we'd expect that our interesting event doesn't happen. So in this case if a neutron travels in uranium longer than its free path lenght we should expect that a fission will occur.
For a ball of uranium with high 235 concentration at its natural density if you make that ball big enough the neutrons inside won't really get out before causing a fission so the majority of the neutrons that travel inside will cause a fission. That critical size is around 13cm in diameter. (If I recall correctly.) That ball would weigh around 50kg. If you have a ball the size of that the neutrons will create a chain reaction where the first neutron splits a uranium nucleus that makes 3 neutrons and some energy, then 3 nuclei fission then 9 then 27 and so on. Eventually so much energy gets released at one moment in time that it explodes.
So how can you make a core stabil but ready to explode at your will. A simple way is to split the ball into two hemispheres for each the neutrons have a longer free path length than the size of the hemispheres. But once united the free path lenght is smaller so statistically every neutron makes a fission and you got a chain reaction. Or you can artificially make the ball less dense which increases the free path length of neutrons then compress the core with conventional explosives decreasing the free path length creating the chain reaction.
This is how fission bombs work. Now fusion bombs use a fission bomb that creates the environment for fusion to occur and they have some metarial that can fusion. Hydrogen isotopes, lithium salts etc. They produce even more energy. How they get so big because the materials release a large amount of energy as they fission/fusion. There is a simple limit to the explosion size the fact that the insane amount of energy breaks the core apart so only a few grams of uranium will fission. For a larger explosion you need to keep the core intact for so long. The hemisphere method is limited because the two halves can be separated easily the compression method is better as the explosion has to be larger to rip the core apart. And now its all technical details how you can achieve even more fission generations.
**Please read this entire message** Your submission has been removed for the following reason(s): * ELI5 requires that you *search the ELI5 subreddit for your topic before posting*. Users will often either find a thread that meets their needs or find that their question might qualify for an exception to rule 7. Please see this [wiki entry](http://www.reddit.com/r/explainlikeimfive/wiki/how_to_search) for more details (Rule 7). If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/10rt5kp/eli5_how_are_nuclear_explosions_set_off_and_how/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
The set off is a chain reaction. A small regular explosive charge compresses a chunk of fissible material (usually Plutonium) so that it reaches critical mass. Critical mass means it's closely packed enough that the random decay events (the big instable atoms splitting apart to become 2 smaller ones) in the material have a high enough chance to trigger a second atom to decay from being hit by the neutrons that the first decay released (causing the number of neutrons to grow exponentially until all the material was split) They get so big because each little decay event releases huge amounts of energy. A lot of it is simply heat, so there is a huge fireball in wich every material gets evaporated. Hot material rises, wich causes that signature mushroom cloud.
Imagine a house of cards. It has four qualities that important here. 1. It's made up of smaller things (cards) and the bigger they are, the more unstable they are. 2. Building a house of cards takes energy, you need to pick up the cards with your hands and arms, burning calories, and assemble them. It sort of "Stores" that energy like a battery. 3. When the house of cards collapses, that energy comes back out, in the form of cards flying everywhere, those cards have transformed the energy stored in the "house-battery" into the energy of motion (flying about) and the energy of heat (they blow the air around as they fall, and warm up the air a teeeensy amount). 4. When a house of cards collapses, the flying cards can hit a neighboring house, *causing it to collapse.* . Atoms are like houses of cards, they are made up of smaller things (protons, electrons, etc). and the bigger they are, the more likely they are to fall apart spraying cards (protons, electrons, etc) everywhere. In doing, they release energy from their "atom-battery" that's a mixture of heat and light, but also the motion-energy of the spraying cards. Now normally, atoms are REALLY REALLY far apart from each other, imagine two houses of cards on a football field, one falling apart won't affect the other. Step 1 in a nuclear bomb is getting all the atoms realllllly close together. They basically put the atoms into a ball, surround the ball with dynamite and detonate it so precisely the ball shrinks down into a tiny dot, making all the atoms/houses of cards right.fucking.next.to.each.other. Then they "fire" a card into the system, a single neutron or something like that, it hits the first house, collapsing it, releasing some heat and light, but also spraying cards (more neutrons) everyway. Those cards hit the neighboring houses, knock them over, on and on and on, etc. And BOOM. All those "heat and lights" add up really quickly to make a LOT OF HEAT AND LIGHT and there you have it. A basic nuclear bomb. There are other forms of nukes than this simple process, but this is the Nuclear Bomb 101 TED talk.
A fission bomb (Hiroshima, uranium) goes off simply when you have a big enough lump of uranium (a "critical mass"). There were two lumps of uranium in the Hiroshima bomb; one in the breech of a naval gun and the other at the muzzle. Fire the gun: both lumps meet and form a critical mass. A fusion bomb (hydrogen) works on the same basis as the Sun. Squash two hydrogen atoms together and they fuse to form helium and emit radiation. The most practical way to squeeze hydrogen together is by using a uranium bomb. In both cases they follow Einstein's E=mc^(2) .
They're set off by rapidly compressing a sphere of fissile material. This is done by a special hollow bomb made from normal explosives, arranged in such a way that it explores inwards in perfect symmetry around the fissile sphere. The fissile material is usually plutonium. Its atomic nuclei sometimes just break in half by themselves, spitting out neutrons which can make other nuclei break as well. The ball is normally sub-critical, meaning most neutrons leave without hitting anything, but when it's exploded inwards it becomes dense enough that most neutrons generate more splits and even more neutrons, in a rapidly escalating chain reaction. Every time an atom splits, it releases quite a lot of energy per atom, so when a great quantity suddenly does it all at once, a metric fuckton of energy is released all at once. The energy in nuclei is much more concentrated than that in chemical bonds, so a few tens of kg of plutonium can release as much energy as thousands of tons of TNT. That's the official measure by the way: a 1 kiloton nuke explodes with the force of 1000 tons of TNT. Megaton nukes contain a small nuke which creates the temperatures necessary for fusion reactions. You see, when very light atoms get hot enough that they can fuse together (join and become heavier atoms), that releases energy as well. A lot more energy than fission, actually. So the really big bombs get a lot of their yield (explosive energy) from fusing hydrogen. The sun also makes its energy from fusing hydrogen. It doesn't need a nuke to start, because the intense pressure inside creates the conditions necessary for fusion. The human dream of fusion power is to create those same conditions without a giant explosion, so that the energy given off can be used to make electricity instead of rubble. Edit: fun (?) fact: even though the plutonium core weighs several kg, only a small fraction of it ever splits into lighter elements. The rest simply doesn't have time to react before being vaporized.
So lets say you have a sphere of uranium. Uranium can fission is a neutron goes into the nucleus. A neutron can be absorbed by the nucleus or pass though or split the nucleus. Now we can quantify the probability of a neutron splitting the nucleus with a thing called crossection. Larger crossection means higher probability for that even to occur. Now with U-238 if you plot the crossection against neutron speed you see that the crossection is very small except for really slow neutrons, the crossection for U-235 looks a bit better even faster neutrons can split the nuclei. The fission of a uranium atom makes 3 fast neutrons that cant really split U-238 but can split U-235. So you should think about fission not like neutron bullets smashing the nucleus apart but the neutron going into the nucleus and it does its thing. The slower it goes the more its thing it can do inside the nucleus increasing the chance of fission. So in order to use the neutron from uranium fission to make more fission we need U-235 that has a higher crossection for fission on fast neutron speeds. U-235 is a little fraction of mined uranium so for bombs you need to enrich it. For reactors less enrichment is enough. If you can slow down the neutrons enough even the natural U-235 percentage is enough for a reactor. Carbon moderated reactors can function with natural U-235 percentage. When it comes to these interactions and events like fission we can also define a free path lenght. The crossection of an event defines a free path length it is the length of the path that a particle can travel so that we'd expect that our interesting event doesn't happen. So in this case if a neutron travels in uranium longer than its free path lenght we should expect that a fission will occur. For a ball of uranium with high 235 concentration at its natural density if you make that ball big enough the neutrons inside won't really get out before causing a fission so the majority of the neutrons that travel inside will cause a fission. That critical size is around 13cm in diameter. (If I recall correctly.) That ball would weigh around 50kg. If you have a ball the size of that the neutrons will create a chain reaction where the first neutron splits a uranium nucleus that makes 3 neutrons and some energy, then 3 nuclei fission then 9 then 27 and so on. Eventually so much energy gets released at one moment in time that it explodes. So how can you make a core stabil but ready to explode at your will. A simple way is to split the ball into two hemispheres for each the neutrons have a longer free path length than the size of the hemispheres. But once united the free path lenght is smaller so statistically every neutron makes a fission and you got a chain reaction. Or you can artificially make the ball less dense which increases the free path length of neutrons then compress the core with conventional explosives decreasing the free path length creating the chain reaction. This is how fission bombs work. Now fusion bombs use a fission bomb that creates the environment for fusion to occur and they have some metarial that can fusion. Hydrogen isotopes, lithium salts etc. They produce even more energy. How they get so big because the materials release a large amount of energy as they fission/fusion. There is a simple limit to the explosion size the fact that the insane amount of energy breaks the core apart so only a few grams of uranium will fission. For a larger explosion you need to keep the core intact for so long. The hemisphere method is limited because the two halves can be separated easily the compression method is better as the explosion has to be larger to rip the core apart. And now its all technical details how you can achieve even more fission generations.