It is when uranium is mixed in with other atoms. It is a very fast process. Faster than a second. You would not be able to see it with the human eye though. You would need a microscope or a telescope. Once all that energy is related it will do mass damage.
Mostly the nuke will have uranium in it. When that is loaded in energy is forming. More and more energy forms to make a big nuke. When the nuke hits the ground all the energy is related causing mass damage. The way nukes get energy is how the sun gets energy.
Nuclear fusion it what helps make a nuke. It is when atoms split apart and bump into uranium to make a ton of energy. Enough to cause a world disaster. It is also a very scary process. That is why the people don't use them a lot. The amount of energy in a nuke depends on how big the nuke is. If is is a huge nuke it will do mass damage. If it is a very very small nuke not a lot of damage will be done.
What's the difference between a fission and a fusion bomb -- and which would be worse to have dropped on you?
At the center of every atom is a nucleus. Nuclear weapons work by splitting those nuclei apart.
The nuke is first loaded up with chemicals. Most likely uranium. Then inside the bomb fusion begins and starts making more energy. The more energy the more explosive. Then when the nuke hits the ground and all the energy is released and makes a huge explosion.
A nuke is a huge bomb that destroys everything in its path. The way they work is nuclear fusion. That happens when a atom slip apart neutron creating more energy that makes the nuke go BOOM. It also happens when a free neutron starts bumping into uranium and plutonium splitting those apart from an atom and nocks a couple neutrons free making more energy it happens almost instantly. Most nukes have uranium plutonium and hydrogen.