Why does an atomic bomb explode, and why doesn’t that start an endless chain reaction?

Question

I understand that an atomic bomb works by detonating fuel (uranium or plutonium) that sparks a chain reaction of fission so every atom causes the fission of more atoms until the fuel is used up. But I don’t understand 2 parts of this process:

1. Why does the fuel (uranium atoms) blow up? I see some sources saying the atoms are being split, but other sources say atoms are being smashed into each other. Which is it? And why does performing that action cause an atom to violently explode?

2. Once the fission is happening and growing exponentially through the fuel, why doesn’t it set off a chain reaction with the atmosphere? Why exactly can’t uranium spark the fission of nitrogen? Why does the chain reaction stop when the uranium is gone?

I know other atomic bomb questions have been asked, but in my research I couldn’t find these answers. Thanks so much!

In: Chemistry

Answer 1

Radioactive atoms are unstable. They will spontaneously split into smaller atoms and release energy and in this case, neutrons. This is fission. You can also combine atoms up till you make iron and also release energy but this is way harder to do. This is fusion and it’s what powers the Sun.

Atom bombs use plutonium or enriched uranium. They will have the fuel in a shape that is almost critical. Sub critical means no chain reaction can happen. Critical means a self sustaining chain reaction is happening and super critical means an exponential chain reaction is happening. These are the implosion and gun type bombs. One of each was dropped on Japan.

Nuclear reactors maintain a critical state to generate power. Atom bombs use explosives to crush the fuel to make it supercritical or can shoot a pellet of fuel at the main batch to make it supercritical.

When the fuel goes supercritical, each atom split is releasing neutrons which then split more atoms. Say for every atom split they split two more atoms. This is the explosion.

This doesn’t blow up the atmosphere as neutrons can’t just split any atom. Non radioactive atoms are stable and will mostly just absorb the neutron or deflect it and take some of its kinetic energy. You’d need the power of the Sun to have enough energy to fuse the atoms in the atmosphere. An atom bomb it just too weak. Even a hydrogen or thermonuclear bomb, which uses fission and fusion, doesn’t have enough energy.