So, like I said, there’s two types of atomic bomb, fission and fusion.
In a fission bomb (the first to be invented) you simply have two chunks of radioactive material.
Fissile material (elements that will undergo nuclear fission) all have a property called a critical mass.
If you have less than the critical mass of the stuff (say, Uranium) then it will be radioactive, but at a fairly slow rate.
If you have more than the critical mass, then the radiation given off is enough to cause other atoms in the lump to give off radiation and you get what is called a chain reaction.
This is really important as it means that the reaction will keep going on it’s own without any input. This is how we generate power in nuclear power stations.
However, if you have /way/ more than the critical mass, the reaction goes haywire and puts out massive amounts of energy in a very short time. This is what happens in an atom bomb.
So the bomb itself is quite simple – two lumps of fissile material, just below the critical mass, a timer, and a trigger. When the timer goes off the trigger pushes the two lumps together, and because they are now way above the critical mass, they give out a massive amount of energy, so fast that it causes an explosion.
A fusion (or H bomb) uses nuclear fusion, which generates even MORE energy than fission, but is a really hard reaction to start. Once it gets going it is way more powerful than a fission bomb and only needs simple fuel, like hydrogen.
So the trick is getting the reaction started – to do this you need very high temperatures… just like in a fission bomb explosion!
To make a fusion bomb then, you make a small fission bomb, which goes off and sets off the fusion reaction. The bombs that have actually been built are a bit more complex than that, but the general principle is quite simple.