quote:

*Originally posted by tony6789*

I am all confused could you simplify it fr me

- Big T

I'm not an expert on nuclear fusion (nor much else), so I'll try and give a layman's eye view.

Uranium 235 (i.e. the type you can use in nuclear bombs – uranium 238 won't work in bombs), or plutonium, can be used in bombs because if an atom of the material is hit by a neutron, it will shatter, and send 3 new neutrons. As you can see, one neutron in, 3 neutrons out, so you can get an ever increasing rate of chain reaction, and while it is doing this, it is generating a lot of energy, that ultimately becomes the basis for your explosion.

The trouble is, the likelihood that a neutron travelling a block of uranium or plutonium will actually hit the nucleus of the atom is very very small, so the greatest likelihood is that the neutrons will just escape out of the uranium/plutonium, and so it simply wont explode.

So what you need to do is have a big enough block of uranium/plutonium to give a very high probability that the neutrons will actually find an atom to hit within the bulk of the material. The minimum size of material you need to have in one block to guarantee that enough neutrons will find enough atoms to hit is known as the critical mass.

In the first atomic bombs, they used a very crude mechanism, where they just had two small pieces of uranium, each below the critical mass, but when brought together, they formed enough uranium to become a critical mass, and start an explosion.

As you try and make bombs that create bigger explosions than this, you need to combine much larger amounts of uranium/plutonium than you can get with just two small pieces or uranium.

As a simple example, lets say you want to bring together a bomb that has almost 4 times the critical mass of uranium. You would need to bring together 4 small blocks of uranium into a single explosive mass. The problem is that even bringing together 2 of these small blocks will cause a small explosion, but would cause the power of the other 2 blocks to be wasted, so you have to make sure that all 4 blocks come together at exactly the same time (within a few microseconds of each other), and so you need explosive charges that with blast these pieces together at exactly the same time.

Even if you only want a smaller explosion, one of the ways to make smaller nuclear explosions is by reducing the critical mass needed before you can start a chain reaction. You do this by increasing the likelihood that a neutron will find another atom to hit, and shatter apart. One of the ways you can do this is by squeezing the uranium block in order to bring the atoms closer together, and thus make it less likely that a neutron will find a path between the atoms where it can escape without hitting an atom. The way you would compress the block of uranium is by packing conventional explosive around it, and when you explode that conventional explosive it will squeeze the uranium into a smaller ball, and so the uranium starts an explosive chain reaction.

The point is, one way or another, you need to use conventional explosive to make a modern nuclear bomb go critical, but that explosive is used to force the uranium or plutonium together. If you think about trying to squeeze a balloon, if you squeeze it on one side more than another, it will simply push the balloon away from you – if you want to squeeze a balloon into a smaller ball, you have to make sure the pressure is equal from all sides. You can think of the explosives trying to squeeze the uranium together as it it was a balloon.

Hope that hasn't confused you even more.

No doubt some nuclear physicists will tell me all the bits I got wrong, but hopefully not wildly wrong.