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plutonium or uranium must be enriched beyond a certain atomic weight. What this means is that individual atoms must be first made heavy and then selected out of all the atoms in a mass of radioactive materials. The most popular way to do this is with centrifuges- imagine a big jar, or perhaps a soup-pot, filled with metal so hot that it's a gas, and kept that hot, so that the gas is just floating in this container.
now make the container spin so fast that the outside, if you let it fall down, it will go 1 kilometer (3/5 of a mile) every second. This is roughly 1,100 yards, if you want to see it like that.
the reason you make this spin is so that the metal- which is in the form of a gas- arranges its atoms according to weight- so that the heavy ones are on the outside, and the light ones are on the inside, and the heavy atoms are then harvested.
now you have the material that the bomb will be made from, so how does it work?
the initial reaction was formerly set of with a gun-like device- a short tube, one piece of enriched material at either end, you bang them into each other. This releases neutrons, protons, electrons,e tc, and the nuclear reaction happens when these pieces hit other atoms, attach for just a billionth of a second, and then the new atom that is formed, and then the new atom breaks apart into a lot of pieces- this is the atomic level beginning of a nuclear reaction, and the two halves of this atom are radioactive debris and become radioactive waste.
I think it should be mentioned that there is more to this. Modern nuclear devices use exactly-machined containers, filled with exact ammounts of exact explosive and given an exact shape- to compress all sides equally within this enclosure, which forces all of the explosions energy inwards, into the enriched material. It is not, as mentioned above, TNT- I don't want to detract from the information above, but there is no way that this chemical is either stable enough or powerful enough to be practical in a nuclear device. This is my only argument with the above explanation. However, other methods- such as tungsten carbide plates- are also used to enclose and reflect radiation. The physical nature of this stuff is very strange, and a highly inert reflective material- like TC- can actually cause a radioactive material to become critical and give off dangerous levels of radiation. The early nuclear testing in the US claimed the lives of two scientists this way.
I guess the shortest way I can say this is "you find heavy things: You make those heavy things very tiny, by squishing them with a gun: Those heavy things get angry and make everything go away".
A nuclear bomb has atoms of uranium or plutonium in it. These atoms when placed in a shell( the bomb casing) start to move really quickly in the shell which causes a huge amount of energy. When the bomb is dropped on its target, on impact the shell breaks thus releasing the built up energy of the atoms. This energy is extremely dangerous and destructive and also since Uranium and Plutonium are radioactive they alter the Genes in a person(s) and also contaminate the surrounding soil,water and air.