Imagine this: 60 kilograms of Uranium-235 can literally generate the same amount of energy (explosion) with 13.000 tons of TNT. This was actually the amount of U-235 that was used in the nuclear bomb in Hiroshima, called "the little boy":
And now think of this: Everyone can dig and find uranium ore. But is it easy to build an A-bomb? As a matter of fact, it is not as simple as mixing salt and pepper. The difficult part is NOT to get the uranium, but to prepare it.
Natural uranium is a metal ore which has basically two types of isotopes: Uranium-238 and Uranium-235, with the first (U-238) being the majority. In fact, only 1 in about every 140 atoms is U-235. The problem is that U-235 is able to sustain a chain reaction - the key to generate energy (and thus explosion).
We call "chain reaction" the process which takes place within the atoms of Uranium, after being bombarded with neutrons. If neutron hits Uranium atom, then the Uranium atom breaks into smaller pieces in a process called "fission". These pieces are called "fission fragments". There are many fission products coming out of this process, many of them being radioactive, but the final mass of these products will be slightly less than the original mass of the uranium atom plus the mass of the neutron... Where is the rest mass? It has been turned into energy, a LOT of energy.
Among the fission fragments, each Uranium atom will produce another 1, 2 or 3 free neutrons. Usually two neutrons come out of the first fission. These two neutrons will find another two Uranium atoms to bombard and cause another 2 fissions. Then there will be 4 neutrons that will cause 4 more fissions, and then 8 etc. This process builds up exponentially in a very short time. So, huge amounts of energy is released within seconds.
The point is that, U-238 is difficult to sustain a chain reaction. It needs very high-energy neutrons to cause fission, and it is not sure that the chain reaction will be sustained. On the other hand, U-235 can initiate and build a chain-reaction with low-energy neutrons. But as we said before, U-235 is just 1/140 of the atoms. So, before all, Uranium must be separated into its isotopes.
Uranium isotopes have identical magnetically and chemical properties, so using magnets or other chemical processes is not a way to do the separation. What they differ at, is their weight. U-238 is slightly heavier than U-235 by about 2%. It is not much, but it is enough to do the separation, and here is how it works:
Suppose that you have a long tube and you put inside tennis balls representing U-238 and ping-pong balls representing U-235. The weight difference is more than 2%, but it is just to get the idea. Then, you apply the same amount of force to all balls, who wins? The lighter atoms of course!
In reality, this tube should be very long, some thousands of kilometers. So scientists figured out this way to do the separation. The uranium is fed with a high-pressure stream into a long perforated tube. Around this tube, there is a low-pressure chamber. Lighter elements go through the perforated walls into the outside chamber, the same way it would happen with the ping-pong balls if they were fed into this tube along with tennis balls. To increase the separation, scientists use many stages of this method. In fact, to achieve 90% of U-235, the uranium must go through some 4000 stages of separation! To make the first A-bomb, the factory in which the separation took place covered an area of 160.000 m2 with 160 km of piping!
Later methods of separation involves the centrifugal force. The gasified Uranium is fed into a stationary perforated tube with an outside housing. Inside the stationary tube there is a rotor rotating at several thousand rpms. The atoms start to rotate very fast inside the tube. The heavier atoms (U-238) are then forced outside the tube, and the U-235 atoms remain clean inside. This method requires only a fraction of the energy from the previous method, but the mechanical parts are very difficult to manufacture due to the high rpm.
Here is a video with Bill Hammack, the Engineering Guy in which he explains all these!