Well you do not need to get perplexed between the similarity of a little boy, fat man and a nuclear power plant. When I refer to the thin and fat guys, I really don’t mean human beings but I refer to the deadly atomic bombs which were dropped on the cities of Nagasaki and Hiroshima of Japan during the Second World War. I can see that you have immediately caught the thread of similarity now between these guys and the nuclear power plant – it’s the nuclear reaction which works underneath both of them. The only difference lies in the utilization of the energy generated during that reaction in a useful or destructive manner.
Little Boy & Fat Man
We know that military operations are usually very secret because of the nature of operations and the implications to national security in case they get leaked. So army men love to use code words and just see the ingenuity of the person who devised the code words little boy and fat man for the bombs that were to be dropped over the Japanese cities. Although I wish this ingenuity should have been utilized in some other area rather than causing mass destruction but then that is a matter of separate debate and I do not want to argue over the decision in this technical article (have learnt a thing or two about being politically correct you see).
I must add here that the little boy that was used to bomb Hiroshima on 6 August 1945 was not as “little" as it sounds, but measured well over 10 feet and a diameter of over one feet, weighing more than 4000 kgs, it was not small by any standards. The fat man which detonated over Nagasaki within a gap of three days was called so because though its other dimensions and weight were nearly comparable to the little boy its diameter was much bigger, hence giving it a fat look.
Although much details of these weapons are not available publicly (nobody puts a recipe to kill thousands of people on the net or for public distribution) and is strictly classified, yet I will try to explain the workings of these bombs.
The bomb consisted of several kgs (little over 60 kgs) of Uranium-235 i.e. un-enriched uranium which was kept separated from a solid target spike. When the bomb was due to be detonated, conventional explosive materials forced them together and hence the mass would reach the supercritical range thus starting the chain reaction. These types of nuclear bombs were highly unsafe in that they could self detonate under various circumstances such as shock, crash, fire and so forth and hence this design is no longer in use. Moreover these weapons were very inefficient in that out of nearly 60 kgs of uranium, hardly less than a kg underwent fission and out of that hardly a few grams was converted into energy. But this should give you an idea of the power inherent in such a small mass energy convergence.
The rest as they say is history, although a very unfortunate history. The only fortunate thing is that such bombs have never been used till date in world history although many countries have piles of these weapons which need to be destroyed.