Fission
Nuclear reactor
The discovery of fission is one of humanity's gains after having grasped Einstein's iconic equation, E=mc2. Simply put, it is the splitting up of a nucleus into smaller fragments called fission products. There is something peculiar about these products though. One that would have been thought utterly impossible had there been no knowledge of Relativity.
When a nucleus splits up, the sum of the masses of the fission products plus that of some released neutrons simply don't add up to the original mass of the whole nucleus. That is, the sum comes out lighter than the original mass. But where does the missing mass go? All this will be clearer if we take a look back at E=mc2.
We notice that this equation directly relates energy and mass. This equation is central to the principle of the conservation of mass and energy. That is, instead of one conservation law for energy and another for mass, the two are taken as one in the Special Theory of Relativity.
Thus, if we talk about the total energy of an isolated system, we'd have to include the energy equivalent of the masses involved as per E=mc2. Thus, when we say that this total energy is conserved, we will have to consider the energy equivalent of all masses in the system as well.
So, going back to the question, "Where does the missing mass go?", we can conclude that it must have been released in the form of energy. True enough, that is what happens. But that is not all. If we look at the equation again, we notice the presence of c, the speed of light in vacuum.
If we recall, c = 3.0 x 108 m/s. Thus, when multiplied with the mass, we can obtain a very large amount of energy. In other words, the missing mass, though small, actually comes out as a huge amount of energy. This realization was enough motivation for scientists like Enrico Fermi, Neils Bohr, and Robert Oppenheimer to focus on harnessing this energy.
To harness this energy in a controlled fashion, nuclear reactors are outfitted with control rods made of neutron-absorbent material like boron or cadmium. As stated earlier, neutrons are among the byproducts of fission. They are also necessary to sustain the nuclear reactions. However too much of them will lead to an uncontrolled nuclear fission reaction which can consequently result in an explosion.
In fact, such an unchecked series of fission reactions is a major ingredient for an atomic bomb.
You can read more about fission here in Universe Today. Dig into the story about NASA's interest in fission reactors for power on the moon. Or the one that talks about NASA's plutonium deficit problem.
Can't get enough of that story? Here are a couple more sources there:
Here are two episodes at Astronomy Cast that you might want to check out as well:
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