Nucleus of the atoms are having a diameter or size of nearly 10-15m. These nucleus has positive charge and the positive charge is due to the presence of the proton. The nucleus of the atom can undergo fission reaction when they are unstable. The nucleus splits into many particle during the fission reaction.
Nuclear fusion is the process in which two or more lighter nuclei
combine to form a nucleus which is heavier than the one that combines.
There may be some other smaller particles that can be evolved.
The nucleus fusion reaction requires large amount of energy
because it has to overcome the repulsion of the positive charge of the
nucleus. When one nucleus comes towards the other the charge on one of
the nucleus which is positive repels the charge of the other nucleus.
So a high temperature is required to fuse the nuclei the high
temperature is normally given the nuclear fission reaction that takes
place in the atom.
The mass of the product nucleus
due to the fusion reaction is always less than the sum of the masses of
the combining lighter nuclei. The difference in the masses is released
as heat. During the fusion reaction enormous amount of heat is
liberated into the atmosphere.
Hydrogen bomb uses the principle of nuclear fusion reaction. It
releases large amount of energy when exploded into the atmosphere which
causes large damages. The temperature required for the purpose of the
nuclear fusion reaction is given by the atom bomb that is the fission
reaction. The nuclear fission reaction in the hydrogen bomb produces a
temperature of over 50 million degree Celsius.