Nuclear fusion has not yet been harnessed as an energy source because

because it has not been truly tested properly and certain precautions need to be addressed first.

Nuclear reactions have two main types: fission reactions and fusion reactions.  When two light nuclei are joined to create a heavier and more stable nucleus, this process is called the fusion reaction. In the fission reaction, the heavy nucleus split into two nuclei with a smaller mass number.  In other words, fission is splitting of atoms although not all atoms undergo fission, only few went through normal circumstances (that is a fact).