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).

Answer;

Scientist are unable to create a vessel to control the heat and pressure needed for the reaction.

The reason as to why nuclear fusion is not currently used as an energy source is because scientist are unable to create a vessel to control the heat and pressure needed for the reaction.

Explanation;

Nuclear fusion is a type of nuclear reaction that forces 2 types of Hydrogen nuclei together.

During nuclear reaction two different types of hydrogen nuclei are forced together to form a Helium nucleus. The nucleus has less mass than the two hydrogen nucleus and the difference in mass is converted into energy.