What changes occur in the nuclei of atoms involved in the reactions that fuel nuclear power plants?

Nuclear fusion and nuclear fission are two different types of energy-releasing reactions that occur in the nuclei of an atom.
Here are the major differences between the two:

1. To differentiate the two, fission is the splitting of an atom into two or more smaller atoms while fusion is the conjoining or fusion of two or smaller atoms into larger one.

2.  Fission does not normally occur in nature while fusion occurs mostly in heavenly bodies such as the stars.

3.Fission produces highly radioactive particles that can be hazardous to both the living things and its habitat or environment while fusion is "clean energy" and "environmental friendly" meaning there are fewer radioactive particles are produced. But if a fission "trigger" is being used, there will  be radioactive particles produced.

Among the two nuclear changes, fission is widely used because this reaction produces heat in nuclear reactor. This heat is used to generate steam which operates the turbines to eventually produce electricity.

10.063g / 10.00 ml = 1.0063 g/ml which rounds off to 4 sig figs as: 1.006 g/ml