Yes, there are nuclear reactions constantly occurring in our bodies, but there are very few of them compared to the chemical reactions, and they do not affect our bodies much. All physical objects are made of molecules. A molecule is a series of atoms linked together by chemical (electromagnetic) bonds. Inside each atom is a nucleus which is a collection of protons and neutrons linked together by nuclear bonds. Chemical reactions are the making, breaking, and rearranging of bonds between atoms in molecules. Chemical reactions do not change the nuclear structure of any atoms. In contrast, nuclear reactions involve the transformation of atomic nuclei. Most of the processes surrounding us in our daily life are chemical reactions and not nuclear reactions. All of the physical processes that take place to keep a human body running (blood capturing oxygen, sugars being burned, DNA being constructed,etc.) are chemical processes and not nuclear processes. Nuclear reactions do indeed occur in the human body, but the body does not use them. Nuclear reactions can lead to chemical damage, which the body may notice and try to fix.