Answer: Mass is converted into energy.
Explanation: There are mainly 2 types of nuclear reactions -
a) Nuclear Fission - in which one heavy nucleus gets break into two or more products releasing tremendous amount of energy especially kinetic energy. The amount of energy released due to mass-energy equivalence principle is higher as some amount of mass of the nucleus gets converted into the energy thus decreasing the mass.
The image of the reaction is attached.
b) Nuclear Fusion - in which 2 or more nuclei fused together to form a heavy nucleus .