Question

How does nuclear fission produce energy?

Answer 1

by mass defect .

binding energy per nucleon for products is more than binding energy per nucleon for reactant molecule ,therefore net energy is released

Answer 2

Yes, it takes less energy to hold together the fission product pieces than it did to hold together the original nucleus.

Answer 3

Just to add - you can use the Einstein's equation to formulate the mass defect.
\[E = mc^2 \]
As the speed of light is a constant, to increase energy, mass has to go down.