Question

How does nuclear fission create energy?

Answer 1

Nuclear fission doesn't create energy; it releases the energy that was stored as mass in the original nuclei.

Nuclear fission releases energy because the total mass of the products is less than the mass of the original nucleus.

You can read about uranium fission in

http://socratic.org/questions/how-does-nuclear-fission-work-with-uranium?source=search

http://socratic.org/questions/can-you-tell-me-in-a-nuclear-fission-reaction-which-has-more-mass-between-the-in?source=search