Fusion releases energy for light nuclei because the binding energy per nucleon increases toward the iron region. Fission releases energy for very heavy nuclei because splitting them moves products toward higher binding per nucleon. The energy release is determined by the mass difference via $Q = \Delta mc^2$.

Fission involves barrier penetration, deformation, saddle-point dynamics, fragment mass distributions, and prompt neutron and gamma emission. It is governed by the competition between macroscopic liquid-drop energetics and microscopic shell effects that stabilize specific fragment splits.

Fusion reaction rates depend on temperature, density, and plasma screening. In controlled fusion, the challenge is achieving sufficient confinement and reaction rate to satisfy the Lawson criterion, $$nT\tau_E$$ large enough for net energy gain.