Nuclear fission and fusion both release energy because mass and energy are equivalent. If the total mass of the products is smaller than the total mass of the reactants, the missing mass appears as energy: $\Delta E=\Delta m{c}^2$. Here $\Delta E$ is in joules, $\Delta m$ is in kilograms, and $c=3.00\times 10^8{m\; s}^{-1}$.