The above is a plot of binding energy per nucleon $E_b$, against the nuclear mass $M$; $A,B,C,D,E,F$ correspond to different nuclei. Consider four reactions:(i) $A+B\to C+\epsilon$ (ii) $C\to A+B+\epsilon$ (iii) $D+E\to F+\epsilon$ (iv) $F\to D+E+\epsilon$ where $\epsilon$ is the energy released. In which reactions is $\epsilon$ positive?

Energy is released ($\epsilon >0$) when the total binding energy of the products is greater than that of the reactants. This occurs when the products have a higher binding energy per nucleon ($E_b$) than the reactants.In reaction (i), $A$ and $B$ (lighter nuclei) combine to form $C$ (heavier nucleus) which has a higher $E_b$. This is nuclear fusion.In reaction (iv), $F$ (very heavy nucleus) splits into $D$ and $E$ (middle-weight nuclei) which have higher $E_b$. This is nuclear fission.In both cases, the system moves towards a more stable state (higher $E_b$), so $\epsilon$ is positive.