Chapter 11: Q12CQ (page 518)
An untrained but perceptive exclaims, 鈥淭hey say that nuclear energy can be released by sticking nuclei together and by breaking them apart. That doesn鈥檛 make sense鈥 Straighten out your friend鈥檚 confusion.
Short Answer
It is because the behavior of binding energy per nucleon is not monotonic as a function of the atomic number that we can either stick small nuclei together or break big nuclei apart to gain nuclear energy.