If nuclear fusion is the reverse of fission, why is energy released in both processes?

Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat. There are basically two ways to release energy from nuclei:
  • nuclear fission, which is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei).  This released energy is the one that is frequently used in steam turbines to produce electricity in nuclear power plants.
Public Domain, https://commons.wikimedia.org/w/index.php?curid=486924
  • nuclear fusion, which is a reaction in which two atomic nuclei fuse to form a heavier nucleus. Nuclear fusion reactors are not yet economically viable, but this technology is currently under research and it could become viable in a few decades.

Deuterium-tritium fusion.svg
By Wykis - Own work, based on w:File:D-t-fusion.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2069575
If splitting a nucleus into two smaller nuclei releases energy, it seems that combining two smaller nuclei into one larger nucleus would require energy, not release it, because it is the inverse process. So, why can we obtain energy from both processes?

 Are you able to resolve this cognitive conflict?

Please, explain your reasoning. You can post your attempted answers in the comment box below. Please, do not use Facebook or Twitter to give your answers.


  1. My guess. Firstly, nuclear fission consists of a nucleus which has been bombarded with an elemental particle to break it down (to break the Strong Interaction between its nucleons) so that energy is emitted in form of radiation (gamma radiation from the photons emitted) and obviously in form of cinetic energy from the resultant nuclei.

    Now, the key to nuclear fusion resides in being able to join two nuclei close enough in order to ignore the electromagnetic repulsion. In this way, the Strong Interaction (extremely effective at short distances) overweights the electromagnetic forces, and nucleons inside the joint nuclei are able to attract even more nucleons. Since the number of total nucleons increases, so does the bond energy per nucleon (because it is directly proportioned to the famous "mc^2" or repose energy, and we have just seen that the total theorical mass "m" increases, because the total number of nucleons increase). That is why the more mass the resultant nucleus of the fusion SHOULD HAVE (in fact, it has less due to the mass decrease), the more bond energy per nucleon there is, which is the result of the mass decrease. That is the "energy emitted" from the fusion.
    Appart from, of course, the cinetic energy that the particles emitted in the reaction have. (In the image above it is clearly seen a neutron being emitted).

    I don't know if my answer is complete or correct, it is just a guess.

    1. Of course in order to split a nucleus we need to apply an activation energy to defeat the attraction coming from the strong force. After that, the electromagnetic repulsion between the two smaller parts will act, giving you back the activation energy and even more. That is why fission is an exothermic process.
      All that has been said in the previous paragraph can be applied to fusion, but changing the signs of all the energies, so your reasoning is not enough to solve the puzzle yet.
      The bond energy per nucleon will give you the key.
      Come on! You are almost there.