Posted by: Kash Farooq | May 27, 2011

Nuclear energy: an introduction to fusion and fission

I’m currently enrolled on the short Open University course: “Inside nuclear energy” (ST174) and I am blogging as I go. See all my nuclear energy posts here.

When two light nuclei fuse (a light nucleus is anything lighter than Iron), the resultant larger nucleus has a mass that is less than the original two nuclei. To put it another way: the larger nucleus has a mass that is less than the sum of its parts.

When a large nucleus splits into two (i.e. undergoes fission), the combined mass of the resultant smaller nuclei is less than the mass of the original nuclei.

In both cases there is said to be a mass defect. The missing mass is released as energy – and the amount of energy released can be calculated using the mass-energy equivalence equation, E=mc2. The total energy that is released when a nucleus is formed is called the binding energy of the nucleus. Conversely, the binding energy can be thought of as the energy that must be supplied to break up a nucleus into its constituent protons and neutrons.

Fission is used to generate energy in nuclear power stations. We use fission to generate the energy required to create steam, which is then used to power the turbines to generate electricity.

Nuclear fission

Fission: a neutron is absorbed by a Uranium-235 nucleus forming the far more unstable Uranium-236 nucleus. This decays to smaller nuclei, neutrons that have large amounts of kinetic energy and energetic γ-rays. Image credit: Open University

Fusion reactions occur in the Sun. Hydrogen nuclei (such as deuterium and tritium – basically, hydrogen with extra neutrons) fuse to form helium, and the mass defect is released as energy.

Nuclear fusion

Fusion: under the high pressure and temperature in the core of a star, it is possible for nuclei to be forced together. In this example the two hydrogen isotopes (deuterium and tritium) are forced together to form helium (and a spare neutron is released). The total mass of the products are less than the mass of the reactants – the lost mass is released as energy. Image credit: Lancaster University.





  1. Nicely written 🙂

  2. Nice blog – are you enjoying/did you enjoy the Open Uni course? I’ve just signed up myself and came across your blog while looking for a review.

    • Thanks!

      Just finished my TMA today 🙂

      Yeah, pretty good. Learned some stuff I didn’t know. Made me think about areas I’d never previously considered.

      I just did this course to get some knowledge in this area after Fukushima (my next “proper course” – i.e. related to by main area of interest – doesn’t start until November, so I had spare time).
      It would be a good course for someone new to OU or new to science.

  3. Thanks for this –

    Hope the TMA went well!!

    I managed to miss this comment. I’ve signed up to the course starting in September and I’m quite happy with the decision. I am studying for a degree in Renewable Energy engineering at the moment so I’m not really new to science, but I am doing this course more through interest (Renewables and Nuclear have such a bizarre relationship that I thought I should learn a bit more about nuclear – I’m fascinated by them both).

    What course are you taking in November?

    • S283 – Planetary Science

  4. Thank you Boss!!!!!!! It was very helpful

  5. nice!!!!! It is very useful for me .


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