Nuclear Fission – Learn

In nuclear fusion, two smaller nuclei fuse to form a larger nucleus releasing energy at the same time. Fission is essentially the opposite of this process, as a larger nucleus divides into two or more smaller nuclei, again releasing energy. Nuclear fission is predominantly stimulated by the injection of a free neutron into a large nucleus. This warps the shape of the nucleus, causing two or more positive centers of mass to push away from each other, resulting in the separation of the nucleus into smaller components.

A nuclear fission has been visualised above. The diagram shows a neutron entering the nucleus of an atom, causing it to split in two. The small “explosion” presented in this image, represents that energy has been released in this reaction, although importantly, this is not as much as a fusion reaction. It can also be seen that several neutrons have been released as byproducts, which has important consequences. Should these free neutrons collide with another fissile nuclei, another reaction could occur, which in turn would release more energy and free more neutrons. This has the potential to start a chain reaction, where one nuclear fission reaction leads to another reaction and then another, and so on. This was hypothesised prior to being observed, although immediately proposed a conundrum, just how much energy could be released through a nuclear chain reaction. 

Uranium-235 is a commonly used isotope capable of undergoing nuclear fission. The nuclear reactions provided below highlight that there are multiple possible fission reactions that can occur, each containing different products (including the number of neutrons released). Some of these reactions occur at slightly different frequencies, and there are yet more different pathways that the reaction can take. 

Energy released in fission reactions

To understand the energy released during a fission reaction, a knowledge of Einstein’s mass-energy equivalence is required. This famous relationship, E=mc2, can be used to explain the apparent disparity in mass between the reactants and products. When an atom undergoes nuclear fission, the mass of the products is always less than the reactants, including any neutrons which are involved. This loss of mass reflects the difference in energy required to hold these nucleus together, which is released as other forms of energy during the reaction, heat and light. Drawing upon an existing understanding of electrostatics, the necessity of a binding energy is apparent, with many positively charged protons confined to a very small space. Quantitative analysis of the energy released in fission reactions is included in the next section from Module 8. 

Uncontrolled chain reactions 

As stated previously, when a nuclear fission reaction is stimulated by the injection of a neutron into the nucleus of an atom releasing more free neutrons, a chain reaction can occur. The free neutrons have the potential to interact with other fissile nuclei, undergoing reactions to release more neutrons, thus creating a positive feedback loop. This spread of this reaction is therefore only limited by the amount of fissile material. 

The diagram above depicts a chain reaction where two neutrons are released by each successive fission. Considering that this reaction can occur extremely quickly, and the number of atoms reacting double each generation, this means that in a fraction of a section billions of atoms could be reacting. Should there be sufficient fissile material, and the chain reaction allowed to proceed exponentially, a very large amount of energy is released. This is how nuclear weapons work, similar to the atomic bombs dropped on Hiroshima and Nagasaki in the second World War, intentionally instigating an uncontrolled chain reaction in a sample that is rich with fissionable nuclei. The term critical mass is used for a sample containing enough fissionable nuclei to sustain an uncontrolled chain reaction.

Controlled nuclear reactions 

In order to harness the power of nuclear reactions productively, they need to be controlled. The first nuclear reactor was built during the nuclear arms race during the second World War, secretly in Chicago. It was the use of a moderator and a neutron absorbing material that enabled the amount and motion of the free neutrons to be controlled, therefore, controlling the number of future fission reactions. This same approach is still used in nuclear reactors today, which are predominantly used for energy generation, medical purposes and scientific research. 

The diagram above shows a basic nuclear reactor, with the image on the left representing a reactor that it not operational, while the image on the right an operating reactor. Each of the coloured segments represent an important part of the reactor, the grey material around the outside is a containment chamber (1), the blue within this the moderator (2), the green rods are the control rods (3),and the red-orange rods in the center are the fuel rods (4). The functioning of each of these has been outlined below.

Containment chamber – This is a protective shield around the reactor that prevents the harmful gamma radiation produced in the reactor escaping into the local environment. It is usually made from thick, steel-reinforced concrete.

Moderator – Nuclear fission reactions are more likely to be successful if the neutron entering the nucleus is travelling relatively slowly, more slowly than those generally produced by other fission reactions. Moderators are materials used to slow down free neutrons in a reactor, to improve the probability of another reaction occurring. Graphite, an allotrope of carbon, and heavy water, water containing deuterium (hydrogen-2) instead of hydrogen-1, are materials generally used in reactors as moderators.

Fuel rods – This is the fissionable material in the reactor, most commonly Uranium 235. These are replaced about once every 6 years, thus contain a lot of fuel for the reactor.

Control rods – Maneuverable rods that control the amount of free neutrons in the reactor. They can be made of a range of materials, although most commonly cadmium, indium and silver are used. These elements are able to absorb neutrons into the nucleus of the atom without undergoing fission themselves. When the control rods are completely inserted into the reactor, it is shut down, as there are insufficient free neutrons to sustain a chain reaction. The rods can be removed different increments to allowing for fusion to occur freely, therefore, controlling the energy released by the reactor.

Spontaneous nuclear fission

Spontaneous nuclear fission is a rare form of nuclear decay, that only occurs in very large nuclei. While alpha decay technically leads to the formation of two nuclei, is not considered an example of fission. For the purpose of the HSC Physics, fission generally refers to the splitting of a nucleus following the injection of a neutron into the nucleus.