Foundations of Nuclear Energy
The law of conservation of energy states that energy cannot be created or destroyed, it only can be changed from one form to another. Thus, energy in different forms must be “stored, released, transformed, transferred, and ‘used’ in both natural processes and man-made devices.”¹ Nature could be viewed in terms of just two basic entities, particles and energy, but even this distinction can be removed because we know matter can be converted into energy and visa-versa.
Energy fundamentals
In order to understand the release of nuclear energy and its conversion into thermal and electrical forms, one must be familiar with some fundamental principles of physics. The ability to do work is associated with a limited number of basic forces that exist, such as gravitational, electromagnetic, and strong and weak nuclear. If a constant force ‘F’ is applied to an object to move it a distance ‘s’ the amount of work done is W = Fs. Once work is done, the object possesses stored energy, also known as ‘potential energy’. Next, a force ’F’ acting on a mass ’m’ provides an acceleration ’a’, given by Newton's Law F = ma. Starting from rest, an object gains a speed ’v’, and, at any instant, an object has a ‘kinetic energy’ or energy of motion in the amount of EK=(½)mv², demonstrating the conservation of energy. Energy takes on many forms, including gravitational energy, exemplified by a waterfall; mechanical energy, associated with the spinning of a turbine which is converted into electrical energy by a generator that includes an electrical potential difference providing the force to move electrons.¹ Electrical supply systems then convert this energy into light, as seen in light bulbs, or into heat for use in homes and businesses. Energy, in its various forms, can be harnessed, transferred, and transformed to power the world around us; the alterations of atomic nuclei allow for a significant energy release from small quantities of matter, especially when compared against other energy sources.
Nuclear efficiency
The concept of energy and matter being interchangeable is fundamental to nuclear power. As expressed by Einstein’s famous equation E=mc², small amounts of matter can be converted into vast amounts of energy (kinetic energy equals the product of mass and the speed of light squared), making nuclear fuel significantly more powerful than chemical fuels, like coal or gas. For instance, the energy released from one kilogram of nuclear fuel is over one million times greater than that from a kilogram of chemical fuel.² In the production of nuclear power, uranium serves as the primary fuel. “Atoms absorb neutrons as they undergo fission, breaking into smaller atoms and releasing significant heat. This heat boils water to produce steam which, in turn, powers turbines to generate electricity.”² The remarkable efficiency and power density of nuclear fuel make it a compelling solution for large-scale energy needs; its principles are grounded in the unique behaviours and transformations of atomic nuclei.
Atomic structure
Atoms, the smallest units of matter, consist of a central nucleus made up of protons and neutrons, surrounded by electrons in orbitals. The number of protons in the nucleus determines the element’s identity and is called the atomic number. Neutrons, together with protons, contribute to the atom’s mass but do not affect its charge.
Isotopes and reactions
Each element can exist in different forms, called isotopes, which vary by the number of neutrons in the nucleus. Uranium, when mined, is composed of roughly 99.3% uranium-238, 0.7% uranium-235. Both contain 92 protons, but U-235 has 143 neutrons, while U-238 has 146, giving them atomic masses of 235 and 238, respectively. This small difference in neutron count affects their properties in nuclear reactions—U-235 is fissile (capable of being split to sustain a nuclear chain reaction), while U-238 is not.
Nuclear processes
Nuclear reactions release energy by altering atomic nuclei, unlike chemical reactions, which involve only the outer electrons of atoms. Two primary types of nuclear reactions are fission and fusion.
Fission
In fission, a heavy atomic nucleus (like uranium-235) splits into two smaller nuclei when struck by a neutron, releasing a large amount of energy and additional neutrons. This process can lead to a chain reaction, where the released neutrons go on to split more atoms, creating a continuous release of energy. Fission is the reaction used in nuclear power plants and is primarily the type of reaction that is associated with nuclear power production.
Fusion
Fusion occurs when two light atomic nuclei, such as hydrogen isotopes, combine to form a heavier nucleus, releasing even more energy than fission. Fusion is the process powering the sun and stars and requires extremely high temperatures and pressures. While promising as a potential energy source due to its abundance and cleaner by-products, fusion has yet to be harnessed for practical power generation on Earth.
Chain reactions
In a chain reaction, neutrons produced by one fission event trigger further fission events, releasing energy and additional neutrons in the process. This self-sustaining sequence can be carefully managed for power generation.
In nuclear power plants, controlled fission is achieved by using materials like control rods, which contain non-fissile elements such as hafnium, boron, etc. to absorb excess neutrons and regulate the reaction rate. When each fission event produces just one new reaction (a stable chain reaction), a steady output of energy is maintained, which is essential for safe power generation.
Criticality and the multiplication factor
Criticality refers to the state of a nuclear chain reaction, determined by the balance of neutrons within the system. This is governed by the multiplication factor (k), which describes the average number of neutrons from one fission event that go on to cause further fissions:
Subcritical (k < 1): Fewer than one neutron per fission event causes further reactions, and the chain reaction gradually dies out.
Critical (k = 1): Exactly one neutron from each fission event triggers another, resulting in a stable, self-sustaining chain reaction. This is the desired state for nuclear reactors.
Supercritical (k > 1): More than one neutron per fission event causes additional reactions, leading to an exponential increase in fission events. This state can be used to increase power rapidly but must be carefully controlled to avoid runaway reactions.
By maintaining a critical state (k = 1) using control rods and other safety measures, nuclear reactors ensure a steady output of energy.¹
Radioactivity
Radioactivity is the spontaneous transformation, or decay, of unstable atomic nuclei, releasing energy and particles. Many heavy elements, especially those with atomic numbers greater than 82 (like lead), are radioactive.¹ There are three main types of radioactive decay: alpha decay, which emits two protons and two neutrons and has low penetration power; beta decay, which emits electrons or positrons with moderate penetration power; and gamma decay, which emits gamma rays or high energy photons released as the nucleus moves to a lower energy state and has high penetration power requiring dense materials for shielding.¹
Half life
This is the time it takes for half of the atoms in a radioactive substance to decay into a more stable form. This period varies widely among isotopes, from fractions of a second to billions of years, and helps determine the longevity and behaviour of radioactive materials in various applications. For example, Iodine-131 has a physical half-life of 8.0 days,⁴ whereas Uranium-238 has a half-life of 4.5 billion years. Since Earth is around 4.6 billion years old, the original amount of Uranium-238 present at its formation has now diminished by half.⁵
Effects of radiation
Ionising radiation has the ability to strip electrons from atoms, forming ion pairs that can disrupt biological molecules, particularly those with covalent bonds, such as those in human tissue. To protect against these effects, basic safety measures are essential. These include: using proper shielding, limiting exposure, and selecting materials that can reduce the biological impact of radiation.¹
Radiation and its uses
Radioactivity has several valuable applications across fields. In medicine, radioactive isotopes play a role in diagnostic imaging, as seen with radiopharmaceuticals, and in cancer treatments through radiation therapy. In archaeology and geology, carbon dating and other radiometric methods rely on the known decay rates of isotopes to estimate the age of ancient materials. In nuclear energy production, the principles of radioactive decay and fission make it possible to sustain controlled chain reactions in reactors, providing a reliable and stable source of power.¹
Footnotes
Raymond L. Murray and Keith E. Holbert, Nuclear Energy: An Introduction to the Concepts, Systems, and Applications of Nuclear Processes, 8th ed. (Amsterdam: Butterworth-Heinemann, 2019).
Uranium Digital, Modernising Uranium Trading, White Paper, June 2024. https://www.uraniumdigital.com/whitepaper
Energy Encyclopedia, "Fission Chain Reaction," accessed November 2024, https://www.energyencyclopedia.com/en/glossary/fission-chain-reaction.
ScienceDirect, "Iodine-131," accessed November 2024, https://www.sciencedirect.com/topics/medicine-and-dentistry/iodine-131#:~:text=This%20iodine%20isotope%20has%20a,2.3%20and%200.6%20mm%2C%20respectively.
Ministry of the Environment, Japan, "Radiation and Radioactivity," accessed November 2024, https://www.env.go.jp/en/chemi/rhm/basic-info/1st/01-02-08.html#:~:text=Uranium%2D238%20has%20a%20half,has%20now%20reduced%20to%20half.
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