A beginner's guide to nuclear energy

This is a short guide that answers a few of the questions I had around nuclear energy. The answers are a summary (often direct copy) of the information found in the sources.

How does nuclear energy work?

  • Nuclear energy works by splitting or combining atoms to produce energy. It all started in the late 1930s, when it was discovered that some particularly large atoms can split in two in a reaction that releases large amounts of energy. In the process, a certain amount of the large atom’s mass is converted to pure energy, following Einstein’s formula E = MC2.
  • When these atoms are arranged properly in a machine, one splitting atom can cause nearby ones to split, creating a chain reaction. All commercial  nuclear power plants  in operation use this reaction to generate heat which they turn into electricity.
  • Fuel is made up of heavy atoms that split when they absorb neutrons and is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause other atoms to split. Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is transferred to outside of the reactor by a coolant, which is most commonly water (the energy from the reactor is dissipated to the water that passes in the circuit). That water turns into steam, which spins a turbine to produce electricity.
  • Reactors use uranium for fuel. The uranium is made into small ceramic pellets and stacked together into sealed metal tubes called fuel rods. Typically, more than 200 of these rods are bundled together to form a fuel assembly. A reactor core is usually made up of a couple hundred assemblies, depending on the power level.  Inside the reactor vessel, the fuel rods are immersed in water which acts as a coolant and moderator. The moderator helps slow down the neutrons produced by fission to sustain the chain reaction.
  • Today, nuclear energy generates about 10% of world's electricity.

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How does a nuclear chain reaction start and end?

  • A nuclear chain reaction is started with a neutron shot from outside with enough energy or by keeping a neutron emitting material like Americium inside the reactor. Once the neutrons hit a fissile material (in a nuclear reactor, normally Uranium-235) then they go through fission reaction and emit two or three fission products and 2 or 3 neutrons. These newly born neutron starts another fission, starting the chain reaction.
  • The nuclear reaction is stopped with control rods. Control rods are made of materials such as Boron, Halfnium, or Cadmium, which have unique properties that allow them to absorb the neutrons used in the chain reactions without breaking into additional atoms. This way the control rods can effectively stop the chain reaction by being pushed into the core of the nuclear reactor.

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What is uranium's role in the nuclear reaction?

  • Uranium is the element of choice in commercial nuclear power plants, because its atoms are easily split apart. It needs to be enriched to be used in nuclear fusion, to ensure that is contains at least 3% of U-235, the isotope* which can sustain a fission chain reaction. U-235 normally makes up about 0.72% of natural uranium.
  • Uranium is found in rocks in concentrations typically between 0.1% and 0.2% and its must be mined to be extracted.

* an isotope is a variant of a chemical element which differs in its number of neutrons. All isotopes of the same chemical element have the same number of protons in its nucleus. 

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What are the most common types of nuclear reactors?

  • The most common type of nuclear reactor is the Pressurized Water Reactor (PWR), which uses regular water as a coolant. It is called "pressurised" because the water is kept in extremely high pressure in order not to boil*.
  • The uranium is hit with nucleus that causes nuclear fision. The atoms are broken down, releasing radiation (which heats the water in the reactor) and releasing more nucleus that cause a chain reaction. Then, a separate water circuit is heated to a boil, which turns it into steam and moves a turbine that created electricity through eletromagnetism. 
  • Despite being the most common, the most famous nuclear accidents (Chernobyl and Fukushima) did not use Pressurized Water Reactors. Fukushima used a Boiling Water Reactor - where the reactor core heats water to a boil - while Chernobyl was a RBMK - where graphite acts as the reactor moderator, instead of water.


Pressurized water reactor - Simple English Wikipedia, the free encyclopedia
Pressurized water reactor diagram- Wikipedia


*The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the surrounding environmental pressure.

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Why is there so much nuclear waste?

  • Waste is generated during the nuclear fission. Current nuclear reactors only generate energy from about 10% of its uranium input. When the atoms split, the two smaller atoms that are formed still have leftover energy to give, in the form of radiation. This radiation can take up to 100,000 years to be fully released, and hence it must be kept away from the biosphere and safely stored.
  • Unlike popular belief, this waste is solid, not liquid. It is composed of the used metal fuel rods that contain the enriched uranium used in the nuclear reaction. 
  • According to the Energy Department of the United States, the total amount of used fuel by the US nuclear plants each year is 2,000 metric tons. All of the used fuel since the 1950s could fit on a single football field at a depth of less than 10 yards. As a reference, In the US in 2019, nuclear energy accounted for 9% of primary energy consumption.
  • According to the World Nuclear Association (an organisation that promotes nuclear energy) not all waste produced by the nuclear power industry is dangerous. They claim that 97% of the waste produced is low- or intermediate-level waste. This type of waste is easily disposable and does not pose a hazardous risk to our health.

US primary consumption by major source - U.S Energy Information Administration


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Why do nuclear accidents happen?

  • Nuclear accidents, as in the case of Chernobyl or Fukushima, typically happen because of a core meltdown accident. 
  • A core meltdown accident happens when heat from the nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. Depending on the root cause of the accident (e.g a problem that stopped coolant from reaching the reactor), the melted nuclear fuel (this would typically be uranium) breaches the different layers of containment of the nuclear reactor. These breaches lead to explosions that contaminate the environment with radioactive material.
  • In Fukushima, in 2011, an earthquake and tsunami flooded the area around the nuclear power plant. In an over simplification, the tsunami flooded parts of the reactors, which caused a failure in the emergency generators and loss of power to the circulating water pumps. This caused a loss of the supply of cooling the reactor, which lead to a nuclear meltdown and 3 subsequent hydrogen explosions and the release of radioactive material to the environment.
  • The Chernobyl accident in 1986 was also a core meltdown accident. A safety test designed to test the plant's ability to provide reactor cooling water circulation during a prolonged electrical power outage. Multiple reasons - including reactor design flaws and serious breaches of protocol - caused the nuclear reactor to overheat, vaporise the pressured water and resulted in a steam explosion (caused by violent boiling of water into  steam) that ruptured the reactor and contaminated the environment.

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What are new alternatives in the production of nuclear energy?

A new generation of nuclear reactors, Generation IV, promises increased safety, energy efficiency and reduced nuclear waste.  They differ from the current nuclear reactor in size (small modular reactors), in its coolant (not based on water) or in the physical reaction that triggers the energy release (fusion). The majority of the new designs are still far away from being deployed, with the exception of the small modular reactors from NuScale. 

Small modular reactors

  • Small modular reactors, or SMR are slimmed-down version of conventional fission systems that promise to be cheaper and safer. The smaller form factor allows these reactors to be built in a  central factory and transported to the site for assembly. Its design promised produce longer fuel cycles, reducing the amount of waste produced. 
  • Since there are multiple SMR designs, there are multiple approaches on safety. Multiple of them as classified as "passive strategies" because they do not depend  on operators intervention in order to bring the reactor to a safe shutdown in case of emergency. These methods rely on natural engineering of the reactors components and leverage phenomena such as gravity, heat convection and other to accomplish its safety functions. 
  • The first SMR is  expected to begin test operations at the Idaho National Laboratory in December 2025, with the first power module operating by 2029.

Alternative coolant reactors

  • Advanced fission reactors differ from traditional nuclear reactors in the use of alternative coolants. These include sodium, molten salt and helium. Some of the main benefits expected from advanced fission reactors are:
  • They can operate at atmospheric pressure (or much lower than in typical PWR) thus reducing the large, expensive containment structures used for PWRs and eliminating a source of explosion risk. By comparison, PWR operate at 75-150 times atmospheric pressure.
  • They operate at higher temperatures than traditional reactors, increasing its energy efficiency and reducing nuclear waste.
  • In the case of molten salt reactors, there are multiple ongoing projects working on this technology around the world. TerraPower, the company famously backed by Bill Gates, is a particular type of molten-salt reactor design, in which liquid natural uranium is mixed with molten chloride coolant in the reactor core, achieving very high temperatures while keeping pressure at atmospheric levels.

Fusion power

  • Finally, there is nuclear fusion. For some, this promises to be the new revolution in nuclear energy. Here, instead of harnessing energy from the split of uranium atoms, energy is generated from the combination of smaller atomic nucleus.  This process mimics what goes on stars such as the sun. On Earth, replicating the fusion conditions means temperatures as high as 150 million °C.
  • Nuclear fusion is expected to have several advantages over fission, including reduced radioactivity and little high-level nuclear waste. Additionally, nuclear fusion reactors are expected to be immune to nuclear meltdowns. These reactors work with very little fuel and with extremely controlled temperature and pressure, and any damage to the reactor would cause the nuclear reaction to stop in seconds or minutes.
  • Despite the promise, no design has been able to be net positive efficient, meaning outputting more power than needed as input.
  • This idea is not new. Since the 1920s much research has been devoted to nuclear fision, with the first man made fusion devices testes in the 1950s. Currently there are multiple ongoing efforts in the field, but none with a clear line of sight beyond experiments.

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