22.6: Nuclear Energy

What is Nuclear Energy? How Does it Work? 

Nuclear energy is the energy that is stored in the nuclei of atoms; it comes in two types. Fission energy is released when large atoms like uranium split apart into smaller atoms. This is constantly taking place within the Earth—and it is part of the source of geothermal energy—and all around us, but the rate is very slow. We can build devices that increase that rate enough to generate heat in a controlled way, but there are some downsides to the technology. The heat generated from nuclear fission is used to boil water, produce steam, and drive turbines that generate electricity. Unlike fossil fuels, nuclear energy does not produce carbon dioxide during electricity generation, which has made it an important part of discussions about low-carbon energy sources. However, nuclear energy does produce radioactive waste that must be carefully isolated and stored for thousands of years, and uranium is a nonrenewable resource. The United States currently generates approximately 18 to 19 percent of its electricity from nuclear power, making it the country's largest source of low-carbon electricity.

Fusion energy is released when small atoms like hydrogen are fused together to make larger atoms. This is what is happening inside the sun, so we use that energy all of the time. We can reproduce nuclear fusion here on Earth in in the form of a hydrogen bomb, but that technology doesn’t allow for a controlled rate of energy release. Over the past several decades governments of several countries have spent tens of billions on research into controllable nuclear fusion. Fifty years ago, commercial nuclear fusion was considered to be less than fifty years away. During that time significant progress has been made in understanding what is needed to make that happen, but in 2021, it is still likely that a proof of the concept of fusion energy production is 5 to 10 years away, and that a fusion energy industry is likely another 25 years beyond that.

Nuclear Fission

Nuclear fission has been used for generating electricity since the late 1950s and there are currently around 440 fission reactors operating in 30 countries, producing about 10% of the world’s electricity. In a few countries, including France, Slovakia and Ukraine nuclear fission makes up more than 50% of electricity generation^[1].

While naturally occurring uranium in rock splits apart only very slowly, the rate of fission is accelerated many thousands of times in a fission reactor because the uranium is highly concentrated, and the fuel is packed close enough together so that the neutrons released by one fission event will almost immediately trigger another fission in a nuclear chain reaction. An important point is that the chain reaction process must be carefully controlled by substances called moderators and, in the event of some irregularity, the reaction can be slowed relatively quickly by inserting control rods that inhibit the chain reaction process. It is important to realize that although a fission reactor can be dramatically slowed by inserting the control rods, the massive reactor core will still remain very hot for several days. This residual heat has been the cause of several nuclear accidents. A diagram of a type of fission reactor is provided on Figure 9.5.1, and the important components are described.

Figure \(\PageIndex{1}\) The Components of a Gas-Cooled Nuclear Fission Reactor . The uranium fuel elements are shown in orange. The graphite moderator, which keeps the reaction from getting out of control, is grey. The control rods that can be used to dramatically slow the chain reaction are black. Heat generated by the fission in the fuel is heats a gas which is circulated through a heat exchanger to boil water that is then used to run a steam turbine.

Modern nuclear reactors have many levels of protection against irregular operating conditions and against more significant failure and out-of-control behaviour. While that might give us some comfort, it cannot be denied that there have been some major failures in the nuclear power industry, at least one of which has had significant human and environmental implications. Three such failures are described in Box \(\PageIndex{1}\). Each of these led to a loss of public confidence in the safety of nuclear power, and to reductions in the number of reactors ordered and constructed in subsequent years.

Box \(\PageIndex{1}\) Nuclear Fission Plant Failures

Three significant nuclear plant failures—Three Mile Island (Pennsylvania), Chernobyl (Ukraine) and Fukushima (Japan)—have shaped the public’s perception of the safety of the nuclear industry, and have also informed the industry’s efforts to make nuclear reactors as safe as possible.

Figure \(\PageIndex{2}\) The Chernobyl Nuclear Fission Reactor Following the Accident in 1986

The Three Mile Island accident in March 1979 involved an operator error, the failure of a cooling system valve and the failure of the system that was meant to warn the operators of the valve’s malfunction. This led to a loss of coolant within the reactor vessel and resulted in partial melting of the nuclear fuel. There were high levels of radioactivity within the plant, and some radioactive materials were released into the environment via steam. Health officials eventually concluded that there were no deaths or significant illnesses that could be attributed to the accident, however that conclusion was controversial at the time, and remains so in the minds of many people.

The Chernobyl accident of April 1986 also involved an operator error and a system failure that resulted in the reactor power level increasing when it should have been decreasing (Figure 9.5.2). This led to a steam explosion that ruptured the reactor core and triggered an open-air reactor fire that continued for 9 days, releasing airborne radioactive material into the environment. A total of 28 plant workers and firefighters died within several days of the accident.^[2] A 2006 United Nations report concluded that as many as 4000 cancer deaths are likely to occur amongst 5 million residents in the contaminated region around the plant over the 80 years following the accident, and that this represents a 0.3% increase over expected cancer deaths for that region.^[3]

The Fukushima accident of March 2011 was the direct result of a magnitude 9 earthquake and a subsequent tsunami. Although none of the reactors was damaged by the earthquake, they were automatically shut down when the earthquake struck and therefore could not provide power to the cooling systems. Six external power supply systems failed because of the earthquake and although an emergency diesel power plant continued to supply power to cooling pumps, this was destroyed 50 minutes later by the 15 metre-high tsunami that overtopped protective barriers and inundated the plant. The result was that there was no cooling available for the still-hot reactors, three of which suffered melting within the reactor cores. No deaths have been attributed to the Fukushima accident, and although it is estimated that approximately 130 deaths may eventually occur because of elevated cancer rates , that number is small compared with the number of expected cancer deaths from other causes.

Nuclear Waste Storage

In a typical reactor the fuel elements will last for 18 months to a few years. Some types of reactors can be refuelled while still operating, but most have to be shut down for periods of up to several weeks. The spent fuel is removed and replaced with fresh fuel. The highly radioactive spent fuel is stored in swimming-pool sized tanks at the reactor site for about 5 years, and then is transferred to dry storage facilities, which are also on-site in most cases. On-surface storage of nuclear waste is not a viable option because the materials remain radioactive for thousands of years and represent both a health risk and security risk (from the perspective of nuclear proliferation). Several countries, including Canada and the United States, have conducted research into the deep underground storage of nuclear waste, but as of 2021, only Finland has an operating long-term storage facility.

Figure \(\PageIndex{3}\): Proposed Yucca Mountain nuclear waste facility. Source: United States Department of Energy (public domain)

Although the waste from nuclear fission is a serious problem, the volumes are not huge. The amount of radioactive waste produced for a person who used nuclear energy for their lifetime supply of electricity is about 3 kg. Nuclear fuel is quite heavy, so that amount would likely fit within a coffee cup.

Uranium Mining and Processing

Uranium, the primary element used to generate nuclear fission energy, is a naturally occurring radioactive element found in trace amounts in most rocks, soils, and even seawater. Although uranium is approximately 100 times more common than silver, the fissile isotope uranium-235, which is the form capable of sustaining a chain reaction, makes up only about 0.7 percent of naturally occurring uranium. The remaining 99.3 percent is uranium-238, which does not directly contribute to fission. Uranium tends to concentrate in the continental crust through geological processes, particularly in granitic rocks and in sedimentary deposits where hydrothermal fluids or groundwater have mobilized and redeposited it. In the United States, the most significant uranium deposits are found across the Colorado Plateau of Utah, Colorado, New Mexico, and Arizona, where groundwater leached uranium from volcanic ash and basement rocks and carried it through permeable sandstone formations. When this uranium-bearing water encountered organic-rich reducing zones within the rock, the uranium precipitated as uraninite, forming the large sedimentary uranium deposits that supplied most of the country's uranium during the Cold War era and remain an important domestic resource today.

Once uranium ore is mined, it is processed at a mill where it is crushed and chemically treated to dissolve the uranium and separate it from the surrounding rock. This produces a solid uranium oxide concentrate commonly called yellowcake, which must be further refined and enriched before it can be used as reactor fuel. Enrichment is necessary because natural uranium contains too little U-235 to sustain the chain reaction required in most commercial reactors.

Media Attributions

• Magnox Reactor Chematic By Emoscopes, CC BY-SA 3.0 Unported, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F..._schematic.svg
• Chernobyl Nuclear Fission Reactor from IAEA image bank, CC BY-SA 2.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...613115146).jpg

1. World Nuclear Association, https://www.world-nuclear.org/ ↵
2. Wikipedia contributors. (2021, November 14). Chernobyl disaster. Wikipedia (accessed November 15, 2021), https://en.Wikipedia.org/wiki/Cherno...iate_aftermath ↵
3. The Chernobyl Forum: 2003–2005. (2006). Chernobyl’s legacy, health, environmental and socio-economic impacts (second revised version). World Health Organization. https://www.who.int/publications/m/i...on-and-ukraine ↵