Key Takeaways
- Nuclear energy is the only type of power used today that harvests the energy stored in atomic nuclei.
- Nuclear fuel is thousands of times more energy-dense than fossil fuels and does not release any carbon emissions or pollution, helping meet climate goals.
- Safety is at the heart of nuclear reactor technology, which prevents nuclear accidents through inherently safe physics, durable materials, redundant approaches to design and operation, highly detailed regulations, and frequent inspections and maintenance.
Nuclear power plants convert the energy stored in atomic nuclei into other useful forms, such as heat and electricity. By releasing the energy stored in atomic nuclei, nuclear power plants can continuously produce large amounts of electricity with zero carbon emissions for months between refuelings. Ounce-for-ounce, nuclear fuel contains thousands of times more energy than fossil fuels. In this article, we’ll dive deeper into the characteristics that make nuclear power unique.
In This Article
The Atomic Nucleus
Atoms are made of protons, neutrons, and electrons. Protons and neutrons are densely packed inside the atomic nucleus, which is surrounded by electrons traveling at fast speeds in diffuse orbits.
Protons have a positive electrical charge, while neutrons are neutral. This means that the atomic nucleus has a strong positive charge, causing protons to repel each other like magnets.
Packed in a dense nucleus on such a tiny scale, the repelling forces inside the nucleus are immense — so why doesn’t the nucleus break apart?
The Strong Nuclear Force
The strong nuclear force holds the atomic nucleus together, overcoming the repulsion between protons. It’s one of the four fundamental forces of nature, and it is 1038 times stronger than gravity. The strong nuclear force shows just how much energy is stored in the atomic nucleus. In the first half of the 20th century, physicists realized the potential of this huge amount of energy and sought a way to harness it. While chemical reactions involve transfers of electrons but not protons or neutrons, nuclear reactions combine, change, and break apart the energy-packed nuclei of atoms. This is why nuclear reactions release thousands of times more energy than chemical reactions, such as gasoline combustion in a car. A single nuclear fuel pellet, the size of a fingertip, contains as much potential energy as one ton of coal.
Fission – Splitting the Atomic Nucleus
Some atomic nuclei are more stable than others. A carbon-12 nucleus, which contains six protons and six neutrons, is relatively small and highly stable. While the strong nuclear force easily holds it together, larger nuclei can be broken apart. Other isotopes of carbon — nuclei of carbon with different numbers of neutrons — are not as stable. For example, carbon-14 is a naturally occurring carbon isotope with two extra neutrons that decays over thousands of years. In nuclear reactors, uranium-235 is the most common fuel isotope. It is fairly stable under normal conditions — but when a low-energy neutron hits a uranium-235 nucleus, the nucleus splits, releasing an enormous amount of energy as heat. Fission is the process of splitting the atomic nucleus to release energy. The first nuclear reactors were designed and tested during the 1940s using fission. From the 1960s onward, hundreds of nuclear power plants were built around the world. Today, the U.S. has 92 operating nuclear reactors that provide around 20% of its electricity by splitting atoms and harnessing their large quantity of stored energy.
How Does a Nuclear Power Plant Work?
Nuclear plants safely accomplish two goals: they sustain a fission reaction and convert the energy the reaction releases into other forms.
Fission Chain Reaction
Nuclear power plants create a fission chain reaction that continuously releases a steady amount of energy. In addition to splitting the nucleus, fissioning a uranium atom releases 2-3 high-energy neutrons and a large amount of heat. Neutrons can be controlled to sustain the chain reaction: a neutron splits an atom, which releases more neutrons that split more atoms. Ideally, only one of the neutrons produced in each fission goes on to cause another fission — otherwise, the reaction would grow or shrink exponentially with each fission. A reactor is in a stable state called “critical” when each fission leads to one neutron that causes one more fission. A few important factors help sustain the fission reaction:
Enrichment
Natural uranium found in the Earth’s crust contains about 0.7% uranium-235 and over 99% uranium-238. But uranium-238 is too stable to sustain a fission reaction in today’s reactors, and natural uranium can’t be used. Instead, a process called enrichment is used to increase the concentration of uranium-235 to sustain fission. Reactors in the U.S. today use nuclear fuel that is enriched to 3-5% uranium-235. This enrichment level is enough for a slow, steady burn in nuclear reactors, but it cannot be used for nuclear weapons, which use highly enriched uranium with more than 90% uranium-235.
Moderation
In today’s reactors, fission produces high-energy neutrons that need to be decreased in energy before they can cause more fissions. This process of decreasing neutron energy is called moderation. High-energy neutrons collide with the atoms in a moderating material, like water, reducing their energy. Once a neutron has collided enough times to reach thermal energy (around 10,000,000 times less energy), the neutron will cause a fission in a uranium-235 nucleus, releasing more energy and high-energy neutrons to start the process over again.
Geometry
The geometry of a nuclear reactor plays a significant role in controlling the reaction. If a reactor is cylindrical, then the center will have the highest concentration of neutrons and, therefore, the highest number of fissions. The perimeter, on the other hand, can “leak” neutrons out of the reactor. Sometimes, a reflector is placed around the outside of a reactor to minimize neutron leakage and raise efficiency. The spacing between fuel rods must also be optimized to balance moderation and heat transfer properties. Extensive simulation tools allow nuclear engineers to precisely predict the neutron behavior properties of nuclear reactors so they can design and build reactors that continuously sustain fission reactions and energy production.
Control Mechanisms
To carefully control the rate of the nuclear reaction and ensure a stable state of criticality, retractable control rods can be used to absorb excess neutrons. Today’s nuclear reactors have many essential safety features that regulate the fission reaction. Every reactor has control rods, which can be inserted into the reactor core to immediately stop the nuclear reaction, in case of emergency. Second, today’s reactors operate with a negative reactivity coefficient, which means that as the fuel heats up, the nuclear chain reaction slows down. This means that a “runaway reactor” situation is physically impossible — a reactor with too many neutrons and fissions will automatically slow down, cooling back to a safe level. Third, some reactors use “burnable poisons” in their coolant. These are materials like boron dissolved in the water that “poison” the nuclear chain reaction by absorbing neutrons like a control rod. As the reactor operates, these materials will absorb neutrons and “burn up”, consuming them.
Electricity Production
Fission releases large amounts of heat in the reactor core. Nuclear plants contain equipment that is designed to capture, transfer, and use this heat to produce electricity.
Coolant
In the majority of operating nuclear reactors around the world today, water is used as both a moderator and a coolant. By passing through the nuclear core, water moderates neutrons to sustain the chain reaction, and it also absorbs the heat produced by the reaction. Water circulates through the reactor core in the primary coolant loop. The hot water enters a heat exchanger, which transfers its energy to a secondary coolant loop that is not exposed to the high radiation field in the nuclear core. This process cools the water in the primary coolant loop, which cycles back into the nuclear core.
Steam Generator, Turbine, and Condenser
In pressurized water reactor designs, the hot water in the secondary coolant loop leads to a steam generator, which converts it into steam. The steam spins a turbine, generating electricity. Typical nuclear plants today have an efficiency of about 35% — in other words, 35% of the energy released in the fission reaction is converted into usable electricity. The steam can be collected and condensed back into liquid water using a cooling tower or other source of cooling water like a river or ocean to reduce the temperature of the steam going into a condenser. This cools the water and cycles it back to the start of the secondary loop. Some reactor designs use the heat of nuclear fission to directly boil water into steam, and use that steam to spin the turbine and generate electricity. These are called boiling water reactors, and represent around a third of the nuclear power reactors in the U.S.
Is Nuclear Energy Safe?
Nuclear energy is one of the safest forms of energy available today. Every reactor is designed, operated, and decommissioned with safety considered at every step. While no technology is perfect, risks can be lowered to acceptable levels. Nuclear power plants are designed to lower the risk of spreading nuclear materials as much as possible, and in a hypothetical worst-case scenario where materials do spread, they are designed to limit risk to human health and the environment as much as possible.
Designed for safety
Today’s reactors are designed with control rods and negative reactivity coefficients, making them easy to shut down instantly in case of emergency and making it impossible for a reactor to lose control. Reactors use at least three strong levels of containment to prevent radioactive material from spreading out of the core. These layers of containment start with the nuclear fuel itself. The fuel pellets are contained in airtight metal tubes made of a strong cladding material, which is stable up to thousands of degrees. These sealed fuel rods help to prevent the spread of radioactive fission products. Outside the fuel and cladding, the reactor operates within a thick-walled pressure vessel, which is designed to withstand high temperatures and pressures. The reactor and pressure vessel are housed inside a large containment building. These are designed with thick, reinforced concrete walls that are capable of withstanding extreme weather and even aircraft impacts without rupturing.
Carefully Monitored
Outside the containment building, all nuclear plants are surrounded by environmental radioactivity detectors that will quickly discover any radiation leaks. If unsafe levels of radiation were to hypothetically escape the containment building, the detectors would quickly find it and shut down the plant immediately. Nuclear plants are also frequently inspected and analyzed for safety by regulatory agencies, like the Nuclear Regulatory Commission (NRC) in the United States, the Nuclear Safety Authority in France, or the Nuclear Safety and Security Commission in South Korea. These organizations are responsible for ensuring the safety of nuclear plants and protecting the public by following high transparency standards.
Minimal Waste
In the U.S., the NRC also ensures that nuclear waste is handled carefully such that radioactive materials can never spread. The NRC tests and certifies waste-storage technologies to guarantee their long-term safety. The U.S. has relatively minimal amounts of nuclear waste. If we accumulated all the used nuclear fuel that the U.S. has ever produced in 60 years of operation, it would fit on a football field and rise about 10 yards high. Future reactor designs may be able to reduce nuclear waste by producing less, or even by fissioning it as fuel.
Can Nuclear Energy Help Meet Environmental Goals?
In the past decade, governments and utilities have taken greenhouse gas emissions more seriously, with many committing to reach net-zero emissions by 2050. Nuclear power appears to be a necessary technology to help achieve this goal. Thanks to the high density and power output from nuclear power plants, nuclear energy presents a reliable, scalable alternative to fossil fuels. Most nuclear reactors operating today produce more than 1 gigawatt of electricity, require just over a square mile of land, and release zero greenhouse gas emissions during operation. Nuclear plants still have remaining challenges such as high construction costs and mixed public perception. New designs and innovations aim to lower costs and construction timelines while making nuclear plants safer and more efficient than ever.
This article is one of many developed by EPRI’s Advanced Nuclear Technology program. Stay up-to-date on the newest nuclear technologies by becoming a member today.