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Nuclear fission

Nuclear fission

Nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts.


Nuclear fission is the subdivision of a heavy atomic nucleus, such as that of uranium or plutonium, into two fragments of roughly equal mass. This process generates a large amount of energy as a reaction to the nucleus of an atom breaks up into two lighter nuclei. This process may take place spontaneously, or it can be induced by the excitation of the nucleus. The excitation can be facilitated by a variety of particles, or with electromagnetic radiation such as gamma rays. This process creates a large quantity of energy, forms radioactive products, and emits several neutrons.

Energy release

When large nuclei, such as uranium-235, fissions, the resulting release of energy is so large there is a measurable decrease in mass, meaning some of the mass is converted to energy. The amount of mass loss in fission is equal to about 3.20 x 10-11 Joules of energy. This is an exothermic reaction, capable of releasing 200 million eV. And because of this, fission is used in electricity generation, especially when compared to the energy release of more traditional electrical generation methods such as burning coal.

With the emitting of neutrons, these neutrons can induce fission in a nucleus of fissionable material and can release more neutrons, causing a chain reaction in which an enormous amount of energy is released. If this energy release is controlled, such as in a nuclear reactor, this chain reaction can provide power generation. While, if uncontrolled, as in the case of an atomic bomb, the fission process leads to an explosion of massive destructive force.


Enrico Fermi split the uranium nuclei in 1934, and in effect, discovered the fission process. He believed certain elements could be produced by bombarding uranium with neutrons, and while he expected the new nuclei to have larger atomic numbers than the original uranium, Fermi found that the formed nuclei were radioisotopes of lighter elements. This discovery opened the potential for nuclear fission and its use for either electricity generation and transportation or massive destruction. The risk and benefit ratio of its applications have generated sociological, political, economic, scientific advances, as well as concerns.

The experiments of Enrico Fermi were interpreted by Lise Meitner and Otto Frisch, two German physicists who used the term fission to describe the disintegration of a heavy nucleus into two lighter nuclei. In 1939, Frédéric Joliot-Curie, Hans von Halban, and Lew Kowarski found that several neutrons were emitted in the fission of uranium-235 which led to the possibility of a self-sustaining chain reaction. Fermi and his coworkers recognized the potential and on December 2, 1942 they succeeded in controlling the reaction in the world's first nuclear reactor. Known as a "pile," this device was built on the University of Chicago campus and consisted of an array of uranium and graphite blocks.

Nuclear fission reactors

A nuclear fission reactor takes the fission reaction and contains and controls the reaction in order to produce heat. The heat generated is used to make steam that spins a turbine and create electricity. These reactors use uranium for nuclear fuel. The process of using the uranium is to process it into ceramic pellets and stack those pellets together into metal tubes called fuel rods. These rods are bundled to form a fuel assembly, and a reactor core is typically made up of a hundred of these assemblies, depending on the power level.

These fuel rods are then immersed in water, which acts as both coolant and moderator. The moderator helps to slow the neutrons produced by fission down and sustain the chain reaction. To further increase the control and slow down the reaction, control rods can be inserted into the reactor core. These rods can also be withdrawn in order to increase the reaction.

Diagram of a pressurized water reactor.

Two of the more common types of these reactors are pressurized water reactors (PWR), which make up more than 65 percent of commercial reactors in the United States, and boiling water reactors (BWR). In a PWR, water is pumped into the reactor core at high pressure to prevent the water from boiling, and is then pumped into tubes inside a heat exchanger, which separate water source to create steam used to turn an electric generator and produce electricity. A boiling water reactor works similarly, except the reactor is used to heat the water and create steam that is fed to a turbine to produce electricity.

A diagram of a boiling water reactor.
Generations of nuclear reactors

Nuclear reactor designs are categorized by generation, with the key attributes characterizing the development and deployment of nuclear reactors offering the differences between the generation of reactors. This includes six key reactor attributes that separate generations, including cost-effectiveness, safety, security and nonproliferation, grid appropriateness, commercialization roadmap, and the fuel cycle.

Key reactor attributes


Commercialization roadmap

Commercialization roadmaps have to include a plausible timeline for deployment with a need for near-term readiness such that these technologies are either previously market tested or close to commercial demonstration.


From a customer perspective, a nuclear kilo-watt hour is indistinguishable from a renewable or fossil-fired kilowatt hour. Nuclear power must therefore be economically competitive and be able to compete with fossil-fired power which continue to set market prices.

Grid appropriateness

The capabilities of either local or national electric grid must match the electric power a proposed reactor will deliver to the grid. Grid appropriateness can be determined by a combination of nameplate capacity and externalities defined by the extant electrical grid.


Several nuclear systems have begun to incorporate passive design features to ensure the safe operation of nuclear reactors, as opposed to active safety systems requiring intervention by human agents. This is due to a variety of technical and public policy reasons, including quantitative risk reductions.

Security and nonproliferation

Nuclear power systems are required to minimize the risks of nuclear theft and terrorism. This includes designs on the international market which can minimize the risks of state-sponsored nuclear weapons proliferation.

The Dresden-I nuclear power plant.

There are three generations of reactors, derived from designs developed for naval use beginning in the late 1940s, operating worldwide.

Generation I

The first generation of reactors are the prototypes and power reactors which essentially launched civil nuclear power. This includes early prototype reactors from the 1950s and 1960s, such as Shippingport in Pennsylvania, Dresden-I in Illinois, and Calder Hall-I in the United Kingdom. These reactors typically ran at power levels considered "proof-of-concept" and, in the United States, were regulated by the Nuclear Regulatory Commission (NRC) pursuant to Title 10, Code of Federal Regulations, Part 50. The final Gen I plant, the Wylfa Nuclear Power Station in Wales, was closed in December 2012.

Generation II

This refers to the second generation of reactors, which were designed to be economical and reliable when compared to the previous generation. These generators were designed for a typical lifetime of forty years, and the prototypical second generation reactor included pressurized water reactors. These reactors are typically referred to as light water reactors and use traditional safety features involving electrical or mechanical operations that are initiated automatically and can be initiated by operators. Some reactors of this generation include passive systems for control or safety features for the possible loss of auxiliary power.

CANDU reactor schematic.

Most generation plants still in operation were manufactured by Westinghouse, Framatome, and General Electric. These reactors began operations in the late 1960s and comprise the bulk of the world over 400 commercial pressurized water reactors and boiling water reactors, including the CANada Deuterium Uranium pressurized water reactor (CANDU), and included gas-cooled reactors and Vodo-Vodaynoi Energetichesky reactors.

Generation III

The third generation of reactors do not vary much from second generation reactors, but rather work to evolve their designs with improvements in the are of fuel technology, thermal efficiency, modularized construction, safety systems, and standardized design. Improvements in the third generation of reactor technology also aimed to extend operational life from forty to sixty years of operation, and to exceed sixty years, prior to a need for a complete overhaul and reactor pressure vessel replacement.

Diagram of the Enhanced CANDU 6 reactor.

One of the first generation III reactor designs was the Westinghouse 600 MW advanced PWR. A parallel development included the GE Nuclear Energy Advanced Boiling Water Reactor, which obtained design certification from the NRC. The first of these reactors went online in Japan in 1996. Other designs include the Enhanced CANDU 6, developed by the Atomic Energy of Canada Limited (AECL) and System 80+, a Combustion Engineering design.

Unlike the previous generations, the third generation of reactors are regulated by NRC regulations pursuant to title 10 CFR Part 52, offering a new regulation for these newer reactors.

Generation III+

The generation III+ reactors are developments of the third generation reactors not considered to be radical or significant enough to mandate a new generation. These include many safety design improvements on the third generation reactor designs certified by the NRC in the 1990s. Many of the generation III+ developments began in the 1990s and built on the operational experience of the American, Japanese, and Western European LWR fleets.

Generation III+ reactor in China.

One of the most significant systems improved upon in the generation III+ designs over second generation designs is the development of passive safety features without requiring active controls or operator intervention, and relying on gravity or natural convection to mitigate the impact of any abnormal event. These passive safety features also suggest a possibility of expediting the certification review process of a reactor and shorten construction schedules. These reactors are also expected to achieve higher fuel burnup than their predecessors.

Generation IV

Generation IV reactor designs are expected to make a commercial debut in 2030, and are being developed through an international cooperation of fourteen countries, including the United States. Part of this cooperation is to support research and development in a wide range of new advanced reactor technologies that could change the nuclear industry, including making systems cleaner, safer, and more efficient than previous generations. This generation also includes three new reactor designs intended to offer more passive safety controls and more modular design. These reactor designs include sodium-cooled fast reactor, very high temperature reactor, and a molten salt reactor.

SFR diagram.

Sodium-cooled fast reactors (SFR) use liquid metal as a coolant instead of water, which allows for the coolant to operate at higher temperatures and lower pressures than previous generation reactors which can improve on the efficiency and safety of previous generation reactors. The SFR uses a fast neutron spectrum, meaning neutrons can cause fission without having to be slowed first, and could allow SFRs to use fissile material and spent fuel from previous generation reactors in the production of electricity.

VHTR diagram.

Very high temperature reactors (VHTRs) are cooled by flowing gas and designed to operate at high temperatures that could produce electricity at higher efficiencies than previous generation reactors. The high temperature gas could also be used in energy-intensive processes that rely on fossil fuels, such as hydrogen production, desalination, district heating, petroleum refining, and ammonia production. Very high temperature reactors also offer improved safety features and can be easy to construct and affordable to maintain.

MSR diagram.

Molten salt reactors (MSRs) use molten fluoride or chloride salts for coolant, which can flow over solid fuel like other reactors or fissile materials can be dissolved into the primary coolant and the fission process directly heats the salt. MSRs are designed to use less fuel and produce shorter-lived radioactive waste than other reactor types. They have the potential to change the safety posture and economics of nuclear energy production by processing fuel online, removing waste products, and adding fresh fuel without lengthy refueling outages. Their operation can be tailored for the efficient burn up of plutonium and minor actinides, which could allow MSRs to consume waste from other reactors.

Reactor safety

From the beginning of the use of nuclear fission as a power source in reactors, there has been an awareness and a fear of the potential hazard of both nuclear criticality and release of radioactive materials from generating electricity with nuclear power. However, and despite a few incidents, the evidence from six decades and more of operating has shown that nuclear power is a safe means of generating electricity, with the risk of accidents in nuclear power plants low and declining, and with the consequences of an accident or attack on a nuclear reactor minimal compared with other commonly accepted risks.

And while every industry has accidents, the high energy density of nuclear power creates an obvious potential hazard. In relation to nuclear power, safety is closely linked with security, and with safeguards, when compared to other industries and power generation processes. Some of these distinctions include the following:

  • A focus on unintended conditions or events which could lead to radiological releases from authorized activities, relating mainly to intrinsic problems or hazards
  • A security focus on the intentional misuse of nuclear or other radioactive materials by non-state elements to cause harm and relates mainly to external threats to materials or facilities
  • Safeguarding which focuses on restraining activities by states that could lead to the acquisition or development of nuclear weapons

Image from the Chernobyl disaster.

In this history of civil nuclear power generation, there have been three significant accidents at nuclear power plants:

  • Three Mile Island in 1979, where the reactor was damaged but radiation was contained and there were no adverse health or environmental consequences
  • Chernobyl in 1986, where the destruction of the reactor by steam explosion and fire killed two people initially plus a further twenty-eight from radiation poisoning within three months, and had significant health and environmental consequences
  • Fukushima Daiichi (2011), where three old reactors were written off after the effects of loss of cooling due to a huge tsunami were inadequately contained and without any deaths or serious injury caused by radioactivity
Preventative measures

Preventative measures are the set of design and operating rules designed to make certain a reactor is operated safely. The nuclear industry in the United States created a design philosophy referred to as defense-in-depth that numerous countries have since adopted. This defense-in-depth model requires all safety systems to be functionally independent, inherently redundant, and diverse in design. Among well-known preventative measures are the reports and inspections for checking to ensure a plant is properly constructed, rules of operation are maintained, and qualification for operating personnel is upheld to ensure they know their jobs. Nuclear reactors are required to operate under a high standard of quality assurance. This includes requiring staff members to evaluate, audit, survey, and verify that all procedures and maintenance is being performed as intended.

Design requirements

An important part of the safety system of any reactor is to adhere to design requirements. This means the reactor must have negative power-reactivity coefficient, the safety rods are required to be injectable under all circumstances, and no single regulating rod should be able to add substantial reactivity. Another important design requirement is that structural materials in the reactor must retain acceptable physical properties over the expected service life. Construction is covered by stringent quality-assurance rules, and both design and construction must be in accordance with standards set by major engineering societies and accepted regulatory bodies.

Minimize risk

The aim of regulations and safety measures to minimize risks to the point in which a reasonable individual could conclude the risks are trivial, even though no human activity can be considered absolutely safe. What this minimum risk value is, and whether it has been achieved by the nuclear industry, is subject to debate. However, it is generally accepted the independent regulatory agencies such as the United State Nuclear Regulatory Commission (NRC), the United Kingdom's Office for Nuclear Regulation (ONR), the International Atomic Energy Agency (IAEA), and similar agencies around the world, are the bodies to make judgements of such matters.

Mitigating measures

Also referred to as safety systems, mitigating measures are systems and structures to prevent accidents from proceeding to a catastrophic outcome. Two of the principal mitigating measures are the safety rod systems that can put a reactor into a subcritical state and prevent a supercritical accident, and the containment structure that prevents radioactive materials from being released into the atmosphere. Other mitigating measures can include emergency core-cooling system to provide cooling of the core and fuel region upon a loss of reactor coolant, and an emergency power system designed to supply electrical power to support systems in the event that normal supply is disrupted. These emergency power systems are necessary to maintain critical components operation, such as detectors, circulating pumps, and valves needed to remove decay heat.

Pros and cons of nuclear power

The pros of nuclear power includes the low carbon emissions associated with nuclear power, which is considered to be at 29 tons of carbon dioxide per gigawatt hour (GWh) of energy produced, which compares favorably with renewable sources like solar which is considered at 85 tons per GWh, wind at 26 tons per GWh, and more favorably with fossil fuels such as lignite at 1,054 tons per GWh, and coal at 888 tons per GWh.

Further, nuclear power plants are cheaper to run than coal or gas rivals, with an estimation that, even with costs such as managing and disposing of radioactive fuel and waste, nuclear plants cost between 33 to 50 percent of a coal plant and 20 to 25 percent of gas combined-cycle plants. The amount of energy produced by a nuclear power plant is also considered better, with the United States Department of Energy estimating that to replace 1 gigawatt nuclear power plant would require 2 gigawatts of coal or 3 to 4 gigawatts from renewable sources to generate the same amount of electricity. Similarly, nuclear power is not intermittent and can run without interruptions for a year making it a reliable source of energy.

As well, the continued development of nuclear fission and acceptance of nuclear fission power generation is expected to help and accelerate the development of nuclear fusion, which is expected to offer practically unlimited energy generation. It is also estimated that the amount of energy released in a nuclear fission reaction is ten million times greater than the amount released when burning fossil fuels.

Runit Island radioactive waste coffin.

With the concerns and awareness of the safety and possible consequences of nuclear reactors, a major con to nuclear power remains that different factors can cause a reactor to go into meltdown and such a moment can have devastating effects. Similarly, nuclear waste, a side effect of nuclear power, is considered a con, with it being estimated that the world produces 34,000 meters cubed of nuclear waste each year, which takes years to degrade. One solution has been the development of concrete nuclear waste coffins, such as the one built on Runit Island, however this coffin has begun to crack and could potentially release radioactive material.

As well, the initial cost of building a nuclear power plant is steep, with the cost of manufacturing a reactor in the United States being estimated to cost $3.5 to $6 billion and with large costs to maintain the facility. Similarly, despite some considering nuclear power generation offering zero carbon emissions, it still has an impact on the environment through the mining for usable uranium and the discharge of water. Uranium mining particularly is known for releasing arsenic and radon.

Nuclear power plants are also water intensive, as water is used for cooling and can consume large amounts of water, with the reactors in the United States in 2015 consuming 320 billion gallons of water to produce nuclear power; which is more than coal processing. As well, nuclear power cannot be considered a renewable resource, as the uranium used is depleted and creates waste, and the uranium is a finite resource.

Fission reactor advancements

There have been continued evolutions in fission reactor technologies and in the possible role it could play in use with other industries. This includes changes to the size of reactors, advancement in reactor safety, and in the versatility of the reactors.

Safer designs

Many find it unsettling to live near a nuclear plant, and to ease these concerns nuclear technologies need to be able to assure safety and put those who find the technology unsettling at ease. As part of this, nuclear companies are planning inherent safety features intended to make potential calamities not just improbable but impossible. These include risks such as a plant malfunction, nuclear fuel being weaponized, and the risk leftover material poses when stored.

The concern of accident has in many ways been mitigated with the experiences of past failures which have led to enhanced designs aimed at removing the possibility of the same problems recurring, even in extremely or unlikely conditions, something known as inherent safety. While the risk of weapons proliferation requires any radioactive substance to be vigilantly handled, but even then the fuel used by nuclear reactors is different from weapons grade nuclear material. The concentration of radioactive uranium is far lower and the enrichment process is complicated and expensive and requires a group of people with specialized expertise which is not readily available.

And the concern of waste, or spent fuel, is also being tackled by the development of reactors known as fast reactors. These reactors use a fast reaction, which converts spent fuel back into usable material and leads to an end product much less radioactive, or potentially no waste material if the process is repeated a sufficient amount of times. Fast reactors are harder to stabilize than other reactors, because other reactors have mechanisms to slow and control the reaction, and these reactors can use weapons-grade plutonium fuel, which also offers a way to get rid of weapon stocks without creating further risk of proliferation.

Another increase in the safety is in the new cooling design developed for newer reactors which use either liquid metal, sodium water, or gas. In the case of a natrium reactor, the plant uses liquid sodium as a cooling agent, which is the cooling system used by the reactors developed by TerraPower. These cooling systems do not rely on outside energy sources to operate in the event of an emergency shutdown of a reactor. Instead, the system works through hot air rising from natural circulation within the system, called a reactor vessel air cooling system. This can prevent accidents similar to those that occurred at the Fukushima Daiichi plant, which saw an earthquake shut down the plant's reactor and the subsequent tsunami disabling the diesel backup generators.

The natrium reactor technology can also store heat in tanks of molten salt for future use, similar to a battery, which could see an increase in a natrium reactor plant's power output from about 345 megawatts to 500 megawatts for up to give hours. As well, these natrium plants target a cost for a commercial plant at around $1 billion, including costs for engineering, procurement, and construction. This is far lower than the traditional cost of constructing a plant, and is partly due to natrium plants operating at lower pressures and do not require the same heavy duty, expensive construction materials. The plants are also smaller and could be more attractive to electrical companies as they could plug into an existing grid. These reactors are also expected to produce less waste, through a more efficient use of radioactive materials.

Smaller reactors

One of the bigger advancements in nuclear energy, other than the increase in safety, has been a reduction in overall size of the reactors. Reactors of the 20th century were large and expensive behemoths. These large plants could generate enough energy to power millions of homes, but they also required decades of time and billions of dollars to build. The trend has been towards small modular reactors which generate a fraction of the energy of traditional reactors but at a fraction of the cost.

Existing nuclear reactors generate 500 megawatts to 1 gigawatt of electricity. Whereas small modular reactors generate less than 300 megawatts, while offering greater flexibility than their predecessors. A single reactor could suit projects with lower energy needs, or combined with multiple reactors could be combined for higher energy needs, similar to hitching cars to a freight train. Some of these smaller reactors include passive safety cooling systems which make the reactors walk-away safe, meaning no action is required by an operator to safely shut the reactor down.


When it comes to nuclear power, electricity generation is a beginning, with more than 80 percent of the world's energy use goes to industrial processes and transportation rather than electricity. In that sense, nuclear plants of the 20th century have been humongous, limited-use generators. Evolutions of the reactors have given them the potential to do more than generate electricity, with the basis of some of those uses being heat based.

Traditional reactors cannot exceed 572 degrees Fahrenheit or the coolant will boil off too quickly and create safety risks. However, that's not a problem for molten salt reactors, and can reach temperatures of 1,292 degrees Fahrenheit, or high-temperature gas reactors, which can reach over 1,652 degrees Fahrenheit. The heat generated could be used for other industrial applications, such as pulling hydrogen from water for hydrogen fuel, or for manufacturing steel and concrete, or generating the heat required to remove carbon dioxide from the atmosphere. These use cases would require the reactors to be close to facilities using the waste heat which would make proper containment procedures ever more critical.

Portable nuclear power and Project Pele

Another advancement of nuclear power generation has been the development of portable nuclear reactors for portable nuclear power. One of the leaders in this research direction has been the United States Department of Defense, which has awarded several contracts for the development of these portable nuclear reactors. The combined $39.7 million in contracts came from the "Project Pele" run through the Strategic Capabilities Office looking for a 1 to 5 megawatt power range reactor.

The department awarded a $13.5 million contract to BWX Technologies, a $11.9 million contract to Westinghouse Government Services, and a $14.3 million contract to X-energy LLC. These contracts were for a two-year competition to build a microreactor capable of being deployed with forward forces outside of the continental United States.

A large part of the Project Pele is the development of tri-structural isotopic (TRISO) fuel, which is expected to be one of the most robust nuclear fuels. TRISO consists of poopy-seed sized kernels of uranium, carbon, and oxygen encapsulated in layers of carbon and ceramic-based materials to block the emission of radioactive fission products. X-energy has developed a propriety form of TRISO, which the company claims is one of the safest available and can greatly reduce risk associated with nuclear fuels.

The funding for Project Pele also works to explore new nuclear technologies capable of power different transportation technologies, including aircraft, trains, and spacecraft. These technologies are intended to help advance energy resiliency and carbon emission reductions.

Companies developing nuclear fission reactors and technology

Nuclear fusion

Both fission and fusion are nuclear reactions that are capable of producing energy in very different processes. Fission, which includes splitting a heavy unstable nucleus, is considered a destructive process creating nuclear waste. While fusion is a process where two light nuclei combine together to release vast amounts of energy, and is considered a less destructive process.

Department of Energy's Nuclear Fusion diagram

Fusion takes place when two low-mass isotopes, typically isotopes of hydrogen, unite under conditions of extreme pressure and temperature to produce a neutron and helium isotope. This process produces an enormous amount of energy, several times the amount produce from fission. So far nuclear fission has been controlled and utilized for power generation, but fusion has been considered a more attractive opportunity for power generation as it creates less radioactive material than fission and has a nearly unlimited fuel supply. However, the fusion reaction is not easily controlled and so far the conditions for a fusion reaction have proven expensive to create.

Companies developing nuclear fusion reactors


Further Resources


3 Advanced Reactor Systems to Watch by 2030


Advanced nuclear: A climate-tech comeback story | Greenbiz


Evolution of nuclear fission reactors: Third generation and beyond


Fission vs. Fusion: What's the Difference?


September 23, 2019


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