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Nuclear fission is a process that involves the splitting of atomic nuclei. When a neutron strikes the nucleus of an atom, such as uranium or plutonium, the nucleus often splits into two smaller nuclei, releasing a substantial amount of energy. In the fuel composition of nuclear reactors, the fission events also release additional high energy neutrons, which can collide with other nuclei and continue a sustained fission reaction within the reactor.
At the subatomic level, fission reactions harness the power of the strong nuclear force, one of the fundamental forces of nature which hold the particles of atomic nuclei together. Atomic nuclei of heavier elements like uranium effectively store an immense amount of energy which can be accessed through a nuclear fission reaction. A nuclear power plant harnesses the heat generated by a fission reaction to produce high pressure steam, which drives a series of turbines that power a generator which produces electricity.
As a comparison, the energy density of nuclear fuel is more than one million times greater than the energy density of traditional fossil fuels. The amount of energy stored within the atomic nuclei of uranium is roughly one million times greater than the chemical energy stored in the molecular bonds of a combustion fuel compared by weight. Even after enrichment and usage within conventional reactors are factored in, the resulting energy density is still several orders of magnitude greater than traditional fossil fuels. (European Nuclear Society, n.d.)
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Uranium has several isotopes and U-235 is the most important for nuclear power because it is fissile, meaning it can undergo sustained nuclear reactions. U-238 is fertile meaning it can capture a neutron and be transformed into fissile plutonium. Only about 0.7% of naturally obtained uranium is U-235, the vast majority is U-238 which is not sufficient for a self-sustaining nuclear reaction. Naturally obtained uranium must be enriched, increasing the concentration of U-235 to a level suitable for a reactor.
The difference in isotope ratios comes down to half-lives, as the atomic composition of terrestrial materials were forged in a massive stellar explosion billions of years ago, even if equal amounts of U-235 and U-238 were made, half of the U-235 would decay every 700 million years while half of the U-238 would take the U-238 four billion years. These differences compound over time resulting in such an extreme difference.
Differences in half-lives are due to differences in radioactive decay rates. A uranium isotope often undergoes an alpha decay, which is when the larger nucleus ejects a helium nucleus, meaning two neutrons and two protons are ejected. However alpha decays do not have enough energy to initiate a fission reaction.
Other isotopes can undergo different kinds of radioactive decay, such as a beta decay which involves a proton becoming a neutron by releasing an electron. This will change the element of the isotope, but not the isotope number. Such as a U-238 isotope capturing a neutron to briefly become U-239, to then undergo 2 beta decays to become Pu-239 as two of the neutrons become protons, increasing the proton count from Uranium’s 92, to Plutonium’s 94.
When enough uranium of a certain composition is positioned within a particular geometric proximity, a nuclear fission reaction can be sustained. However this chain reaction must be initiated by a startup neutron source, which is often a rod centrally positioned in the reactor containing radioactive material that can decay through spontaneous fission, releasing the high energy neutrons needed to start the fission reaction.
The fission reaction in a pressurized water reactor (PWR), is managed by control rods positioned within the fuel assemblies inside the reactor. Control rods contain neutron-absorbing materials, such as boron or cadmium, which can be inserted or withdrawn from the reactor core to control the neutron flux and the rate of fission. The more neutrons that get absorbed by the control rod materials, the less neutrons are available to sustain the fission reaction.
During normal operation, the control rods are partially inserted into the reactor core to maintain a steady-state fission reaction, producing a steady and optimal amount of heat. The control rods can be withdrawn to increase the rate of the reaction which increases heat or inserted further to reduce the reaction rate which decreases the heat output.
When the nuclei of nuclear fuel are split into smaller isotopes from a fission reaction, many of these remaining isotopes can be unstable, leading some to undergo radioactive decay releasing additional heat energy. For example, a U-235 isotope can fission out into krypton and barium isotopes in addition to more neutrons. Barium has several stable isotopes while krypton-85 has a half life of less than 11 years, making it particularly active, yet krypton-85 decays into rubidium-85 which is stable.
Decay heat is also a factor to account for in the operation of a nuclear reactor as it will linger long after the fission reaction is stopped. Fortunately nuclear engineers are very knowledgeable and experienced with managing decay heat, which is why reactors are designed to run cooling systems for days or weeks following the shut down of a reactor. The elaborate infrastructure and technical management of spent fuel is also built around managing decay heat from these fission products.
The composition of the initial fission products can vary widely depending on the initial fuel composition, configuration and how the reactor was operated over time, every fission product has a unique set of probabilities and possible decay chains. Isotopes decay at the rate of known half-lives however the decay of one isotope could create another isotope that has a different half life, which follow paths that are well understood in calculating cool down rates. There are elaborate flow charts of fission products with probabilities of decay types and sequential chains of resulting isotopes mapped out from decades of research and scientific measurement.
Fission reactions can vary based on how the neutrons interact with nuclei across a spectrum that ranges from “thermal” to “fast”. Most nuclear reactors are thermal spectrum reactors, meaning that the neutrons generated by the fission reaction are moderated, or slowed down to thermal energy levels before they are able to induce further fission events. Slowing the neutrons down allows for much higher probabilities in colliding with other nuclei in the fuel composition. Fast spectrum reactors maintain fission reactions where neutrons are not moderated and remain at fast or high-energy levels. There can be some advantages to this since different isotopes can respond differently to fast neutrons, in some cases increasing fuel efficiency, composition and energy release.
Thermal spectrum reactor designs are favored because low-enriched uranium fuel is cheaper, readily available and difficult to divert for weapons production. The slower neutrons in thermal spectrum reactors are easier to work with from an engineering standpoint as they can be absorbed more easily by the fuel and the presence of a moderator provides additional methods of incorporating safety systems. Thermal spectrum reactors are typically fueled with low enriched uranium or mixed oxides containing plutonium, while fast spectrum reactors often use uranium enriched to a higher composition, plutonium, or thorium.
All fission reactors undergo two distinct yet interrelated processes referred to as “breeding” and “burning”. Depending on the reactor design, operating cycle and fuel composition, there will be a specific “breed to burn” ratio.
Breeding refers to a neutron capture event when a fertile nucleus absorbs a neutron and can eventually become a fissile nucleus. A high enough breeding ratio can result in more fissile material being produced over time. Some reactor designs are called “breeder reactors” due to their breeding ratio exceeding the threshold of producing more fissile material than it consumes. One common sequence involves a fertile uranium-238 isotope capturing a neutron and undergoing two beta decays to become plutonium-239 which is fissile. There are many different pathways neutron capture and subsequent decay reactions can produce different isotopes and yield different energy outputs. The breeding process ultimately increases the fuel inventory within the reactor, allowing it to sustain a substantial power output for a longer period of time.
Burning casually refers to the fission reactions taking place within the reactor, commonly used in terms like “breed and burn reactor”. In a fission reaction, a neutron collides with a heavy nucleus, typically uranium-235 or recently bred plutonium-239, causing the nucleus to break apart into two lighter elements, releasing energy and additional neutrons continuing the fission reaction.
Uranium has an exceptionally high energy density from the basis of pure physics, the actual release of that energy can be much lower in a nuclear reactor, however still several orders of magnitude higher than every other source of non-nuclear fuel. The fuel usage in a reactor is often called burnup and is measured in GigaWatt Days per Metric Ton of Uranium (GWD/MTU). According to the NRC the average burnup of nuclear reactors was 35 GWD/MTU two decades ago, but today that figure has improved to 45 GWD/MTU. (NRC, n.d.)
The importance of fission cannot be understated as fission reactors provide clean affordable energy that can scale to serve human needs on a global level. Fissile fuel is cheap, abundant and offers a high energy density, making it a valuable source of electricity generation. Nuclear power plants have always been a reliable source of low-carbon energy, helping to mitigate the impacts of climate change by reducing our dependence on fossil fuels. The benefits of nuclear power can expand across the entire energy sector if there is enough demand for this change.
Beyond energy, fission reactions have a wide range of applications in nuclear medicine, including the production of radioisotopes for medical imaging and cancer treatment. Radioisotopes save countless lives through their medical applications alone. (NRC, n.d.) Fission is also useful in research and development for material sciences, where radiation sources composed of fission products or actinides can provide methods of precise measurement and testing for different materials. Radiation sources also have a range of other technical applications, for instance Americium is an element that is produced inside fission reactors through neutron capture, and it has been a critical component to the function of many smoke detectors over the years. Ultimately fission has shaped our world as an important technology providing clean, safe, and reliable energy in addition to supporting medical advances, industrial development and driving scientific discovery.
As exciting as developments in fusion can be, we have already figured out and commercialized fission for a range of energy applications, and that legacy can continue to scale. Fission has been the driving force behind nuclear energy with decades of operational history across many countries and industries.
Decades of industry operations have built up significant familiarity, knowledge and experience with fission reactors. Many nuclear reactors utilize fine tuned enrichment ratios and standard sizing for fuel rods fitted within standardized fuel assemblies. There can be some variation across different reactor designs but the general concepts of fuel rods and control rods are present across the vast majority of civilian nuclear power plants. Even within the same reactor design, there can be slight differences in fuel enrichment ratios and positioning within the reactor, however the rod size and fuel assemblies remain consistent. Additional innovations in fuel have worked within these specifications, such as the incorporation of Mixed OXides (MOX) fuel which utilizes the addition of recycled plutonium that can be used as reactor fuel. Cladding around the fuel has also been improved upon allowing reactors to run with higher power outputs while maintaining operating conditions within designated safety thresholds.
Uranium as a fuel is abundant on our planet and therefore relatively cheap. Accessing this energy through fission reactors and the associated infrastructure of nuclear power plants can also be made affordable by improving the delivery model of plant construction.
The supply chain for standard fuel bundles with known enrichment ratios already exists and can be accessed as needed. The manageable ratios and standardization of the fuel takes advantage of modularity in manufacturing and economies of scale.
Electricity from Nuclear power is generated by heating water to spin a turbine, yet the heat can be applied to other uses. Desalination, district heating and other industrial processes that need heat can benefit from a PWR-20 as a reliable heat source. This can be cheaper and cleaner than burning fossil fuels onsite or operating grid purchased electrical heating systems.
The high energy density of uranium results in a smaller footprint of a PWR-20 system. A smaller footprint allows more flexibility in placement as it can be installed in more places and scale in parallel where multiple units can be assembled.
One reactor worth of fuel can operate for 7 years, functioning like a high powered long cycle nuclear battery. This long fuel cycle is resilient to supply chain disruptions and reduces the need for downtime related to refueling. Once refueled, the reactor can continue with additional 7 year fuel cycles.
Decades of industrial experience with PWRs have developed intricate knowledge of procedural management and operations with nuclear reactors, enabling comprehensive automation in routine operations and safety systems.
European Nuclear Society. (n.d.). Fuel comparison - ENS. European Nuclear Society. Retrieved February 6, 2023, from https://www.euronuclear.org/glossary/fuel-comparison/
NRC. (n.d.). Backgrounder On High Burnup Spent Nuclear Fuel | NRC.gov. Nuclear Regulatory Commission. Retrieved February 6, 2023, from https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bg-high-burnup-spent-fuel.html
NRC. (n.d.). Uses Of Radiation | NRC.gov. Nuclear Regulatory Commission. Retrieved February 6, 2023, from https://www.nrc.gov/about-nrc/radiation/around-us/uses-radiation.html
