Rajan Wilcox
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei, releasing a large amount of energy in the form of heat and radiation. This energy is produced by the fission products, with about 85% of it being the kinetic energy of the fission fragments. The most common nuclear fuels are 235U (uranium-235) and 239Pu (plutonium-239). These fuels break apart into a range of chemical elements with atomic masses. Nuclear reprocessing aims to recover usable material from spent nuclear fuel to enable uranium supplies to last longer and reduce waste. While nuclear fission produces a large amount of energy, it also results in radioactive waste that needs to be carefully managed and isolated from the environment.
| Characteristics | Values |
|---|---|
| Energy produced from 1 kg of coal | 8 kWh |
| Energy produced from 1 kg of mineral oil | 12 kWh |
| Energy produced from 1 kg of uranium-235 | 24,000,000 kWh |
| Percentage of natural uranium that is fissile | 0.7% |
| Percentage of natural uranium that goes into fuel as the fissile part | 0.49% |
| Percentage of natural uranium that is actually fissioned | 0.394% |
| Percentage of U-238 turned into Pu-239 that is fissioned | 0.2% |
| Percentage of original natural uranium that is utilized | 0.6% |
| Percentage of energy released as kinetic energy of fission fragments | 85% |
| Percentage of energy released as gamma rays | 15% |
| Percentage of energy released as kinetic energy of neutrons | 15% |
| Percentage of prompt neutrons | 0.66% for U-235, 0.27% for U-233, 0.23% for Pu-239 |
| Half-life of the longest delayed neutron group | About 56 seconds |
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What You'll Learn
Nuclear fuel cycle
The nuclear fuel cycle is a series of industrial processes that involve the production of electricity from uranium in nuclear power reactors. Uranium is a relatively common element found throughout the world and is mined in several countries. It must be processed before it can be used as fuel for a nuclear reactor. The nuclear fuel cycle consists of two phases: the front end and the back end.
The front-end steps involve the preparation of uranium for use in nuclear reactors. This includes mining and milling, conversion, enrichment, and fuel fabrication. Uranium is first mined and milled to produce uranium concentrate, which can be used as fuel. The uranium concentrate is then converted and enriched to increase the level of U-235, the specific type of uranium used for nuclear fission due to its atoms being easily split apart. After enrichment, the uranium is sent to a fuel fabrication plant where it is made into reactor fuel pellets and fuel rods. These fuel rods are then arranged into a fuel assembly ready for introduction into a reactor.
The back-end steps involve the safe management, preparation, and disposal of spent but still highly radioactive nuclear fuel. Spent fuel may undergo temporary storage, reprocessing, and recycling before the waste produced is disposed of. Reprocessing involves examining the used fuel to learn more about the processes that occur during its use and to potentially recycle it for new fuel. The waste from the nuclear fuel cycle is categorized as high-, medium-, or low-level based on the amount of radiation it emits. High-level waste includes the highly radioactive fission products separated in reprocessing and, in many countries, the used fuel itself.
Transport is an integral part of the nuclear fuel cycle, as nuclear materials are often transported between different stages of the cycle and to specialized facilities that provide fuel cycle services. Most nuclear fuel cycle materials are transported in solid form, with the exception of uranium hexafluoride (UF6), which is considered a gas.
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Fission products
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The products of nuclear fission are called fission products. These are the atomic fragments left after a large atomic nucleus undergoes nuclear fission.
Small amounts of fission products are naturally formed as the result of either spontaneous fission of natural uranium, or as a result of neutrons from radioactive decay or reactions with cosmic ray particles. About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 (90%) or tritium (7%).
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Nuclear waste
The nuclear fuel cycle, which involves the processing of uranium to produce fuel and the reprocessing of used fuel, is a significant contributor to nuclear waste. The waste from the front end of the cycle includes alpha-emitting waste from uranium extraction, often containing radium and its decay products. Uranium dioxide concentrate from mining is highly radioactive and undergoes a series of processes to become usable fuel.
Used nuclear fuel, or high-level waste, contains highly radioactive fission products and actinides. These include isotopes like uranium-234, neptunium-237, plutonium-238, and americium-241, with half-lives ranging from hundreds to millions of years. While nuclear reprocessing can recycle spent fuel, the remaining 4% becomes waste, including minor actinides and fission products.
Management strategies for nuclear waste include direct disposal and reuse in reactors for low-carbon electricity generation. Short-term storage methods involve segregating and storing waste on or near the surface, while long-term solutions favor burial in deep geological repositories. The nuclear industry takes full responsibility for its waste, and facilities for low-, intermediate-, and high-level waste management are in operation or under development.
Compared to other energy sources, nuclear power produces a relatively small amount of waste. On average, the waste generated from supplying a person's electricity needs for a year would be about the size of a brick. Additionally, nuclear waste repositories are designed to be safe, and the radioactivity of the waste is expected to decay to safe levels within a few hundred years.
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Nuclear chain reaction
The specific nuclear reaction that triggers this chain reaction is often the fission of heavy isotopes, such as uranium-235 (U-235). This is the most common nuclear fuel and it can be found in rock formations worldwide, though it only makes up approximately 0.7% of all naturally occurring uranium. Nuclear fuel cycles aim to recover usable material from spent nuclear fuel to enable uranium supplies to last longer.
The process of a nuclear chain reaction involves the release of neutrons in fission, which then produce an additional fission in at least one further nucleus. This new nucleus also releases neutrons, and the process repeats. This is known as neutron multiplication and was discovered by Frédéric Joliot-Curie, H. Von Halban, and L. Kowarski in Paris. If each neutron released in this process causes two more neutrons to be released, the number of fissions doubles each generation, leading to an explosive energy release.
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Neutron cross-section
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The most common nuclear fuels are 235U (the isotope of uranium with mass number 235) and 239Pu (the isotope of plutonium with mass number 239). These fuels break apart into a bimodal range of chemical elements with atomic masses. Nuclear reprocessing aims to recover usable material from spent nuclear fuel to enable uranium (and thorium) supplies to last longer and to reduce waste.
The neutron cross-section is a crucial concept in understanding nuclear fission. It is defined as the area in cm^2 where the number of neutron-nucleus reactions is equal to the product of incident neutrons passing through and target nuclei. The standard unit of measurement for the neutron cross-section is the barn, equal to 10^-28 m^2 or 10^-24 cm^2. A larger neutron cross-section indicates a higher likelihood of a neutron reacting with the nucleus.
The neutron cross-section is further classified into the scattering cross-section and the absorption cross-section. The scattering cross-section can be subdivided into coherent and incoherent scattering, depending on the sample's isotope composition and spin dependence. The absorption cross-section relates to neutron absorbers, which capture neutrons and either decay or retain them in their nuclei. Isotopes that undergo beta decay, such as 232Th becoming 233*Th and then 233Pa, transmute from one element to another.
The fission cross-section, a type of neutron cross-section, measures the probability of a compound nucleus forming and subsequently breaking apart, releasing neutrons, gamma rays, and energy. Uranium-235, a common nuclear fuel, exhibits a higher fission cross-section at low neutron energies, which is why nuclear reactors use neutron moderators to reduce neutron energy and increase the probability of fission.
In summary, the neutron cross-section is a fundamental concept in nuclear fission, influencing the behaviour of neutrons and their interactions with nuclei. The scattering, absorption, and fission cross-sections play specific roles in understanding neutron behaviour and the overall nuclear fission process.
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Frequently asked questions
Nuclear fission uses nuclear fuels like 235U (uranium with mass number 235) and 239Pu (plutonium with mass number 239). Only 0.7% of natural uranium is capable of undergoing fission. In terms of quantities, 1 kg of uranium-235 can generate around 24,000,000 kWh of energy.
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. This process releases a large amount of energy in the form of heat and radiation.
Nuclear fission can generate a significant amount of energy from a small amount of fuel. For example, 1 kg of natural uranium can produce the same amount of energy as nearly 10,000 kg of mineral oil or 14,000 kg of coal.
