⚛️Intro to Applied Nuclear Physics Unit 5 – Nuclear Fission and Chain Reactions

Basic Concepts of Nuclear Physics

• Atomic structure consists of a dense, positively charged nucleus surrounded by a cloud of negatively charged electrons
• Nucleus contains protons (positively charged) and neutrons (neutral) held together by the strong nuclear force
• Isotopes are variations of an element with different numbers of neutrons in the nucleus
• Radioactive decay occurs when an unstable nucleus emits particles or energy to reach a more stable configuration
• Half-life represents the time required for half of a given quantity of a radioactive isotope to decay
• Mass-energy equivalence, expressed by Einstein's equation $E=mc^2$, states that mass and energy are interchangeable
• Binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
  • Nuclei with the highest binding energy per nucleon are the most stable

Nuclear Fission: Process and Mechanics

• Nuclear fission is the splitting of a heavy atomic nucleus into two or more lighter nuclei
• Fission reactions are typically induced by the absorption of a neutron by a fissile nucleus (e.g., uranium-235 or plutonium-239)
• The absorbed neutron causes the nucleus to become unstable and split into fission fragments
• Fission fragments are the lighter nuclei produced by the splitting of the original heavy nucleus
• The fission process releases additional neutrons, which can trigger further fission events in nearby nuclei
• Fission reactions also release a large amount of energy in the form of kinetic energy of the fission fragments and gamma radiation
• The probability of a fission reaction occurring depends on factors such as the energy of the incident neutron and the cross-section of the target nucleus
  • Cross-section is a measure of the likelihood of a particular nuclear reaction occurring

Chain Reactions Explained

• A chain reaction occurs when the neutrons released from one fission event trigger additional fission events in nearby nuclei
• For a self-sustaining chain reaction to occur, at least one neutron from each fission event must cause another fission event
• The multiplication factor (k) represents the average number of fission events caused by each neutron in a generation
  • If k > 1, the chain reaction grows exponentially (supercritical)
  • If k = 1, the chain reaction is self-sustaining (critical)
  • If k < 1, the chain reaction dies out over time (subcritical)
• The critical mass is the minimum amount of fissile material required to sustain a chain reaction
• Neutron moderators (e.g., water or graphite) slow down fast neutrons, increasing the likelihood of them causing fission in nearby nuclei
• Neutron absorbers (e.g., control rods) can be used to control the rate of the chain reaction by absorbing excess neutrons
• Prompt criticality occurs when the chain reaction is driven by prompt neutrons released immediately after fission, leading to a rapid increase in power

Key Players in Nuclear Fission

• Fissile isotopes (e.g., uranium-235 and plutonium-239) can sustain a chain reaction and are the primary fuel for nuclear fission reactors
• Fertile isotopes (e.g., uranium-238 and thorium-232) can be converted into fissile isotopes through neutron capture and subsequent radioactive decay
• Neutron moderators slow down fast neutrons, increasing the probability of them causing fission in nearby nuclei
  • Common moderators include water (in light water reactors) and graphite (in graphite-moderated reactors)
• Neutron absorbers (e.g., control rods made of boron or cadmium) are used to control the rate of the chain reaction by absorbing excess neutrons
• Fission products are the lighter nuclei produced by the fission process, many of which are highly radioactive
• Delayed neutrons are emitted by certain fission products and play a crucial role in controlling the chain reaction in nuclear reactors
  • Delayed neutrons allow more time for control systems to respond to changes in reactor conditions

Energy Release and Applications

• Nuclear fission releases a large amount of energy, primarily in the form of kinetic energy of the fission fragments and gamma radiation
• The energy released per fission event is approximately 200 MeV (3.2 × 10^-11 joules)
• Nuclear power plants harness the heat generated by controlled fission reactions to produce electricity
  • The heat is used to generate steam, which drives turbines connected to electrical generators
• Nuclear propulsion systems use the heat from fission reactions to generate thrust, enabling the development of nuclear-powered submarines and ships
• Radioisotope thermoelectric generators (RTGs) use the heat from radioactive decay of fission products to generate electricity for spacecraft and remote terrestrial applications
• Nuclear fission has military applications, such as the development of atomic bombs and nuclear warheads
  • Uncontrolled chain reactions in fissile materials can lead to the rapid release of enormous amounts of energy

Controlling Nuclear Reactions

• Nuclear reactors control fission chain reactions to maintain a steady power output and prevent runaway reactions
• Control rods, made of neutron-absorbing materials (e.g., boron or cadmium), are inserted into the reactor core to absorb excess neutrons and regulate the reaction rate
  • Inserting control rods deeper into the core slows down the reaction, while withdrawing them allows the reaction to accelerate
• Neutron moderators (e.g., water or graphite) slow down fast neutrons, increasing their likelihood of causing fission in nearby nuclei
• Burnable poisons (e.g., gadolinium or boron) are added to the reactor fuel to help control the reaction rate over the fuel's lifetime
• Reactor shutdown systems, such as emergency control rod insertion or injection of neutron-absorbing solutions, can quickly stop the chain reaction in an emergency
• Reactivity coefficients describe how the reactor's power output responds to changes in various parameters (e.g., temperature, pressure, or coolant density)
  • Negative reactivity coefficients help to stabilize the reactor by reducing power output when these parameters increase

Safety and Environmental Considerations

• Nuclear safety is paramount in the design, operation, and maintenance of nuclear facilities to prevent accidents and minimize risks
• Containment structures are designed to prevent the release of radioactive materials into the environment in the event of an accident
• Redundant safety systems, such as emergency cooling and backup power, ensure the reactor can be safely shut down and cooled in an emergency
• Radiation protection measures, including shielding and personal protective equipment, are used to minimize exposure to workers and the public
• Nuclear waste management involves the safe handling, storage, and disposal of radioactive waste generated by nuclear facilities
  • High-level waste (e.g., spent fuel) requires long-term isolation in deep geological repositories
  • Low-level waste can be disposed of in near-surface facilities with appropriate containment and monitoring
• Environmental monitoring programs track the presence of radioactive materials in the air, water, and soil around nuclear facilities to ensure public safety
• Decommissioning is the process of safely dismantling a nuclear facility at the end of its operational life and restoring the site for other uses

Future of Nuclear Fission Technology

• Advanced reactor designs, such as small modular reactors (SMRs) and Generation IV reactors, aim to improve safety, efficiency, and economic viability
  • SMRs are smaller, factory-built reactors that can be deployed more quickly and with lower upfront costs
  • Generation IV designs focus on enhanced safety features, fuel efficiency, and waste reduction
• Thorium-based nuclear fuel cycles have the potential to reduce waste generation and improve resource utilization compared to traditional uranium-based fuels
• Nuclear fusion, the joining of light atomic nuclei to form heavier nuclei, promises virtually limitless clean energy if technical challenges can be overcome
• Hybrid fission-fusion systems could combine the advantages of both technologies, using fusion neutrons to drive fission reactions and reduce waste
• Nuclear medicine continues to advance, utilizing radioisotopes produced by fission for diagnostic imaging and cancer treatment
• International cooperation and knowledge sharing are essential for the safe and peaceful development of nuclear fission technology worldwide
  • Organizations like the International Atomic Energy Agency (IAEA) play a crucial role in promoting nuclear safety, security, and non-proliferation