⚛️Nuclear Physics Unit 9 – Nuclear Fission and Nuclear Reactors

Key Concepts and Principles

• Nuclear fission involves splitting heavy atomic nuclei into lighter fragments, releasing energy in the process
  • Typically involves heavy elements like uranium and plutonium
  • Requires a critical mass of fissile material to sustain a chain reaction
• Neutron-induced fission occurs when a neutron collides with a fissile nucleus, causing it to split
  • Releases additional neutrons that can trigger further fissions, leading to a self-sustaining chain reaction
• Fission products are the lighter nuclei formed after a heavy nucleus splits
  • Includes elements like barium, krypton, and strontium
  • Highly radioactive and generate significant heat
• Neutron moderators slow down fast neutrons to increase the likelihood of inducing fission
  • Common moderators include water, heavy water, and graphite
• Neutron absorbers (control rods) are used to regulate the fission rate by capturing excess neutrons
  • Typically made of materials like boron, cadmium, or hafnium
• Fission cross-section represents the probability of a fission reaction occurring
  • Depends on factors like neutron energy and target nucleus properties
• Criticality refers to the state where the number of neutrons produced by fission equals the number lost through absorption and leakage
  • Necessary for a self-sustaining chain reaction in a nuclear reactor

Historical Background

• Nuclear fission was discovered in 1938 by German chemists Otto Hahn and Fritz Strassmann
  • Bombarded uranium with neutrons and observed barium as a product
• Lise Meitner and Otto Frisch provided the theoretical explanation for fission in 1939
  • Proposed that the nucleus split into lighter fragments due to the repulsive electrostatic force overcoming the strong nuclear force
• Enrico Fermi and his team achieved the first self-sustaining nuclear chain reaction in 1942 at the University of Chicago
  • Constructed the Chicago Pile-1 (CP-1) reactor using graphite as a moderator and natural uranium as fuel
• The Manhattan Project, led by the United States during World War II, developed the first nuclear weapons based on fission
  • Culminated in the atomic bombings of Hiroshima and Nagasaki in 1945
• After the war, nuclear fission was pursued for peaceful applications like power generation
  • The first nuclear power plant began operation in Obninsk, Soviet Union, in 1954
• Subsequent decades saw the proliferation of nuclear power plants worldwide
  • Faced challenges related to safety, waste management, and public perception, especially after accidents like Three Mile Island (1979) and Chernobyl (1986)

Nuclear Fission Process

• Fission reactions can be induced by neutrons or occur spontaneously in certain unstable isotopes
• Neutron-induced fission is the primary mechanism in nuclear reactors
  • Fissile isotopes like uranium-235 and plutonium-239 readily undergo fission with low-energy (thermal) neutrons
• When a neutron is absorbed by a fissile nucleus, it becomes highly unstable and deforms
  • Repulsive electrostatic force between protons causes the nucleus to elongate and eventually split
• The fission process releases energy in the form of kinetic energy of the fission fragments and neutrons
  • Typically around 200 MeV per fission event
• Fission fragments are highly excited and undergo radioactive decay, emitting beta particles and gamma rays
  • Decay heat contributes significantly to the overall energy output of a reactor
• Neutrons released during fission have high energies (fast neutrons) and must be slowed down by a moderator to increase the likelihood of inducing further fissions
• The fission process also yields neutron-rich isotopes that can undergo radioactive decay, contributing to the long-term radioactivity of spent nuclear fuel

Types of Nuclear Reactors

• Pressurized Water Reactors (PWRs) are the most common type of nuclear reactor
  • Use ordinary water as both coolant and moderator
  • Water is kept under high pressure to prevent boiling in the reactor core
• Boiling Water Reactors (BWRs) are similar to PWRs but allow water to boil in the reactor core
  • Steam is generated directly in the core and used to drive turbines for electricity production
• Heavy Water Reactors (HWRs) use heavy water (deuterium oxide) as a moderator
  • Allows the use of natural uranium as fuel, which is less enriched than the fuel used in PWRs and BWRs
• Gas-Cooled Reactors (GCRs) use graphite as a moderator and carbon dioxide or helium as a coolant
  • Operate at higher temperatures than water-cooled reactors, enabling higher thermal efficiency
• Fast Breeder Reactors (FBRs) do not use a moderator and rely on fast neutrons to sustain the chain reaction
  • Can "breed" more fissile material (plutonium-239) from fertile isotopes like uranium-238
• Molten Salt Reactors (MSRs) use a molten salt mixture as both coolant and fuel
  • Operate at high temperatures and low pressures, with potential safety and efficiency benefits

Reactor Components and Design

• Reactor core contains the fuel assemblies, control rods, and moderator
  • Fuel is typically in the form of uranium dioxide pellets encased in metal cladding (fuel rods)
  • Control rods are inserted or withdrawn to regulate the fission rate
• Pressure vessel is a thick-walled steel container that houses the reactor core
  • Designed to withstand high pressures and temperatures
• Coolant system removes heat from the reactor core and transfers it to the steam generators (in PWRs) or directly to the turbines (in BWRs)
  • Pumps circulate the coolant through the core and heat exchangers
• Steam generators (in PWRs) transfer heat from the primary coolant loop to a secondary loop, producing steam to drive the turbines
• Containment structure is a reinforced concrete or steel building that encloses the reactor and primary coolant system
  • Designed to contain radioactive material in the event of an accident
• Turbine generator converts the thermal energy of the steam into electrical energy
  • Steam spins the turbine, which is connected to an electrical generator
• Condenser cools the steam exiting the turbine, converting it back into water for recirculation
  • Typically uses water from a nearby river, lake, or cooling tower

Safety and Control Mechanisms

• Control rods are the primary means of regulating the fission rate in a reactor
  • Inserted into the core to absorb neutrons and slow down the reaction
  • Automatically inserted to shut down the reactor in an emergency (SCRAM)
• Burnable poisons are materials that absorb neutrons and are gradually consumed during reactor operation
  • Help to control reactivity and flatten the power distribution in the core
• Reactor protection systems monitor various parameters (temperature, pressure, neutron flux) and initiate safety actions if limits are exceeded
  • Include redundant sensors, logic circuits, and actuation mechanisms
• Emergency core cooling systems (ECCS) provide cooling water to the core in the event of a loss-of-coolant accident (LOCA)
  • Consist of high-pressure injection, low-pressure injection, and accumulators
• Containment systems prevent the release of radioactive material to the environment in case of an accident
  • Include the containment structure, isolation valves, and spray systems
• Passive safety features rely on natural phenomena (gravity, convection) rather than active components to ensure safety
  • Examples include gravity-driven cooling systems and passive heat removal

Environmental Impact and Waste Management

• Nuclear power plants do not emit greenhouse gases during operation, making them a low-carbon energy source
  • Lifecycle emissions (including construction and fuel processing) are comparable to renewable energy sources
• Nuclear accidents like Chernobyl and Fukushima have raised concerns about the safety and environmental impact of nuclear power
  • Resulted in the release of radioactive material into the environment and long-term evacuation of surrounding areas
• Spent nuclear fuel remains radioactive for thousands of years and requires safe storage and disposal
  • Typically stored in pools or dry casks on-site at nuclear power plants
  • Long-term disposal options include deep geological repositories, but no such facilities are currently operational
• Uranium mining and milling can have environmental impacts, including water pollution and habitat disruption
  • Proper management and remediation of mining sites are essential
• Decommissioning of nuclear power plants at the end of their operational life involves the safe dismantling and disposal of radioactive components
  • Process can take decades and requires significant financial resources

Applications and Future Developments

• Nuclear power currently provides around 10% of the world's electricity
  • Seen as a potential solution to reduce greenhouse gas emissions and combat climate change
• Small Modular Reactors (SMRs) are a promising development in nuclear technology
  • Smaller, standardized reactors that can be manufactured in factories and transported to site
  • Potential benefits include lower capital costs, enhanced safety, and flexibility in deployment
• Generation IV reactor designs aim to improve safety, efficiency, and sustainability
  • Include concepts like the Very High Temperature Reactor (VHTR) and the Molten Salt Reactor (MSR)
  • Some designs can operate on alternative fuel cycles (thorium) or use nuclear waste as fuel
• Nuclear fusion, the process that powers the sun, is a potential future source of virtually limitless clean energy
  • Involves fusing light elements (hydrogen isotopes) into heavier elements, releasing energy
  • Technically challenging due to the extreme conditions required (high temperature and pressure)
  • Research efforts like the ITER project aim to demonstrate the feasibility of fusion power