## **22.6 NUCLEAR REACTORS** A nuclear chain reaction can release a massive amount of energy very quickly. A **nuclear reactor** is a special machine designed to control this powerful reaction, allowing us to harness its energy safely and steadily, mainly to generate electricity. This section will take a look inside a reactor to explain its most important parts, such as the fuel, the control rods that act like brakes, and the moderator that keeps the reaction going efficiently. ### **Nuclear Reactor Definition** A device designed to initiate and control a nuclear fission chain reaction, releasing energy for useful purposes like electricity production and radioisotopes. ### **Main Components of a Nuclear Reactor** * **Core**: The central part of the reactor where nuclear fission occurs, containing the fuel and designed to sustain a controlled chain reaction and generate heat. * **Fuel Rods**: * Contain fissile material, typically *Uranium-235* or *Plutonium-239*, in small pellets arranged into long rods (approx. 1 cm diameter). * Most reactors use uranium enriched to about $3\%$ U-235. * **Control Rods (Safety Rods)**: * Made of neutron-absorbing materials like *boron* or *cadmium*. * Function: Control the extent of the fission reaction by absorbing excess neutrons, ensuring only one neutron from each event causes further fission. * Can be moved into or out of the core as needed. * **Moderator**: * Material that *slows down fast neutrons* (from MeV range to ~0.025 eV) emitted during fission, making them more likely to cause additional fission. * Commonly used materials: *water* and *graphite*. * **Coolant (Heat Exchanger)**: * A fluid (e.g., *water*, *helium*) that circulates through the core, carrying away the heat produced by fission. * This heat is used to produce steam, which drives turbines to generate electricity. * Also cools the fuel rods and moderators, preventing core meltdown. * **Shielding Unit**: * A thick layer of concrete or other material surrounding the reactor vessel. * Purpose: Protects workers and the environment from harmful strong radiations and energetic particles (by-products of fission). ### **Enrichment of Uranium** * **Natural Uranium Isotopes**: * *Uranium-238 (U-238)*: ~99.3% of natural uranium, non-fissile but can convert to fissile Plutonium-239 (Pu-239) by capturing neutrons. * *Uranium-235 (U-235)*: ~0.7% of natural uranium, *fissile* (can sustain a chain reaction by absorbing neutrons). * **Definition**: The process of increasing the proportion of the fissile U-235 isotope in uranium fuel. (Note: The text also states, "The process of converting non fissile material like uranium-238 into a fissile material like uranium-235 is called enrichment," which refers to the broader goal of increasing fissile content). * **Importance**: Natural uranium's low U-235 concentration (~0.7%) is insufficient to sustain a controlled nuclear chain reaction in most reactors. Enrichment (typically to 3-5% U-235) is crucial for achieving criticality and efficient operation. * **Methods**: * **Gaseous Diffusion**: Uranium converted to a gas (Uranium hexafluoride). Lighter U-235 molecules pass through porous barriers more easily than heavier U-238, gradually increasing U-235 concentration. * **Gas Centrifugation**: Uses rapidly spinning centrifuges to separate uranium isotopes based on differences in their masses. * **Laser Enrichment**: Uses lasers to selectively ionize U-235 atoms for separation.