Unit
- Energy conversion process: Nuclear fission releases heat energy that is transferred to a coolant, which then produces steam to drive turbines connected to electrical generators. This thermal-to-mechanical-to-electrical conversion follows the same Rankine cycle used in fossil fuel plants.
- Fission basics: A neutron strikes a fissile nucleus, typically uranium-235 or plutonium-239, causing it to split into lighter nuclei (fission products) and releasing 2-3 fast neutrons and approximately 200 MeV of energy per event. The neutrons can then cause further fissions, sustaining a chain reaction.
- Neutron moderation: Fast neutrons from fission must be slowed down (thermalized) to increase the probability of causing further fissions. Moderators such as light water, heavy water, or graphite reduce neutron kinetic energy to thermal levels.
- Control mechanisms: Control rods containing neutron-absorbing materials (boron, cadmium, hafnium) are inserted into the core to regulate the neutron population and thereby the power level. Their positioning determines the reactor's reactivity.
- Heat removal: The primary coolant circulates through the reactor core, absorbing heat, and transfers it to a secondary system via steam generators (in PWRs) or directly to turbines (in BWRs). Efficient heat removal prevents core damage.
- Containment: Multiple physical barriers prevent radioactive release: the fuel pellet, fuel rod cladding, reactor vessel, and containment building. Each barrier is monitored and maintained independently.
- Reactor types: Pressurized Water Reactors (PWR), Boiling Water Reactors (BWR), Pressurized Heavy Water Reactors (PHWR), and Advanced Gas-cooled Reactors (AGR) are the main commercial designs, each with distinct coolant, moderator, and operational characteristics.
- Global deployment: Over 440 nuclear reactors operate in 32 countries, providing about 10% of world electricity. The United States, France, China, and Russia lead in installed capacity.
- Operational roles: Reactor operators, shift supervisors, and engineers work under strict licensing requirements, undergoing extensive simulator training and periodic requalification.
- Fuel cycle: Uranium is mined, milled, converted, enriched, and fabricated into fuel pellets. After irradiation, spent fuel is stored in pools or dry casks before eventual reprocessing or permanent disposal. The closed fuel cycle in some countries recycles plutonium and uranium for mixed-oxide (MOX) fuel.
- Decay heat: After reactor shutdown, fission products continue to emit radioactive decay energy, generating about 7% of full power initially. Residual heat removal systems must operate continuously to prevent fuel melting, even after control rods are inserted.
- Safety systems: Engineered safety features include emergency core cooling systems, containment spray systems, and passive cooling mechanisms. These redundant, diverse systems ensure core integrity under accident conditions as required by defense-in-depth principles.
Process Flow
- Step 1 - Fission Event: Neutron absorption by fissile nucleus causes splitting and release of energy and neutrons.
- Step 2 - Heat Transfer: coolant circulates past fuel rods, absorbing thermal energy through convection.
- Step 3 - Steam Generation: primary coolant transfers heat to secondary system; water boils to steam.
- Step 4 - Turbine Drive: high-pressure steam expands across turbine blades, spinning the rotor and generator.
- Step 5 - Condensation: exhaust steam is condensed back to water and returned to steam generator or reactor vessel.
- Step 6 - Power Regulation: control rods adjust reactivity; coolant pumps maintain flow rate; feedwater system controls steam pressure.
Quick Revision
- Nuclear fission releases heat used to generate steam and electricity via turbines.
- Moderators slow neutrons; control rods absorb neutrons to regulate power.
- Multiple barrier layers prevent radioactive release.
- Common commercial types include PWR, BWR, PHWR, and AGR.
- Operators require specialized training and licensing.