Nuclear reactors come in various types, each with unique designs and components. Pressurized Water Reactors and Boiling Water Reactors are the most common, using water as both coolant and moderator. Fast Breeder Reactors, on the other hand, use liquid metal coolants and produce more fuel than they consume.

The heart of a nuclear reactor is its core, containing fuel rods and control elements. Coolant systems remove heat, while moderators slow neutrons in thermal reactors. Safety features, including containment structures and emergency systems, are crucial to prevent accidents and protect the public.

Reactor Types

Pressurized and Boiling Water Reactors

• Pressurized Water Reactor (PWR) uses high-pressure water as both coolant and moderator
  • Primary coolant loop circulates water through the reactor core at high pressure (around 150 atmospheres)
  • Secondary loop generates steam for turbines, separated from radioactive primary loop
  • Most common reactor type worldwide (French nuclear fleet, many U.S. plants)
• Boiling Water Reactor (BWR) allows water to boil directly in the reactor core
  • Single loop system where steam is generated directly in the reactor vessel
  • Operates at lower pressure than PWR (around 70 atmospheres)
  • Simpler design with fewer components (no steam generators)
  • Used in several countries (General Electric designs in the U.S. and Japan)

Fast Breeder Reactors

• Fast breeder reactor utilizes fast neutrons without moderation
  • Designed to produce more fissile material than it consumes
  • Uses liquid metal coolant (usually sodium) instead of water
  • Can convert fertile uranium-238 into fissile plutonium-239
  • Higher neutron economy allows for more efficient use of uranium resources
  • Examples include Russia's BN-series reactors and France's Phénix and Superphénix

Reactor Core Components

Fuel and Control Elements

• Reactor core houses the nuclear fuel and controls the fission reaction
  • Typically cylindrical in shape, containing fuel assemblies arranged in a lattice
  • Core size varies depending on reactor type and power output
• Fuel rods contain fissile material used in the nuclear reaction
  • Usually made of uranium dioxide (UO2) pellets encased in zirconium alloy tubes
  • Arranged in fuel assemblies, each containing multiple fuel rods (200-300 rods per assembly)
  • Enrichment levels vary (3-5% U-235 for most commercial reactors)
• Control rods regulate the fission rate within the reactor
  • Made of neutron-absorbing materials (boron, cadmium, hafnium)
  • Inserted or withdrawn to control reactor power output
  • Emergency shutdown achieved by fully inserting all control rods (SCRAM)

Coolant and Moderator Systems

• Coolant removes heat generated by fission reactions
  • Water most common in thermal reactors (PWRs, BWRs)
  • Liquid metals (sodium, lead-bismuth) used in fast reactors
  • Gas coolants (helium, carbon dioxide) employed in some designs (Advanced Gas-cooled Reactors)
• Moderator slows down fast neutrons to increase fission probability
  • Water serves dual purpose as coolant and moderator in most reactors
  • Other moderators include graphite (RBMK reactors) and heavy water (CANDU reactors)
  • Fast reactors do not use moderators, relying on high-energy neutrons for fission

Reactor Containment and Safety

Containment Structures and Safety Systems

• Containment structure encloses the reactor to prevent release of radioactive materials
  • Typically consists of a steel liner inside a reinforced concrete dome
  • Designed to withstand internal pressures and external impacts (aircraft crashes)
  • Multiple barriers: fuel cladding, reactor vessel, containment building
• Safety systems protect against accidents and mitigate consequences
  • Emergency Core Cooling System (ECCS) prevents core meltdown in loss of coolant accidents
  • Passive safety features in modern designs rely on natural processes (gravity, convection)
  • Redundant and diverse systems ensure reliability (multiple backup power sources)
• Regulatory bodies oversee nuclear safety (Nuclear Regulatory Commission in the U.S.)
  • Establish safety standards and conduct regular inspections
  • Require extensive operator training and emergency preparedness plans

Power Generation Components

Steam Generation and Turbine Systems

• Steam generator transfers heat from reactor coolant to secondary loop in PWRs
  • Large heat exchanger with thousands of tubes
  • Produces steam to drive turbines without direct contact with radioactive primary coolant
  • BWRs generate steam directly in the reactor, eliminating need for separate steam generators
• Turbine converts thermal energy of steam into mechanical energy
  • Multiple stages (high-pressure, low-pressure) maximize energy extraction
  • Typically operates at 1800 or 3600 rpm (60 Hz electrical systems)
  • Steam quality and moisture content critical for turbine efficiency and longevity

Electrical Generation and Grid Connection

• Generator converts mechanical energy from turbine into electrical energy
  • Large synchronous generators produce three-phase alternating current
  • Cooling systems (hydrogen, water) maintain generator efficiency
  • Output voltages typically 20-25 kV, stepped up for transmission
• Switchyard connects the nuclear plant to the electrical grid
  • Transformers increase voltage for long-distance transmission (345 kV, 500 kV, or higher)
  • Circuit breakers and disconnect switches provide grid isolation capability
  • Auxiliary power systems ensure plant can operate during grid disturbances