Nuclear Reactors — Types of Reactors in Use | How Do Reactors Work? | Key Components Of a Reactor

What are nuclear reactors?

A nuclear reactor is a device used to initiate and control a sustained nuclear chain reaction. Most commonly, they are used to generate electricity by harnessing the massive amount of heat released during nuclear fission (the splitting of heavy atomic nuclei).

How do they work?

1) FISSION: Heavy atoms, usually Uranium-235 or Plutonium-239, are struck by neutrons. This causes them to split into smaller atoms, releasing more neutrons and a tremendous amount of thermal energy (heat).

2) CHAIN REACTION: The newly released neutrons strike neighboring atoms, causing them to split as well. This creates a continuous chain reaction.

3) ELECTRICITY GENERATION: The heat produced by this reaction is used to boil water into pressurized steam. This steam spins a massive turbine connected to a generator, which produces electricity.

(Created using Gemini AI)

Key Components of a Reactor

1) FUEL: The radioactive material (like enriched uranium) packed into fuel rods.

2) MODERATOR: A substance (often regular water, heavy water, or graphite) that slows down the fast-moving neutrons. Slow neutrons are much more efficient at causing further fission.

3) CONTROL RODS: Rods made of neutron-absorbing materials (like boron or cadmium). They can be inserted into or removed from the reactor core to speed up, slow down, or completely shut down the chain reaction.

4) COOLANT: A fluid (usually water, but sometimes liquid metal or gas) that circulates through the core to absorb the heat and carry it away to generate steam.

5) CONTAINMENT STRUCTURE: A thick, robust shell of steel and reinforced concrete designed to prevent any radioactive materials from escaping into the environment in an emergency.

In short, a nuclear reactor acts like a highly sophisticated, incredibly powerful boiler. Instead of burning coal or gas to make steam, it splits atoms.

Types of Reactors in Use:

Gas Cooled Thermal Reactors

All use graphite as the moderator

1) Magnox Reactors:

These use uranium metal rods as fuel, enclosed in a Mg/Al (magnox) casing. The fuel is natural, that is, not enriched, and CO2 gas is used as the coolant. Most of the early reactors in the UK are of this type. These have been completely shut down now with the last one being Wylfa 1 which was shut down on December 30, 2015.

2) Advanced Gas Cooled Reactors (AGR):

These use UO2 (Uranium dioxide) pellets enriched to 2% as fuel, with CO2 as the coolant.

3) High Temperature Reactor (HTR):

These are used for military purposes, such as submarines. The fuel is UC2 (Uranium dicarbide), which is enriched to over 90%, thus allowing the reactor to be small. The coolant is helium, and the control rods are made up of Cadmium (Cd).

Water Cooled Thermal Reactors

(All use H2O/D2O as moderator)

1) Canadian Deuterium Uranium Reactor (CANDU):

These are a Canadian design, and use natural (not enriched) UO2 as fuel, and heavy water D2O as both moderator and coolant.

2) Pressurized Water Reactor (PWR):

These are a US design, and use UO2 pellets enriched to 3% as fuel. Water is used as both moderator and coolant. The casing must withstand the huge pressure from the steam produced, and is typically 10 inches of stainless steel surrounded by concrete.

3) Boiling Water Reactor (BWR):

This is similar to the PWR except that the UO2 fuel is only enriched to 2.2%. It works at a much lower pressure, and so the reactor casing need not be so strong.

4) Steam Generating Heavy Water Reactor (SGHWR):

These use UO2 pellets enriched to 2.3% as fuel, with D2O as moderator and H2O as coolant.

Fast Breeder Reactors

These do not use any moderator

These are much less developed than thermal reactors. They use Plutonium(IV) Oxide (PuO2) as fuel. Enrichment is not necessary as all isotopes of Pu are fissile (capable of undergoing nuclear fission). No moderator is used, and so the neutrons in the reactor are "fast neutrons". Some reactors use liquid Na as coolant, while others use He gas under a high pressure. If depleted UO2 is put in such a reactor, then the non fissile Uranium is converted into fissile Plutonium. The name 'breeder reactor' arises because more fissile material is produced than is used in the process. Potentially this process could provide an unlimited source of energy.

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