Nuclear Reactors

A nuclear power reactor makes use of nuclear fission chain reactions to produce electricity. When nuclear fission occurs, energy is released and used to turn a turbine to produce the electricity. By controlling the nucler reaction (by making it critical), subsequent fissions can occur.

The key components of a nuclear reactor are the fuel, moderator, coolant, boiler and turbine labelled in red in the diagram below:

The fuel is the source of the nuclear fission reaction. In some reactors, the fuel is enriched to have a higher percentage of, for example, \(^{235}U\). Natural uranium has a concentration of 0.71% \(^{235}U\). Fuel can be in the form of solid pellets, liquids with dissolved fuel, etc.

The moderator is used to slow down neutrons to aid in reactions that are more likely to occur at lower energies. Examples of moderators include water (light and heavy), graphite (carbon), lithium and beryillium.

The coolant removes heat from the fuel and transfers it to the boiler. Heat is produced from the fission reactions. If the heat is not removed, the fuel can melt and release radioactive material. Types of coolants include water (light and heavy), helium gas and molten salts.

The boiler creates steam from the hot coolant. Creating steam for a turbine is a common process in electricity power plants, not just nuclear power plants.

The turbine produces electricity from steam and is not unique to nuclear reactors.

Key characteristics of nuclear reactors are summarized below:

Reaction Type Fission can occur from thermal (slow) or fast neutrons.
Fuel Type Nuclear fuel can be Uranium, Plutonium, Thorium, etc. Fuel can be in the form of solid pellets, liquids with dissolved fuel, etc
Enrichment Nucluear fuel can be natural or enriched.
Moderator Moderators can be water, graphite, etc.
Coolant Coolant can be water, gas, etc.
Boilers Reactors can have a pressurized, isolated heat transport loop that transfers heat to the non-radioactivesecondary side to produce steam or the steam is produced directly from the primary side.
Generation Most operating reators are classified as Generation II or III/III(+). Generation IV reactors are currently under research.

Common reactor types are listed below.

PWR Pressurized Water Reactor - high pressure light water coolant and moderator contained in a pressure vessel. Hot coolant heats up water in the steam generators on the secondary side to produce steam for the turbine.
BWR Boiling Water Reactor - low pressure light water coolant and moderator contained in a reactor vessel. Water boilers within the vessel. Steam goes directly to the turbine.
PHWR Pressurized Heavy Water Reactor - high pressure heavy water coolant and moderator. Natural Uranium fuel and coolant is contained in hundreds of pressure tubes.
CANDU CANadian Deterium Uranium - Canadian designed PHWR.
GCR Gas Cooled Reactors - gas coolant with graphite moderator.
RBMK Reaktor Bolshoy Moschnosti Kanalniy - high pressure coolant with graphite moderator. This reator uses a fuel channel design.
LMFBR Liquid Metal Fast Breeder Reactor - Lead or sodium coolant reactor with no moderator that breeds more fuel than it consumes. This reactor type undergoes fission from fast neutrons.
PBR Pebble Bed Reactor - Gas-cooled and graphite moderator. The fuel is in the form of small ceramic balls.
MSR Molten Salt Reactor - Fuel (sometimes Thorium) can be dissolved in a flouride salt coolant.
SMR Small Modular Reactor - Smaller reactors gaining much attention in recent years for powering remote areas and energy intensive systems.