The Inner Workings of a Nuclear Fission Reactor

Pictured below is the most common type of nuclear reactor, the Pressurized Water Reactor.

Schematic used courtesy of the World Nuclear Association

The components common to most types of nuclear fission reactors are as follows:

Fuel rods:  Usually pellets of uranium oxide, in which the uranium-235 has been enriched from its natural 1% level to about 3%, are contained in tubes made from a protective material, usually a zirconium alloy, to form fuel rods.  The rods are arranged into fuel assemblies in the reactor core.

Note: Naturally occurring uranium is less than 1% uranium-235. The vast majority is uranium-238, a non-fissile isotope that absorbs neutrons, which helps control the chain reaction.

Control rods:  These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to slow or stop the reaction. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)

Moderator:   This is a material, such as water, heavy water, graphite, or carbon dioxide, which slows down the neutrons released from fission so that they can collide with the uranium at a speed conducive to a reaction.

Coolant:  A liquid or gas that is pumped through the reactor core and carries the heat produced by the fission reaction to a steam generator and turbine, where it is transformed into electricity.

Steam generator:  The part of the cooling system where the heat from the reactor is used to make steam for the turbine.

Turbine: Steam turns the turbine, which can in turn propel a naval submarine or generate electricity.

Pressure vessel:  The steel vessel containing the reactor core and moderator.

Containment:   The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation or any malfunction inside, usually 1 to 3 meters of high-density concrete.

To recap, nuclear fission reactions take place in the reactor core, fueled by fuel rods and controlled by control rods. A moderator, such as water, inside the core slows down the neutrons so that they can react with the uranium fuel. A coolant, such as water, is pumped through the reactor, carrying the heat from the core to the steam generator, where water boils producing steam, and to the turbine, where the steam turns the turbine to create electricity or propel a submarine. The coolant is often pressurized within the steel pressure vessel to keep it from boiling (in the case of water). All of this is contained within a concrete structure to protect both the reactor and the people.

The links below will take you to descriptions of the five most common types of nuclear reactors.

Written and created by Cami Idzerda. Last updated 11/29/2001. 

Email: CIdzerda@aol.com