Primary Loop Components

Reactor Core and Reactor Vessel

The process of producing electricity in a nuclear energy plant begins in the reactor core. This is where the nuclear chain reaction occurs.

The reactor core is located inside the reactor vessel, a thick steel container that holds the fuel assemblies, coolant and control rods. The chain reaction heats water that circulates through the reactor vessel and pipes carry the heated water to the steam generator.

Reactor Coolant Pumps

The reactor coolant pumps move the chemically infused water, also known as the coolant, through the primary loop. This water removes and transfers the heat produced in the reactor core.

Steam Generator

Heated water from the reactor vessel travels through pipes to the steam generator. The water enters through a nozzle at the bottom of the steam generator and travels under high pressure through tubes surrounded by a different body of water. This turns this second water surrounding the tubes into steam.

The water that entered the steam generator exits through another nozzle to return to the reactor vessel allowing it to be re-heated.

The steam generator’s tubes act as a barrier, keeping radioactive water from the core contained while efficiently transferring heat to create steam.


The pressurizer maintains a balance of water and steam in the primary loop. If pressure in the system increases, the spray line releases cold water, which turns the steam in the pressurizer into water and reduces the pressure. If the pressure in the system decreases, the electrical heaters come on to turn more water into steam and increase the pressure.

Carefully controlling the pressure allows the water within the system to reach a higher temperature without boiling.

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