How does a PWR nuclear reactor work?

There are different types of nuclear reactors, but there are two designs that account for more than 80% of all those in operation in the world: the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR).

In both cases, water acts as both moderator and coolant and circulates through the core, raising its temperature. The basic difference is that, for PWRs, because they operate at very high pressure, the water does not boil and remains liquid throughout the reactor coolant circuit, whereas in BWRs, the water leaves the core in the form of steam.

As an example, in a PWR reactor, there are three coolant circuits isolated from each other:

• Primary circuit. This is the one in contact with the fuel.
• Secondary circuit. Water-steam.
• External cooling circuit. This is the one in contact with the environment.

This figure shows how the water circulates through the so-called "primary circuit" by means of pumps that push it through the reactor core where it is heated before being directed to the steam generators. The pressure in the primary circuit is kept high by the "pressurizer", thus preventing the formation of steam within the primary circuit.

In these plants, the control rods are located in the reactor vessel cover and are inserted into the core at the top. In the event of a demand for rapid reactor shutdown, the reactor is shut down by gravity when the existing electromagnetic locking mechanisms are released.

The water leaving the reactor vessel circulates through the primary circuit until it enters the inverted U-shaped tubes of the steam generators. Water from the secondary circuit flows through the outside of these tubes and is heated to steam. The steam is directed to the turbine where it expands, making it rotate and move the alternator, which is the component in which electricity is produced, which is sent to the transformer station and, from there, to the external electricity grid.

As in any thermal power plant, it is necessary to condense the steam from the turbine exhaust for its return to the water/steam cycle. The condensation is done by an external cooling circuit that uses a large flow of water circulating inside the condenser tubes, transporting the unusable energy to the heat sink of the power plant (river, reservoir, sea, or cooling towers). The steam that condenses on contact with the condenser tubes is driven by the condensate and feedwater pumps to be preheated before being sent to the steam generators.

The pressure in the condenser is lower than that of the external cooling circuit so that in the event of a leak in the tubes it would be the water from the external cooling circuit that would enter the condenser, thus avoiding possible leaks into the environment.