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Pressurised Water Reactor
Pressurised Water Reactors, or PWRs, are the most common nuclear reactors in the world today.
PWRs make up 70% of the global fleet and PWR technology forms the basis of most modern reactor designs being built currently. This includes the new reactors being built at Hinkley Point C in the UK. The main reason for this is that they are easy to operate safely.
How do they work?
As the name suggests, PWRs use water as their coolant and as a moderator.
The design’s primary cooling circuit contains water which acts as a coolant by flowing through the core of the reactor. Water in the primary circuit is hot, around 300°C, and is prevented from boiling by being kept at high pressure.
The secondary circuit also uses water as a coolant. Water in the secondary circuit is under less pressure and therefore boils after interacting with the heat exchanger, which is held inside the Steam Generator. The steam from the boiled water is used to turn the turbine to generate electricity.
Simplified 2D PWR system. Image: World Nuclear Association.
Due to its high temperature, water within the primary circuit must be kept at high pressure if it is to remain liquid. The pressure in the primary circuit is controlled by the Pressuriser, which ensures the cooling water stays in a liquid state. The Pressuriser is a separate vessel connected to the primary circuit. It contains water and steam at a typical pressure of 16 MPa, controlled by changing the temperature inside the pressuriser. Temperature control within the pressuriser is achieved by either water being sprayed in to cool it down or electric heaters being used to heat it up.
The primary circuit water acts as the moderator in the reactor. It is made up of light water and a small concentration of Boron. This slows down the neutrons to enable more fission reactions to happen. Control rods are used to control the amount of fission that happens, so control the power output. PWRs use enriched Uranium as fuel.
Water is a good moderator because it is passively safe due to what’s called ‘Thermal Feedback’. This means that as the water gets hotter the power output goes down, and if the water cools down the power output goes up. This is the main reason why PWRs are so safe and widely used.
Fuel
Fuel
Fun fact: PWRs can last up to 2 years without being refuelled!
Moderator
Moderator
Coolant
Coolant
PWR Safety Systems
PWRs have many safety systems that can be deployed in case of an emergency.
