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What is a Neutron Moderator?
The power generation in a nuclear reactor begins with the fuel undergoing fission. This is the chain reaction that releases the energy we need. For successful fission reactions to take place, the fast neutrons released by the fuel need to be slowed down. Fast neutrons are less effective at causing fission reactions so need to be moderated to increase their chances of leading to more reactions. The moderator in a nuclear reactor slows down these neutrons.
How do they work?
So why don’t fast neutrons cause more fission reactions? This is similar to what happens in putting when the golf ball moves towards the hole but is moving too quickly to drop in, it will jump straight over or maybe run around the rim.
Moderators slow down the neutrons (golf balls) so they can effectively cause another reaction (drop in the hole) and not just fly straight past another fissionable molecule.
The neutrons will collide with atoms within the moderator at high speeds, and pass some of their energy onto that atom. When the neutron passes some of its energy away, it slows down.
Examples of Neutron Moderators
Water is used in some reactors such as PWRs. Neutrons travel through the water colliding with molecules, and pass some of their kinetic energy to the water. This slows down the neutrons making them much more effective at causing fission in the reactor fuel. Water is used as both a moderator and a coolant in PWRs such as Hinkley Point C.
Graphite is good moderator as it can slow down neutrons without absorbing them. Fast neutrons interact with carbon atoms within graphite when they penetrate the material. The neutrons are slowed and release their energy. Graphite is used as a moderator in Advanced Gas-Cooled Reactors (AGRs) such as Hartlepool.
Explore Further
Choose from the articles below to continue learning about nuclear.
Nuclear Fuel – The source of incredible energy
Reactor Core – The beating heart of a nuclear plant
Control Rods – How to control a nuclear reactor
Reactor Coolant – Controlling vast amounts of energy
Did you know? Explore Nuclear also offers great careers information and learning resources.
Below you can find references to the information and images used on this page.
Content References
- NSAN – GAIN Exam Material
- US Department of Energy Office of Scientific and Technical Information, Analysis of essential nuclear reactor materials – Graphite Introduction and History.
- United States Nuclear Regulatory Commission https://www.nrc.gov/reading-rm/basic-ref/glossary/moderator.html
- WNA – Nuclear Power Reactors https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors.aspx
Image References
- Graphite: World Nuclear Association
- Golf: Wikipedia Commons – Licence CC BY 4.0 DEED
- PWR: Wikipedia Commons – Licence CC BY 4.0 DEED
- Graphitkugel fuer Hochtemperaturreaktor – Stefan Kühn – CC BY-SA 2.5
- Water Drop – Free to use
