Principle of a moderator. Credit: European Nuclear Society.
A moderator is a substance used in nuclear reactors to reduce the speed of fast neutrons produced by nuclear fission. These substances consist of atoms of light elements, such as deuterium in heavy water, graphite, and beryllium, to which neutrons are able to impart some of their energy on collision, without being captured. Neutrons that have been slowed down in this way are much more likely to cause new fissions of uranium-235 than they are to be captured by uranium-238.