What is the moderator in US nuclear power plants?

Answer 1

Neutrons released from the fissioning, or splitting, of Uranium or Plutonium atoms collide with the molecules of the moderator, losing some of their energy in the process. This slows the neutrons down (slow neutrons are referred to as "thermal" neutrons") so that they are more likely to cause fission when colliding with another Uranium or Plutonium atom. In the most common types of reactors the moderator used is ordinary distilled water which is also used as the coolant for the reactor. Graphite and heavy water can also be used, and there are types of reactors which use no moderator at all - they operate on "fast" neutrons alone.

Answer 2

The moderator in a nuclear reactor is a medium that slows down neutrons in the core. Fission releases neutrons, and these neutrons come away with a very high kinetic energy. If they collide with other nuclei in the fuel, they can cause more fissions, but without a moderator, they move so fast that they have a low probability of this. The moderator slows down the neutrons and allows them to be "captured" and collide with more nuclei thus the probability of more fissions is greatly increased.

Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types and nuclear weapons, and hydrocarbons have been suggested as another possibility - although the flammability of most hydrocarbons suggest this would pose a risk that may be unacceptable.

Answer 3

Light water (ie ordinary natural water)

Answer 4

In a nuclear reactor, the moderator is: