Nuclear Power

Nuclear fission releases a great deal of thermal energy, allowing electricity generation from a steam turbine.

Nuclear fuel is typically a fissile nuclide such as uranium-235 that produces more than one neutron per fission on average. Fast neutrons released by fission must be slowed down by neutron moderators because thermal neutrons start chain reactions in fissile fuel most efficiently.

Water is a good moderator because hydrogen nuclei and neutrons have comparable sizes, ensuring that neutrons lose substantial kinetic energy in the collision. Heavy water is even better, as deuterium already has a neutron and is unlikely to absorb another.

Moderators also function as a neutron reflector to keep neutrons in the core in an even distribution.

Because the spontaneous fission of uranium-235 or 238 is unpredictable, a neutron source is installed in a reactor to ensure controlled initiation of the chain reaction.

The status of the chain reaction is described by the neutron multiplication factor, k: the ratio of the number of neutrons produced by fission in a generation to the number of neutrons produced by fission in the previous generation.

When k is less than 1, the reactor is subcriti