原子力

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 subcritical and the energy output is decreasing. When k is 1, the reactor is critical and the energy output is steady. When k is greater than 1, the reactor is supercritical and the energy output is increasing. The chain reaction is controlled with control rods made of neutron-absorbing materials like boron or cadmium. Fully inserted control rods absorb a large number of neutrons, keeping the reactor subcritical. Withdrawing the control rods allows more and more fissions to occur. Coolant, such as water, transfers heat away from the reactor core to make steam for the turbine. As the reactor heats up, the neutrons move faster and are less likely to cause fissions, which helps avoid overheating. The core is shielded by materials like water and thick concrete layers. The overall core design and containment structure both depend on the specific type of reactor.