A nuclear power plant is an AC power generation station, whose energy derives from fission nuclear reactions that take place in the uranium-235 rods. These nuclear fuel rods are located in the core of the reactor, which is in turn is contained in a vessel. The uranium nuclear rods produce heat, which converts water into steam in a steam generator. The pressurized steam runs out of the containment shell to flow through a steam turbine, setting it in motion and turning it at high rpm speed. The turbine is coupled to an AC generator rotor, turning it at the same speed. The generator rotor generates alternating current on the generator stator winding. Therefore nuclear power is produced by fission reactions, which is harnessed to produce steam for electric power production. Fission reactions involve the breakup of the nucleus of high-mass atoms, triggering an energy release.
The nuclear reactor in a nuclear power plant is a device in which a controlled nuclear chain reaction takes place accompanied by the release of energy, which is used to produced pressurized steam. The first nuclear reactor was built in the United States in December 1942 under the direction of the Italian physicist Enrico Fermi. In Europe the first nuclear reactor was started up in December 1946 in Moscow under the direction of I. V. Kurchatov. By 1978, about 1,000 nuclear reactors of different types had been built around the world. Every nuclear reactor has a core, which contains the nuclear fuel and which is usually surrounded by a neutron reflector; a coolant; a system for controlling the chain reaction; radiation shielding; and a remote control system. The principal characteristic of a nuclear reactor is its power. A power of 1 megawatt corresponds to a chain reaction in which 3 × 1016 fission occur per 1 sec.
Condition determines the size of the nuclear reactor. For instance, in the case of a natural-uranium, graphite-moderated reactor, v = 2.4, ∊ ≈ 1.03, and ∊φθ ≈ 0.44, whence k∞ = 1.08. This means that for k∞ > 1, P must be less than 0.93, which corresponds to reactor core dimensions of about 5–10 m (as reactor theory shows). The volume of a modern power reactor reaches hundreds of cubic meters and is determined mainly by the possibilities of heat removal rather than by conditions of criticality. The volume of the reactor core in the critical state is called the critical volume, and the mass of fissionable material is called the critical mass. Nuclear reactors with fuel in the form of solutions of salts of pure fissionable isotopes in water and with a water neutron reflector have the smallest critical mass, which is 0.8 kg for U-235 and 0.5 kg for Pu-239. 251Cf has the lowest critical mass (theoretically 10 g). The critical parameters of a graphite-moderated nuclear reactor that uses natural uranium as fuel are as follows: mass of uranium, 45 tons, and graphite volume, 450 cu m. To reduce neutron leakage, the core is spherical or close to spherical, for example, a cylinder, with a height of the order of the diameter, or a cube (minimum ratio of surface to volume).
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| A schematic image of a fission nuclear power plant. The uranium fuel rods are in red in the reactor core. |