Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by bombardment with neutrons has six components: nuclear fuel consisting of fissionable material, a nuclear moderator, a neutron source, control rods, reactor coolant, and a shield and containment system.
Nuclear fuel consists of a fissile isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction. In most pressurized water reactors, each fuel assembly consists of fuel rods that contain many thimble-sized, ceramic-encased, enriched uranium (usually UO2) fuel pellets. Modern nuclear reactors may contain as many as 10 million fuel pellets.
Uranium-235 is a useful fuel because it produces more than one neutron per fission on average, but its natural abundance is about 0.7 percent by weight. Most power reactors require their fuel to be enriched to at least 3 to 5 percent uranium-235 by weight.
Neutrons produced by nuclear reactions move too fast to cause U-235 fission reliably. They must first be slowed to be absorbed by the fuel and produce additional nuclear reactions. A nuclear moderator is a substance that slows the neutrons to a speed that is low enough to cause fission. Early reactors used high-purity graphite as a moderator. Modern reactors typically use heavy water or light water as moderators.
As neutrons have a size similar to that of hydrogen nuclei, when they hit the hydrogen atoms in water molecules, they lose a substantial amount of kinetic energy. Heavy water is a better moderator, as deuterium already has a neutron and is unlikely to absorb another neutron the way that hydrogen-1 sometimes will. Moderators like water and graphite also function as a neutron reflector to keep neutrons in the core in an even distribution.
Although uranium-238 and uranium-235 fission spontaneously, the process is unpredictable, and these intrinsic sources generate very few neutrons. Thus, a reactor requires a neutron emitter to initiate the fission chain reaction. A neutron source like beryllium-9 paired with an alpha emitter such as americium-249 or plutonium-239 is installed in a reactor to produce neutrons for the initiation of the chain reaction.
The power level of the reactor is described by the neutron multiplication factor, denoted by k. It is 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 equals 1, the reactor is critical and the energy output is steady; and when k is greater than 1, the reactor is supercritical and the energy output is increasing.
Nuclear reactors use control rods to control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level. Control rods are made of boron, cadmium, hafnium, or other elements that are able to absorb neutrons.
When control rod assemblies are inserted into the fuel element in the reactor core, they absorb a larger fraction of the slow neutrons, thereby slowing the rate of the fission reaction and decreasing the power produced. Conversely, if the control rods are removed, fewer neutrons are absorbed, and the fission rate and energy production increase. In an emergency, the chain reaction can be shut down by fully inserting all of the control rods into the nuclear core between the fuel rods.
In a pressurized water reactor, the reactor coolant is used to carry the heat produced by the fission reaction to an external boiler and turbine, where it is transformed into electricity. Two heat-exchanging coolant loops are often used to prevent the transfer of contaminated coolant to the steam turbine and cooling tower. Most commonly, water is used as a coolant. Other coolants in specialized reactors include molten sodium, lead, a lead–bismuth mixture, or molten salts. A large, hyperboloid cooling tower condenses the steam in the secondary cooling circuit and is often located at some distance from the actual reactor.
Shield and Containment System
Pressurized water reactors are equipped with a containment system (or shield) that typically consists of three parts: (i) a steel shell that is 3–20 centimeters thick; the moderator within the shell absorbs much of the neutron radiation produced by the reactor; (ii) a main shield of 1–3 meters of high-density concrete that absorbs γ rays and X-rays; (iii) additional shielding to absorb incident radiation from the shielding processes of (i) and (ii). In addition, pressurized water reactors are often covered with a steel or concrete dome that is designed to contain any radioactive materials that might be released by a reactor accident.
This text is adapted from Openstax, Chemistry 2e, Section 21.4: Transmutation and Nuclear Energy.