Fig. 14-1. Fission of ²³⁵U after absorption of a thermal neutron.

The relevant nuclear reactions can be written as follows:

²³⁵U + ¹n Æ fission products + neutrons + energy (~200 MeV) (1)

²³⁸U + ¹n Æ ²³⁹U + gamma rays (2)

²³⁹U Æ ²³⁹Np Æ ²³⁹Pu (a series of beta decays). (3)

An integer number of neutrons, for example, either two, three or four, are emitted in the reactions leading to different pairs of fission products described in reaction (1). The average number is 2.43. Some of the neutrons in reaction (1) can be used to induce fission in another ²³⁵U nucleus, thus continuing a controlled, self-perpetuating nuclear chain reaction. Some fraction of the remaining neutrons from reaction (1) are utilized in reaction (2) to produce ²³⁹Pu. The rest are absorbed in other nuclei without further effect.

The isotope ²³²Th, although not fissionable with thermal neutrons, is a possible energy source because it absorbs thermal neutrons to produce long-lived ²³³U, which also undergoes thermal neutron fission with a high probability. Thus the "big three" readily fissionable nuclei are: ²³⁵U, ²³⁹Pu, and ²³³U.

Fig. 14-2. The reactor vessel of a commercial reactor is inside this containment building.

A typical pressurized (or boiling) water nuclear reactor consists of a core of fissionable material (UO2, enriched to 3.3% to 4% in ²³⁵U) in which the chain reaction takes place. A picture of a reactor is shown in Fig. 14-2. The energy released in the fission process, which is primarily in the form of the kinetic energy of the fission fragments, heats the water. The water serves both as a neutron moderator (it slows down the fission neutrons to thermal energies), and as a heat transfer fluid. The chain reaction is controlled by rods of neutron-absorbing material inserted into the core. The thermal energy is removed from the core by the water to an external thermal-energy converter. In the pressurized water reactor (PWR), the thermal energy produces steam for the turbine through the use of a heat exchanger, whereas in a boiling water reactor (BWR), the steam is produced for direct use in the turbine.

Nuclear reactions liberate a large amount of energy compared to chemical reactions. One fission event results in the release of about 200 MeV of energy, or about 3.2 ´ 10^-11 watt-seconds. Thus, 3.1 ´ 10¹⁰ fissions per second produce 1 W of thermal power. The fission of 1 g of uranium or plutonium per day liberates about 1 MW. This is the energy equivalent of 3 tons of coal or about 600 gallons of fuel oil per day, which when burned produces approximately 1/4 tonne of carbon dioxide. (A tonne, or metric ton, is 1000 kg.)

Nuclear reactors manufacture their own fuel, since they produce ²³⁹Pu from ²³⁸U. With the total worldwide installed nuclear capacity of 3.4 ´ 10⁵ MW_e (megawatt electrical), one can estimate that more than 100 tonnes of ²³⁹Pu are produced each year in reactors whose primary energy source is the fission of ²³⁵U. This ²³⁹Pu can be reprocessed from used fuel rods and used to power other reactors.

It is actually possible to generate more ²³⁹Pu than is used up in the reactor by surrounding the core with a uranium blanket and generating ²³⁹Pu in this blanket. This is called a breeder reactor. A breeder reactor needs to be operated with fast neutrons, a so-called "fast breeder" reactor. In a fast-breeder reactor, water cannot be used as a coolant because it would moderate the neutrons. The smaller fission cross sections associated with the fast neutrons (as compared with thermal neutrons) leads to higher fuel concentrations in the core and higher power densities, which, in turn, create significant heat transfer problems. Liquid sodium metal may be used here as a coolant and heat-transfer fluid. Research on breeder reactors has essentially stopped in the United States because of concerns over nuclear proliferation since the plutonium bred in the reactor might be used for making weapons. Due to such concerns and the complexities of construction and operation, it is unlikely that breeder reactors will ever come into general operation within the next several decades, if ever.

In a yearly operating cycle of a typical (1000 MW_e) pressurized water reactor, the spent fuel contains about 25 tonnes of uranium and about 250 kg of ²³⁹Pu. Some 40% of the energy produced in the course of a nuclear fuel cycle comes from ²³⁹Pu. Since about 20% of the electricity generated in the United States comes from nuclear power plants, about twice as much electricity is generated from ²³⁹Pu as is generated from oil-fired electrical generating plants.

Some 435 nuclear power plants operating around the world generate about 345,000 MW_e of electricity in 32 countries, about one-sixth the world’s electricity supply. Some countries depend vitally on the electricity generated by nuclear energy. France generates 76% of its electricity from nuclear power plants; Belgium–56%, South Korea–36%, Switzerland–40%, Sweden–47%, Finland–30%, Japan–33%, and the United Kingdom–25%. Bulgaria generates 46% of its electricity from nuclear power, Hungary–42%, and the Czech Republic and Slovakia combined–20%. Although the United States is not a leader in percentage, it has the largest total electric output from nuclear power: 98,000 MW_e from 105 plants, generating around 20% of US electric power.

However, the United States, which led the world in early nuclear electric power development, was also first to be affected by its decline. Over the past two decades the US nuclear electric power industry has received no new domestic orders due to concerns over reactor safety, waste disposal, regulatory uncertainty, increased costs, and decreased electrical demand growth. These same pressures are now affecting nuclear power worldwide, although countries such as France and South Korea still have vigorous programs. A rebirth may be imminent because of impending solutions to some of these problems and because of the problem of global warming due to the release of greenhouse gases in the combustion of fossil fuels.

The public has become suspicious of nuclear energy partly because of the elaborate methods used to address safety concerns. Most worrisome is the need for external electrical power to supply the pumps used in emergency core cooling systems in the types of reactors now used in the United States. In an accident, this external supply could possibly be disrupted leading to a release of radioactivity. New reactor designs use passive safety to address this problem. Passive safety features can be thought of as characteristics of a reactor that, without operator intervention, will tend to shut or cool a reactor down, keep it in a safe configuration, and prevent release of radioactivity. These features fall into two broad categories–features that are designed to prevent accidents, and those that mitigate the effects of accidents. Many current problems arise from the huge scale of reactor construction projects. Typically, 1000 MW reactors have been constructed on site. The new types of reactors are smaller (e.g. 600 MW) and may be constructed in factories where uniformity and quality control can produce reactors and operating procedures less prone to failure. Examples of these new designs are two types of Advanced Passive Light Water Reactors (APLWRs), the Liquid Metal Reactor (LMR), and the Modular High Temperature Gas-cooled Reactor (MHTGR). The APLWR is the most developed of these new reactor types. The AP-600 (Advanced Passive 600 MW_e), a pressurized water reactor. It runs at lower temperatures and with larger water inventories than current light water reactors, and has a passive emergency core-cooling system that utilizes gravity. It uses passive natural circulation for the removal of heat from the core.

Nuclear fission reactors, usually pressurized water reactors with energy conversion based on a steam-turbine cycle, have been used extensively to power ships. The United States has built and launched an impressive number of nuclear powered ships–a total of 155 attack and missile submarines, 9 guided missile cruisers (18 reactors), and 5 aircraft carriers (with a total of 24 reactors) and has operated 9 prototype reactors on land. Of this total, 22 submarines have been decommissioned or are non-operational. Shipboard reactors are constructed smaller than similar installations ashore, with special attention given to maintenance, protection from collision, and leakage. Although the primary use of nuclear power has been in submarines and other military vessels, a few non-military marine installations have been demonstrated. Examples include the Russian icebreaker Lenin and the United States demonstration merchant ship NS Savannah.