Nuclear energy is released during the splitting or fusing of atomic nuclei. The energy of any system, whether physical, chemical, or nuclear, is manifested by its ability to do work or to release heat or radiation. The total energy in a system is always conserved, but it can be transferred to another system or changed in form.
The atom consists of a small, massive, positively charged core (nucleus) surrounded by electrons. The nucleus, containing most of the mass of the atom, is itself composed of neutrons and protons bound together by very strong nuclear forces, much greater than the electrical forces that bind the electrons to the nucleus. The mass number A of a nucleus is the number of nucleons, or neutrons and protons, it contains; the atomic number Z is the number of positively charged protons.
The two key characteristics of nuclear fission important for the practical release of nuclear: first, the energy per fission is very large; second, the fission process initiated by the absorption of one neutron in uranium-235 releases about 2.5 neutrons, on the average, from the split nuclei. The neutrons released cause the fission of two more atoms, thereby releasing four or more additional neutrons and initiating a self-sustaining series of nuclear fissions, or a chain reaction.
Naturally occurring uranium contains only 0.71 percent uranium-235; the remainder is the non-fissile isotope uranium-238.
A variety of reactor types, characterized by the type of fuel, moderator, and coolant used, have been built throughout the world for the production of electric power. In the United States, with few exceptions, power reactors use nuclear fuel in the form of uranium oxide isotopically enriched to about 3 percent uranium-235. The moderator and coolant are highly purified ordinary water. A reactor of this type is called a light water reactor (LWR).
In the pressurized water reactor (PWR), a version of the LWR system, the water coolant operates at a pressure of about 150 atmospheres. It is pumped through the reactor core, where it is heated to about 325° C. The superheated water is pumped through a steam generator, where, through heat exchangers, a secondary loop of water is heated and converted to steam.
This steam drives one or more turbine generators, is condensed, and pumped back to the steam generator. The secondary loop is isolated from the reactor core water and, therefore, is not radioactive. A third stream of water from a lake, river, or cooling tower is used to condense the steam.
In the boiling water reactor (BWR) the water coolant is permitted to boil within the core. The steam is radioactive, there is no intermediate heat exchanger between the reactor and turbine to decrease efficiency.
The power level of an operating reactor is monitored by a variety of thermal, flow, and nuclear instruments. Power output is controlled by inserting or removing from the core a group of neutron-absorbing control rods. The position of these rods determines the power level at which the chain reaction is just self-sustaining.
In the initial period of nuclear power development in the early 1950s, enriched uranium was available only in the United States and the Union of Soviet Socialist Republics (USSR). This limitation led Canadian engineers to develop a reactor cooled and moderated by deuterium oxide (D2O), or heavy water.
Nuclear power plants similar to the PWR are used for the propulsion plants of large surface naval vessels such as the aircraft carriers and for submarines.
A variety of small nuclear reactors have been built in many countries for use in education and training, research, and the production of radioactive isotopes. These reactors generally operate at power levels near 1 MW, and are more easily started up and shut down than larger power reactors. A widely used type is called the swimming pool reactor.
The natural resources of uranium are small. That’s why special attention is paid to the breeder reactor. Its key feature is that it produces more fuel than it consumes. It does this by promoting the absorption of excess neutrons in a fertile material. When uranium-238 absorbs neutrons in the reactor, it is transmuted to a new fissionable material, plutonium, through a nuclear process called beta decay.
The breeder system that has had the greatest development effort is called the liquid metal fast breeder reactor (LMFBR).
Any electric power generating plant is only one part of a total energy cycle. Energy cycle include: underground mines, conversion plant, an isotope enrichment plant, a fuel fabrication plant, and reactor power plant.
A typical 1000-Mw pressurized water reactor has about 200 fuel elements, one-third of which are replaced each year because of the depletion of the uranium-235 and the buildup of fission products that absorb neutrons. The discharged fuel is placed in water storage pools at the reactor site for a year or more.
At the end of the cooling period the spent fuel elements are shipped in heavily shielded casks either to permanent storage facilities, or to a chemical reprocessing plant. The hazardous fuels used in nuclear reactors present handling problems in their use.
Radioactive materials emit penetrating, ionizing radiation that can injure living tissues. Radiological hazards can arise in most steps of the nuclear fuel cycle.
The release of nuclear energy can occur through the coalescence of two light nuclei into a heavier one. Nuclear fusion was first achieved on earth in the early 1930s by bombarding a target containing deuterium with high-energy deuterons in a cyclotron. In the typical fusion reaction the reacting nuclei both have a positive electric charge, and the natural repulsion between them, called Coulomb repulsion, must be overcome before they can join. This occurs when the temperature of the reacting gas is sufficiently high-50 to 100 million ° C. The substance with such temperature is in the state called a plasma.
A plasma hot enough for fusion cannot be contained by ordinary materials.
Numerous schemes for the magnetic confinement of plasma have been tried since 1950 in the United States, the former USSR, Great Britain, Japan, and elsewhere. One of them is the tokamak, originally suggested in the USSR by Igor Tamm and Andrey Sakharov.
The confinement chamber of a tokamak has the shape of a torus, with a minor diameter of about 1 m and a major diameter of about 3 m. A toroidal magnetic field of about 50,000 gauss is established inside this chamber by large electromagnets. A longitudinal current of several million amperes is induced in the plasma by the transformer coils that link the torus. The resulting magnetic field lines, spirals in the torus, stably confine the plasma.
Progress in fusion research has been promising, but the development of practical systems for creating a stable fusion reaction that produces more power than it consumes will probably take decades to realize. The research is expensive, as well.