When nuclear reactions were first discovered in the 1930s, many scientists doubted they would ever have any practical application. But the successful initiation of the first controlled reaction at the University of Chicago in 1942 quickly changed their views.
In the first controlled nuclear reaction, scientists discovered a source of energy greater than anyone had previously imagined possible. They discovered that the nuclei of uranium (U) isotopes could be split, thus releasing tremendous energy. The reaction occurred when the nuclei of certain isotopes of uranium were struck and split by neutrons. This is now known as nuclear fission, and the fission reaction results in the formation of three types of products: energy, neutrons, and smaller nuclei about half the size of the original uranium nucleus.
Neutrons are actually produced in a fission reaction, and this fact is critical for energy production. The release of neutrons in a fission reaction means that the particles required to initiate fission are also a product of the reaction. Once initiated in a block of uranium, fission occurs over and over again, in a chain reaction. Calculations done during these early discoveries showed that the amount of energy released in each fission reaction is many times greater than that released by the chemical reactions that occur during a conventional chemical explosion.
The possibility to release such high energies with nuclear reactions was used in the development of the atomic bomb. After the dropping of this bomb brought World War II to an end, scientists began researching the harnessing of nuclear energy for other applications, primarily the generation of electricity. In developing the first nuclear weapons, scientists only needed to find a way to initiate nuclear fission--there was no need to control it once it had begun. In developing the peacetime application of nuclear power, however, the primary challenge was to develop a mechanism for keeping the reaction under control once it had begun so that the energy released could be managed and used. This is the main purpose of nuclear power plants--controlling and converting the energy produced by nuclear reactions.
There are many types of nuclear power plants, but all plants have a reactor core and every core consists of three elements. First, the fuel rods; these are long, narrow, cylindrical tubes that hold small pellets of some fissionable material. At present only two such materials are in practical use, uranium-235 and plutonium-239 (Pu-239). The uranium used for nuclear fission is known as enriched uranium, because it is actually a mixture of uranium-235 with uranium-238. Uranium-238 is not fissile and the required chain reaction will not occur if the fraction of uranium-235 present is not at least 3 percent.
The second component of a reactor core is the moderator. Only slow-moving neutrons are capable of initiating nuclear fission, but the neutrons produced as a result of nuclear fission are fast moving. These neutrons move too fast to initiate other reactions, thus moderators are used to slow them down. Two of the most common moderators are graphite (pure carbon) and water.
The third component of a reactor core are the control rods. In operating a nuclear power plant safely and efficiently, it is of the utmost importance to have exactly the right amount of neutrons in the reactor core. If there are too few, the chain reaction comes to an end and energy ceases to be produced. If there are too many, fission occurs too quickly, too much energy is released all at once, and the rate of reaction increases until it can no longer be controlled or contained. Control rods decrease the number of neutrons in the core because they are made of a material that has a strong tendency to absorb neutrons. Cadmium (Cd) and boron (B) are materials that are both commonly used. The rods are mounted on pulleys, allowing them to be raised or lowered into the reactor core as need may be. When the rods are fully inserted, most of the neutrons in the core are absorbed and relatively few are available to initiate a chain reaction. As the rods are withdrawn from the core, more and more neutrons are available to initiate fission reactions. The reactions reach a point where the number of neutrons produced in the core are almost exactly equal to the number being used to start fission reactions, and it is then that a controlled chain reaction occurs.
The heat energy produced in a reactor core is used to boil water and make steam, which is then used to operate a turbine and generate electricity. The various types of nuclear power plants differ primarily in the way in which heat from the core is used to do this. The most direct approach is to surround the core with a huge tank of water, some of which can be boiled directly by heat from the core. One problem with boiling-water reactors is that the steam produced can be contaminated with radioactive materials. Special precautions must be taken with these reactors to prevent contaminated steam from being released into the environment. A second type of nuclear reactor makes use of a heat exchanger. Water around the reactor core is heated and pumped to a heat exchange unit, where this water is used to boil water in an external system. The steam produced in this exchange is then used to operate the turbine and generator.
There is also a type of nuclear reactor known as a breeder, or fast-spectrum reactor, because it not only produces energy but also generates more fuel in the form of plutonium-239. In conventional reactors, water is used as a coolant as well as a moderator, but in breeder reactors the coolant used is sodium (Na). Neutrons have to be moving quickly to produce plutonium, and sodium does not moderate their speed as much as water does. Another design, the Canada deuterium uranium reactor (CANDU), uses deuterium oxide (D2O, which is heavy water) as moderator and natural uranium as fuel. With the uranium fuel surrounded by heavy water, chain reaction fission takes place, releasing energy in the form of heat. The heat is transferred to a second heavy water system pumped at high pressure through the tubes to steam generators, from which the heat is transferred to ordinary water that boils to become the steam that drives the turbine generator.
Nuclear power plants could never explode with the power of an atomic bomb, because the quantity of uranium-235 required is never present in the reactor core. However, they do pose a number of well-known safety hazards. From the very beginning of the development of nuclear reactors, safety was an important consideration as scientists and engineers tried to anticipate the dangers associated with nuclear reactions and radioactive materials. Thus, control rods were developed to prevent the fission reactions from generating too much heat. The reactor and its cooling system are always enclosed in a containment shell made of thick sheets of steel to prevent the escape of radioactive materials. Nuclear power plants are highly complex facilities, with backup systems for increased safety, which are in turn supported by other backup systems. But the components of these systems age, and human errors can and do occur; safety measures do not always function the way they were designed.
On December 2, 1957, the first nuclear power plant opened in Shippingport, Pennsylvania, and to many, the nuclear age seemed to have begun. Over the next two decades, more than fifty plants were commissioned, with dozens more ordered. But safety problems plagued the industry. An experimental reactor in Idaho Falls, Idaho, had already experienced a partial meltdown as a result of operator error in 1955. In October 1957, just months before the Shippingport plant came on line, a production reactor near Liverpool, England, caught fire, releasing radiation over Great Britain and northern Europe.
The most critical event in the history of nuclear power in the United States was the accident at the Three Mile Island nuclear reactor near Harrisburg, Pennsylvania. In March 1979, fission reactions in the reactor core went out of control, generating huge amounts of heat, and a meltdown resulted. Fuel rods and the control rods were melted; the cooling water was turned to steam and the containment structure itself was threatened but held. Until 2012, no new plants were ordered in the United States, and sixty-five plants on order at that time were cancelled. The explosion at the Chernobyl reactor near Kiev, Ukraine, dealt a second, more global, blow to the industry. After Japan's 2011 Fukushima nuclear accident caused by the Tohoku earthquake and subsequent tsunami, several nations reevaluated their nuclear power industries, taking reactors temporarily offline for inspections or technological upgrades. In May 2012, Japan shut down its last operating reactor for routine maintenance. The shutdown of the final reactor meant that the nation was without operating nuclear power facilities for the first time since the 1970s. The period was temporary, however: The first reactors cleared for restart resumed operations the following month. Germany announced in May 2011 that it was permanently closing all of its reactors then offline for maintenance and ceasing all nuclear power operations in the country by 2022.
Even without these accidents, another problem with nuclear power would remain. This is the problem of spent radioactive wastes. About one-third of the 10 million fuel pellets used in any reactor core must be removed each year because they have been so contaminated with fission by-products that they no longer function efficiently. These highly radioactive pellets must be disposed of in a safe manner, but almost seventy years after the first controlled reaction, no method has yet been discovered to address this issue. These wastes are most commonly stored on a temporary basis at or near the power plant itself. Many have argued that further development of nuclear power should not even be considered until better methods for radioactive waste management have been developed.
A federal repository for nuclear power radioactive wastes and high-level defense waste was proposed in Yucca Mountain, Nevada. The site was approved by Congress and then-President George W. Bush (1946-) in 2002. In 2008, the U.S. Department of Energy (DOE) submitted an application to the Nuclear Regulatory Commission (NRC) for approval to initiate construction. The application is under review by the NRC; however, in 2009, U.S. Secretary of Energy Steven Chu (1948-) reported to the Senate that the Yucca Mountain site was no longer being considered for radioactive waste storage.
The International Nuclear Safety Center (INSC) has the mission of improving nuclear power reactor safety worldwide. The INSC is dedicated to developing improved nuclear safety technology and promoting the open exchange of nuclear safety information among nations, sponsoring scientific research activities as collaborations between the United States and its international partners. Safety issues are addressed at several levels, including the following: risk assessment, containment, structural integrity of reactors, assessment of their seismic reliability, equipment operability, fire protection, and reactor safeguards.
The security of nuclear facilities has also been a point of concern. In 1991, the NRC instituted an operational safeguards response evaluation (OSRE) program, which evaluated the ability of nuclear facility security personnel to withstand a staged commando-style attack by intruders. Unfortunately, six of the eleven evaluations performed in 2000 and 2001 resulted in the attackers being able to penetrate security and simulate damage to reactor equipment.
In the present terrorist attack environment, the vulnerability of America's 96 commercial nuclear power reactors at 58 facilities are a critical national security concern. All nuclear facilities were placed on high alert immediately following the September 11, 2001 attacks, and in early 2002, the NRC issued interim confidential security orders for all licensees to comply with. In addition, decommissioned nuclear plants and spent fuel storage facilities were also required to implement the security orders. The NRC also conducted a thorough review of its internet website, taking the site offline temporarily to analyze content and remove all documents deemed sensitive to national security. In April 2002, the NRC announced the establishment of a dedicated department for plant security, the Office of Nuclear Security and Incident Response.
In 2018, nuclear power plants provided 19.4 percent of electricity in the United States. Comparatively, France obtained about 75 percent of its electricity from nuclear power. In 2005, Congress passed the Energy Policy Act and promoted the development of clean (low or no greenhouse gas emissions) technologies for nuclear power generation based on the Generation IV Nuclear Energy Systems Initiative. The two technologies that have possibilities are the very-high temperature reactors (VHTRs) and the sodium-cooled fast reactors (SFRs). Scientists are pursuing VHTR technology as a priority for the U.S. Next Generation Nuclear Plant (NGNP) because of its potential for advanced electricity and hydrogen production, high temperature process heat applications, and safe integration into industry without greenhouse gas emissions.
In a show of support for the development of nuclear power, President Obama (1961-) pledged to more than triple the proposed level of federal government loan guarantees from levels specified in the Energy Policy Act of 2005. Obama and nuclear power proponents claimed that nuclear power must be a part of the development of clean energy technologies. Critics in the environmental advocacy community countered that nuclear power, with its waste disposal controversies and perils, should not be considered clean technology.
In 2012, the NRC approved the construction of two new nuclear reactors at Plant Vogtle in Waynesboro, Georgia. The reactors, which were preceded by two units completed in 1987 and 1989, marked the first new nuclear construction projects in the United States in more than thirty years. They are scheduled for completion in 2021.