Power Reactors

Type of physical science: Nuclear physics

Field of study: Nuclear reactors

A nuclear power reactor is an installation in which a nuclear chain reaction progresses under controlled conditions and the thermal energy released in the reaction is transformed into electrical energy. Nuclear fission reactors have been in use around the world since the 1950's, and in the early 1990's, they accounted for roughly a sixth of the world's total electrical energy production.

Overview

All substances are made of atoms. An atom consists of a nucleus and a number of negatively charged electrons that spin around the nucleus. The nucleus of an atom is a collection of positively charged protons and electrically neutral neutrons. Some atoms can exist with the same number of protons but different number of neutrons. These are the "isotopes" of that atom.

A substance that is composed entirely of atoms with the same number of protons (or electrons) is called an "element." Close to one hundred different types of elements occur naturally on Earth, and a number of others can be made artificially. The simplest of all elements is hydrogen, which has one proton in the nucleus and one electron. The heaviest element found in nature in abundance is uranium. Its atomic nucleus has 96 protons and 137, 139, or 142 neutrons, corresponding respectively to its three isotopes uranium 233, uranium 235, and uranium 238.

(The number appearing next to "uranium" is the atomic weight, which is the total number of protons and neutrons in the atom.) Protons and neutrons in the nucleus of an atom are held together by nuclear binding forces. Because of the large number of protons and neutrons in the nuclei of heavy atoms, the binding forces in such nuclei are not strong enough to hold the protons and neutrons together tightly. Therefore, addition of a neutron to the nucleus of a heavy atom makes the nucleus even heavier and more unstable and could cause it to split into two or more lighter atoms. This splitting of heavy nuclei is called "fission" and the new atoms produced are referred to as "fission products." In contrast to fission, in nuclear "fusion," nuclei of light elements such as hydrogen are combined (or fused) together to form heavier elements.

The common characteristic of fission and fusion is that both reactions are accompanied by energy release. In both reactions, the total mass of the products is slightly less than the total mass of the reacting materials. It is this mass difference that converts to energy and is released in the process. In principle, both fission and fusion can be used for large-scale energy production.

Commercial energy production by fission, using power reactors, has been in use for a number of decades. The fusion technology is more complex than fission technology, and the feasibility of controlled energy production by fusion is still in the experimental and research stage. All existing atomic power reactors therefore operate based on nuclear fission.

Among all the heavy elements and their isotopes, there are three that can be easily fissioned. These are uranium 233, uranium 235, and plutonium 239; they are referred to as "fissionable" or "fissile" materials. Uranium 235 is found in nature and the other two are manufactured by adding neutron to thorium 232 and uranium 238, respectively. These latter two materials, which become fissionable upon neutron absorption, are called "fertile." The nuclear energy industry mainly uses uranium 235 as the standard fuel for nuclear power reactors.

The basis on which a nuclear fission power reactor operates is a sustained and controlled "chain fission" process. A chain fission process is a sequence of fission reactions in which neutrons produced in one reaction cause subsequent fission reactions. For fission of an atom of uranium 235, one neutron is used and on the average roughly 2.5 neutrons are produced.

If all neutrons produced were to be captured by other uranium 235 nuclides and cause fission, the number of available neutrons would more than double for every generation of fission process, and the energy released would exponentially increase. In practice, however, some of the neutrons produced escape the region containing the fissionable materials and get captured by nonfissionable materials around the fission area. If the neutron loss becomes excessive, the fission reaction chain would cease to continue. In a reactor core, the neutron production and the fission rate are controlled in such a way that only sufficient neutrons become available for fission. Initiation and maintenance of a chain reaction in the core requires a certain minimum amount of fissionable materials; this is referred to as the "critical mass." In order to maintain a balanced rate of chain reactions, and to prevent a possible overheating during reactor operation, strong neutron absorbing materials, usually in the form of long rods, called "control rods," are pushed in and out of the core.

Another component in most nuclear power reactors is the "moderator." Neutrons released by fission move at very high speeds (on the average about one-tenth of the speed of light). The likelihood of neutron absorption by fissionable uranium 235 is low at such high neutron speeds. To enhance the likelihood of their capture by fissionable uranium 235, the velocities of the neutrons need to be reduced. This retarding of neutron velocities is achieved by incorporating a neutron moderating material in the reactor core.

A constituent common to all nuclear power reactors is the cooling system. The function of the cooling system is the effective removal of the heat generated by the fission process. The cooling system consists of the coolant and the cooling equipments (pipes, pumps, compressors, heat exchangers, and so forth). Coolant is the fluid (liquid or gas) that is circulated through the core in order to remove the heat released.

To generate electricity, the thermal energy removed from the reactor core by the coolant is taken to a turbogenerator. In some reactors, this is done directly (that is, in one loop).

In others, the thermal energy of the coolant is transferred to a second fluid in a heat exchanger.

The fluid in the second loop is then taken to the turbogenerator. After exiting the turbine, the fluid is cooled in a condenser and pumped back to the core or the heat exchanger, and this cycle of thermal energy removal from the primary system and electrical energy production in the secondary system is repeated.

In order to prevent corrosion of the fuel and the escape of radioactive materials from the fuel to the coolant, nuclear fuel is coated (cladded, or canned) with other materials, and is often manufactured in the form of "fuel rods." A number of such fuel rods are grouped together and are called "fuel assembly." Fuel in the form of fuel assembly is then placed inside the core.

In practice, as the chain reaction continues, more fission products are produced. The fission products absorb some of the neutrons without releasing any neutrons; in this way, they cause the chain reaction to slow down and even to die out. To prevent such phenomena, the spent fuel from an operating reactor is periodically replaced with fresh fuel.

An important issue concerning nuclear power reactors is the potential for radiation release. Different kinds of radiation take place inside the reactor core. These include α, β, γ, and neutrons. While certain types of radiation, such as &α; rays, can be easily blocked, others, particularly the γ rays, have an extremely strong penetrating power, and their confinement requires thick and strong barriers. The "reactor shield," usually a thick concrete wall with metal lining, intercepts such radiations. To isolate the nuclear activities in a reactor further and to reduce the potential environmental impacts in case of accidents, the primary system is often housed in a "containment building."

Applications

Based on the physical principles, power reactors with varied features have been developed and used for large-scale energy production worldwide since the 1950's. In the United States, research and development for civilian nuclear power technology started with the establishment of the Atomic Energy Commission in 1946. With the Atomic Energy Act of 1954, private participation in the development of nuclear power began. By 1959, three fission reactors with a total capacity of 350 megawatt-electricals were operating in the United States.

Commercial electrical power production using fission reactors in the United Kingdom and in the Soviet Union also began in the mid-1950's.

The choice of specific type of nuclear power reactor depends on a number of factors, the most important ones being safety and environmental issues, economic considerations, and operational characteristics. The most common categories of nuclear power reactors are thermal reactors, light-water reactors, boiling-water reactors, pressurized-water reactors, heavy-water reactors, and gas-cooled reactors.

Most commercial power reactors belong in the category of thermal reactors. The common feature of these reactors is that fission reaction in them is caused by thermal (or slowed) neutrons. Thermal reactors are therefore equipped with moderators. The majority of nuclear power reactors currently in use around the world are thermal reactors.

Often, in a power reactor, the coolant also functions as the moderator. Such is the case for the power reactors with ordinary water as coolant. In nuclear engineering technology, ordinary water is referred to as "light" water; reactors that use ordinary water for cooling and moderating are referred to as "light-water reactors," or LWRs. The vast majority of the thermal nuclear power reactors presently in use around the world are light-water reactors.

In some LWRs, the heated fluid from the core is taken directly to drive a turbogenerator. In this case, the reactor is called a boiling-water reactor (BWR). Alternatively, the fluid from the core can be taken to a heat exchanger to heat a fluid in the second loop (the secondary fluid), which then generates electrical power in the turbogenerator. In this case, to prevent the water from boiling in the core, and to produce a high-quality steam in the secondary loop, a high pressure is maintained in the core. This corresponds to a pressurized-water reactor (PWR).

The fuel technology for BWRs and PWRs is similar. They both use enriched uranium 235. Yet, the percentage of enrichment is slightly higher for PWRs than for BWRs (typically 2 to 3 percent for BWRs and 3 to 3.5 percent for PWRs).

Electrical power capacities of existing LWRs typically range from a few hundred to around 1,300 megawatt-electricals. A large BWR would typically have a core diameter of 5 meters and a core weight of 250 tons. The coolant temperature and coolant pressure in the core of a BWR would be on the order of 300 degrees Celsius and 7 megapascals, respectively (1 megapascal is approximately 10 atmospheres). The corresponding values for a PWR of comparable capacity are 3.5 meters, 120 tons, 300 degrees Celsius, and 15 megapascals.

About 80 percent of the nuclear power reactors now operating around the world are of LWR type. Of these, about two-thirds are pressurized water reactors. PWRs have also been widely used around the world for naval ships. The U.S. Navy uses this type of reactor exclusively in its nuclear ship program.

An alternative to using ordinary water as the moderator and coolant of a thermal reactor is to use "heavy" water for one or both of these purposes (heavy water is made of heavy hydrogen, also called deuterium, which has one neutron and one proton in its nucleus; ordinary water contains about 0.03 percent heavy water). Heavy water, while a good neutron moderator, absorbs fewer neutrons than ordinary water. Therefore, compared to LWRs, heavy-water reactors (HWRs) leave more neutrons to be absorbed by the fissionable material. As a result, such reactors can function with natural uranium as the fuel. The high moderating property of heavy water has another advantage: It allows keeping the dimensions of the HWRs down to smaller values compared to LWRs. Nevertheless, the lower fuel cost and the lower initial cost associated with HWRs is compensated by the high production cost of the heavy water.

One type of HWR is the so-called CANDU (Canadian deuterium uranium) reactor, which uses heavy water as the moderator and as the primary coolant. The secondary fluid in a CANDU reactor is ordinary water. Canadian nuclear power reactors are all of the CANDU type.

Another type of heavy-water reactor, a product of the British nuclear program, is the "steam generating heavy-water reactor" (SGHWR). This type of HWR uses heavy water as the moderator and ordinary water as the coolant. Steam is generated in the core and is taken directly to the turbogenerator. The power generation cycle of a SGHWR is therefore similar to a BWR.

Compared to a CANDU reactor, this system uses slightly enriched uranium for fuel. Certain gases have also been used as coolant in nuclear power reactors. Among them are helium and carbon dioxide. To have slow neutrons in the core, there is still the need for moderators in such gas-cooled reactors (GCRs). Graphite (carbon) is often used as the structural material and the moderator in GCRs. One advantage of gas-cooled reactors is that they do not have the corrosion problems within the core that the liquid coolant reactors have. The main disadvantage of GCRs is their high capital cost. The electrical energy production in these reactors can be accomplished either in one cycle, by taking the heated gas to a gas turbine, or in two cycles, by first transferring the thermal energy of the gas to a fluid in a heat exchanger and then taking the secondary fluid to a turbine. The majority of existing gas-cooled nuclear power reactors are of the latter type, with water as the secondary fluid. GCRs are especially popular in Great Britain, where they constitute the majority of nuclear power reactors in operation.

Another category of nuclear power reactors uses graphite as the moderator and ordinary water as the coolant. Reactors in this category are usually referred to as light-water graphite-moderated reactors (LGRs).

The water-cooled reactors and the gas-cooled thermal reactors discussed above are "burners"; that is, they consume fissionable fuel and produce very little fissionable material. In the early years of nuclear energy expansion, the expectation that the fast growth in the use of nuclear power would result in the depletion of natural fissionable materials prompted the development of another class of nuclear reactors called "breeder" reactors. The principal distinction between breeder reactors and thermal reactors is that the former do not have moderators. As a result, they operate with fast neutrons; and hence they are called fast-breeder reactors (FBRs). Such reactors convert the fertile materials into fissionable materials while producing electrical energy.

According to 1991 data, in all there were 412 nuclear power reactors (30 megawatt-electricals and larger) operational worldwide. Electrical power production by these reactors accounted for roughly 15 percent of the total electrical energy. In the United States, there were 109 nuclear power reactors in operation, supplying close to 20 percent of the nation's total electrical energy consumption. Of the total power reactors operational worldwide, 233 were PWRs, 87 BWRs, 39 GCRs, 29 HWRs, 20 LGRs, and 4 FBRs.

Context

With the discovery of the neutron by James Chadwick in England in 1932, speculation and research concerning the possibility of nuclear fission by neutron bombardment of atomic nuclei began. The first proof of fission came about by an experiment by Otto Hahn and Fritz Strassmann in Germany in 1939. Energy production using nuclear fission, however, remained a matter of academic interest for many years, because, though energy was produced by this method in laboratory experiments, even more energy usually had to be used to perform the fission reaction. It was just before World War II when it was realized that, in appropriate conditions, if atoms of uranium were bombarded with neutrons, the fission process might be made self-sustaining. The first achievement of a self-sustaining fission reaction process was made by Enrico Fermi and his coworkers in the United States, in 1942. This opened the era of atomic energy development.

The notion of atomic energy being a clean, inexpensive, and abundant source of energy, combined with the extensive application of nuclear technology in the weapons industry, helped the fission technology develop rapidly in its early years. Energy production nuclear fission has been used since the 1960's; its once rapid expansion has come to a halt, however, since the late 1980's. The decline in the use of nuclear fission as a source of energy should be mainly attributed to the safety and environmental issues that have been haunting the nuclear industry since its birth. The main issue continues to be the potential and consequences of accidents during the operation of reactors, which would result in the escape of radioactive materials into the environment. Such accidents could be caused by either internal disturbances (such as equipment failure and human error) or external disturbances (such as a destructive earthquake) to the system. Another adverse effect is the escape of radioactive materials to the environment by less visible ways of fluid leakage, equipment contamination, and fuel handling and fuel processing activities. The next major issue, and probably the most controversial one, is concerned with the nuclear waste materials. These materials, which include the spent fuels from power reactors, are toxic and highly radioactive, and their safe disposal requires their complete isolation from the living environment for tens of thousands of years.

Despite extensive quality control during design, construction, and operation of nuclear plants, which has always been an integral part of the atomic energy industry, a number of accidents have been reported so far. Among the most severe were at Three Mile Island in Pennsylvania and Chernobyl in the Soviet Union. Such accidents, combined with the discovery of an active geologic fault in the vicinity of the Diablo Canyon reactors in California, and especially the growing problems with the existing nuclear wastes (such as in Hanford, Washington), have contributed to making the future of nuclear fission as a source of energy a questionable one. A major worldwide energy shortage could bring nuclear fission as a source of energy into the picture once again. Unfortunately, the existing negative public opinion concerning nuclear energy would likely continue, at least until a satisfactory solution to the problem of existing nuclear wastes is found.

The concept of energy production by fusion is more attractive, mainly because of its less adverse environmental impacts. Yet, despite the many years of research and experiment, there is no indication at this time that commercial energy production by fusion will materialize in the twentieth century.

Principal terms

CONTROL SYSTEM: the system used for dealing with operation abnormalities in the primary system; used for regulation and shutdown; includes control rods, shutdown rods, and an emergency cooling system

FUEL ENRICHMENT: the process by which the concentration of fissionable material in the fuel is increased

MEGAWATT-ELECTRICAL: the most common unit of electrical power used for nuclear power reactors; equals 1 million watts

NATURAL URANIUM: uranium as is found in nature; consists of 0.7 percent fissionable uranium and 99.3 percent nonfissionable uranium

PRIMARY SYSTEM: the portion of the nuclear plant in which the nuclear reactions take place and thermal energy is produced; also called nuclear steam supply system

REACTOR CORE: the region in the reactor that contains the nuclear fuel

SECONDARY SYSTEM: the portion of the nuclear plant in which the conversion from thermal energy to electrical energy takes place

Bibliography

Belchem, R. F. K. A GUIDE TO NUCLEAR ENERGY. New York: Philosophical Library, 1957. An excellent book which gives, in a concise manner, an introduction to the theory of nuclear reactions and the way in which nuclear reactors function. A splendid book for the general reader.

Fermi, Laura. ATOMS FOR THE WORLD. Chicago: University of Chicago Press, 1957. This book offers a detailed account of the early progress in the science of atomic energy and the U.S. plans and participations in peaceful uses of atomic energy.

Marples, D. R. THE SOCIAL IMPACT OF THE CHERNOBYL DISASTER. New York: St. Martin's Press, 1980. This book is a comprehensive account of the Chernobyl accident. The theme of the book is the need for proper operation and safety of nuclear power reactors.

Nervo, A. V., Jr. A GUIDEBOOK TO NUCLEAR REACTORS. Berkeley: University of California Press, 1979. This excellent book is a presentation of the basic concepts of nuclear reactors in a simple language. Covers different types of nuclear reactors, including the advanced reactors, or the reactors of the future, which were still in research and development stage in the 1990's. Some of the underlying physical principles are also discussed.

Rahn, Frank J., A. G. Adamantiades, J. E. Kenton, and C. Braun. A GUIDE TO NUCLEAR POWER TECHNOLOGY. New York: John Wiley & Sons, 1984. This book describes the various aspects of nuclear power technology. It is remarkably comprehensive and detailed. No special background on the subject is needed.

United States. President's Commission on the Accident at Three Mile Island. THE NEED FOR CHANGE: REPORT OF THE PRESIDENT'S COMMISSION ON THE ACCIDENT AT THREE MILE ISLAND. New York: Pergamon Press, 1979. The book is a summary of the findings of the commission concerning the causes and the nature of the accident and its implications.

Zinn, W. H., F. K. Pittman, and J. F. Hogerton. NUCLEAR POWER, U.S.A. New York: McGraw-Hill, 1964. This book is a survey of U.S. progress in power generation using nuclear power from its beginning until the mid-1960's. The simple language of the book and the many excellent photographs make it especially interesting to the general reader. In particular, the book gives a photographic tour of the early U.S. nuclear power industry.

The Structure of the Atomic Nucleus

Electrons and Atoms

Fission and Thermonuclear Weapons

Nuclear Reactors: Design and Operation

The Periodic Table and the Atomic Shell Model

Reactor Fuels and Waste Products