I –INTRODUCTION
Nuclear Energy, energy released during the splitting or
fusing of atomic nuclei. The energy of any system, whether physical, chemical,
or nuclear, is manifested by the system’s 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.
Until about 1800 the principal fuel was wood, its energy
derived from solar energy stored in plants during their lifetimes. Since the
Industrial Revolution, people have depended on fossil fuels—coal, petroleum,
and natural gas—also derived from stored solar energy. When a fossil fuel such
as coal is burned, atoms of hydrogen and carbon in the coal combine with oxygen
atoms in air. Water and carbon dioxide are produced and heat is released,
equivalent to about 1.6 kilowatt-hours per kilogram or about 10 electron volts
(eV) per atom of carbon. This amount of energy is typical of chemical reactions
resulting from changes in the electronic structure of the atoms. A part of the
energy released as heat keeps the adjacent fuel hot enough to keep the reaction
going.
II -THE ATOM
The atom consists of a small, massive, positively charged
core (nucleus) surrounded by electrons (see Atom). 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 protons and neutrons, it contains; the atomic number Z
is the number of positively charged protons. A specific nucleus is designated
as ¿U the expression ¯U, for example, represents uranium-235. See Isotope.
The binding energy of a nucleus is a measure of how tightly
its protons and neutrons are held together by the nuclear forces. The binding
energy per nucleon, the energy required to remove one neutron or proton from a
nucleus, is a function of the mass number A. The curve of binding energy
implies that if two light nuclei near the left end of the curve coalesce to
form a heavier nucleus, or if a heavy nucleus at the far right splits into two
lighter ones, more tightly bound nuclei result, and energy will be released.
Nuclear energy, measured in millions of electron volts (MeV), is released by
the fusion of two light nuclei, as when two heavy hydrogen nuclei, deuterons
(ªH), combine in the reaction producing a helium-3 atom, a free neutron (¦n),
and 3.2 MeV, or 5.1 × 10-13 J (1.2 × 10-13 cal). Nuclear energy is also
released when the fission of a heavy nucleus such as ¯U is induced by the
absorption of a neutron as in producing cesium-140, rubidium-93, three
neutrons, and 200 MeV, or 3.2 × 10-11 J (7.7 × 10-12 cal). A nuclear fission
reaction releases 10 million times as much energy as is released in a typical
chemical reaction. See Nuclear Chemistry.
III -NUCLEAR ENERGY FROM FISSION
The two key characteristics of nuclear fission important for
the practical release of nuclear energy are both evident in equation (2).
First, the energy per fission is very large. In practical units, the fission of
1 kg (2.2 lb) of uranium-235 releases 18.7 million kilowatt-hours as heat.
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 in this manner quickly 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, which results
in continuous release of nuclear energy.
Naturally occurring uranium contains only 0.71 percent
uranium-235; the remainder is the nonfissile isotope uranium-238. A mass of
natural uranium by itself, no matter how large, cannot sustain a chain reaction
because only the uranium-235 is easily fissionable. The probability that a
fission neutron with an initial energy of about 1 MeV will induce fission is
rather low, but the probability can be increased by a factor of hundreds when
the neutron is slowed down through a series of elastic collisions with light
nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for the
design of practical energy-producing fission reactors.
In December 1942 at the University of Chicago, the Italian
physicist Enrico Fermi succeeded in producing the first nuclear chain reaction.
This was done with an arrangement of natural uranium lumps distributed within a
large stack of pure graphite, a form of carbon. In Fermi's “pile,” or nuclear
reactor, the graphite moderator served to slow the neutrons.
IV -NUCLEAR POWER REACTORS
The first large-scale nuclear reactors were built in 1944 at
Hanford, Washington, for the production of nuclear weapons material. The fuel
was natural uranium metal; the moderator, graphite. Plutonium was produced in
these plants by neutron absorption in uranium-238; the power produced was not
used.
A -Light-Water and Heavy-Water Reactors
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 three 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 (about
620° F). 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 is
pumped back to the steam generator. The secondary loop is isolated from the
water in the reactor core and, therefore, is not radioactive. A third stream of
water from a lake, river, or cooling tower is used to condense the steam. The
reactor pressure vessel is about 15 m (about 49 ft) high and 5 m (about 16.4
ft) in diameter, with walls 25 cm (about 10 in) thick. The core houses some 82
metric tons of uranium oxide contained in thin corrosion-resistant tubes clustered
into fuel bundles.
In the boiling-water reactor (BWR), a second type of LWR,
the water coolant is permitted to boil within the core, by operating at
somewhat lower pressure. The steam produced in the reactor pressure vessel is
piped directly to the turbine generator, is condensed, and is then pumped back
to the reactor. Although the steam is radioactive, there is no intermediate
heat exchanger between the reactor and turbine to decrease efficiency. As in
the PWR, the condenser cooling water has a separate source, such as a lake or
river. 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. During operation, and even after shutdown, a large,
1,000-megawatt (MW) power reactor contains billions of curies of radioactivity.
Radiation emitted from the reactor during operation and from the fission
products after shutdown is absorbed in thick concrete shields around the
reactor and primary coolant system. Other safety features include emergency
core cooling systems to prevent core overheating in the event of malfunction of
the main coolant systems and, in most countries, a large steel and concrete
containment building to retain any radioactive elements that might escape in
the event of a leak.
Although more than 100 nuclear power plants were operating
or being built in the United States at the beginning of the 1980s, in the
aftermath of the Three Mile Island accident in Pennsylvania in 1979 safety
concerns and economic factors combined to block any additional growth in
nuclear power. No orders for nuclear plants have been placed in the United
States since 1978, and some plants that have been completed have not been
allowed to operate. In 1996 about 22 percent of the electric power generated in
the United States came from nuclear power plants. In contrast, in France almost
three-quarters of the electricity generated was from nuclear power plants.
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). The nuclear power programs in
Canada, France, and the United Kingdom therefore centered about natural uranium
reactors, in which ordinary water cannot be used as the moderator because it
absorbs too many neutrons. This limitation led Canadian engineers to develop a
reactor cooled and moderated by deuterium oxide (D2O), or heavy water. The
Canadian deuterium-uranium reactor known as CANDU has operated satisfactorily
in Canada, and similar plants have been built in India, Argentina, and
elsewhere.
In the United Kingdom and France the first full-scale power
reactors were fueled with natural uranium metal, were graphite-moderated, and
were cooled with carbon dioxide gas under pressure. These initial designs have
been superseded in the United Kingdom by a system that uses enriched uranium
fuel. In France the initial reactor type chosen was dropped in favor of the PWR
of U.S. design when enriched uranium became available from French
isotope-enrichment plants. Russia and the other successor states of the USSR
had a large nuclear power program, using both graphite-moderated and PWR
systems.
B -Propulsion Reactors
Nuclear power plants similar to the PWR are used for the
propulsion plants of large surface naval vessels such as the aircraft carrier
USS Nimitz. The basic technology of the PWR system was first developed in the
U.S. naval reactor program directed by Admiral Hyman G. Rickover. Reactors for
submarine propulsion are generally physically smaller and use more highly
enriched uranium to permit a compact core. The United States, the United
Kingdom, Russia, and France all have nuclear-powered submarines with such power
plants.
Three experimental seagoing nuclear cargo ships were
operated for limited periods by the United States, Germany, and Japan. Although
they were technically successful, economic conditions and restrictive port
regulations brought an end to these projects. The Soviet government built the
first successful nuclear-powered icebreaker, Lenin, for use in clearing the
Arctic sea-lanes.
C -Research Reactors
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 one
MW, and they are more easily started up and shut down than larger power
reactors.
A widely used type is called the swimming-pool reactor. The
core is partially or fully enriched uranium-235 contained in aluminum alloy
plates, immersed in a large pool of water that serves as both coolant and
moderator. Materials may be placed directly in or near the reactor core to be
irradiated with neutrons. Various radioactive isotopes can be produced for use
in medicine, research, and industry (see Isotopic Tracer). Neutrons may also be
extracted from the reactor core by means of beam tubes to be used for
experimentation.
D -Breeder Reactors
Uranium, the natural resource on which nuclear power is
based, occurs in scattered deposits throughout the world. Its total supply is
not fully known, and may be limited unless sources of very low concentration
such as granites and shale were to be used. Conservatively estimated U.S. resources
of uranium having an acceptable cost lie in the range of two million to five
million metric tons. The lower amount could support an LWR nuclear power system
providing about 30 percent of U.S. electric power for only about 50 years. The
principal reason for this relatively brief life span of the LWR nuclear power
system is its very low efficiency in the use of uranium: only approximately one
percent of the energy content of the uranium is made available in this system.
The key feature of a breeder reactor is that it produces more fuel than it
consumes. It does this by promoting the absorption of excess neutrons in a
fertile material. Several breeder reactor systems are technically feasible. The
breeder system that has received the greatest worldwide attention uses
uranium-238 as the fertile material. When uranium-238 absorbs neutrons in the
reactor, it is transmuted to a new fissionable material, plutonium, through a
nuclear process called β (beta) decay. The sequence of nuclear reactions is In
beta decay a nuclear neutron decays into a proton and a beta particle (a high-energy
electron).
When plutonium-239 itself absorbs a neutron, fission can
occur, and on the average about 2.8 neutrons are released. In an operating
reactor, one of these neutrons is needed to cause the next fission and keep the
chain reaction going. On the average about 0.5 neutron is uselessly lost by
absorption in the reactor structure or coolant. The remaining 1.3 neutrons can
be absorbed in uranium-238 to produce more plutonium via the reactions in
equation (3).
The breeder system that has had the greatest development
effort is called the liquid-metal fast breeder reactor (LMFBR). In order to
maximize the production of plutonium-239, the velocity of the neutrons causing
fission must remain fast—at or near their initial release energy. Any
moderating materials, such as water, that might slow the neutrons must be
excluded from the reactor. A molten metal, liquid sodium, is the preferred
coolant liquid. Sodium has very good heat transfer properties, melts at about
100° C (about 212° F), and does not boil until about 900° C (about 1650° F).
Its main drawbacks are its chemical reactivity with air and water and the high
level of radioactivity induced in it in the reactor.
Development of the LMFBR system began in the United States
before 1950, with the construction of the first experimental breeder reactor,
EBR-1. A larger U.S. program, on the Clinch River, was halted in 1983, and only
experimental work was to continue (see Tennessee Valley Authority). In the
United Kingdom, France, and Russia and the other successor states of the USSR,
working breeder reactors were installed, and experimental work continued in
Germany and Japan.
In one design of a large LMFBR power plant, the core of the
reactor consists of thousands of thin stainless steel tubes containing mixed uranium
and plutonium oxide fuel: about 15 to 20 percent plutonium-239, the remainder
uranium. Surrounding the core is a region called the breeder blanket, which
contains similar rods filled only with uranium oxide. The entire core and
blanket assembly measures about 3 m (about 10 ft) high by about 5 m (about 16.4
ft) in diameter and is supported in a large vessel containing molten sodium
that leaves the reactor at about 500° C (about 930° F). This vessel also
contains the pumps and heat exchangers that aid in removing heat from the core.
Steam is produced in a second sodium loop, separated from the radioactive reactor
coolant loop by the intermediate heat exchangers in the reactor vessel. The
entire nuclear reactor system is housed in a large steel and concrete
containment building.
The first large-scale plant of this type for the generation
of electricity, called Super-Phénix, went into operation in France in 1984.
(However, concerns about operational safety and environmental contamination led
the French government to announce in 1998 that Super-Phénix would be
dismantled). An intermediate-scale plant, the BN-600, was built on the shore of
the Caspian Sea for the production of power and the desalination of water. The
British have a large 250-MW prototype in Scotland.
The LMFBR produces about 20 percent more fuel than it
consumes. In a large power reactor enough excess new fuel is produced over 20
years to permit the loading of another similar reactor. In the LMFBR system
about 75 percent of the energy content of natural uranium is made available, in
contrast to the one percent in the LWR.
V -NUCLEAR FUELS AND WASTES
The hazardous fuels used in nuclear reactors present
handling problems in their use. This is particularly true of the spent fuels,
which must be stored or disposed of in some way. A -The Nuclear Fuel Cycle
Any electric power generating plant is only one part of a
total energy cycle. The uranium fuel cycle that is employed for LWR systems
currently dominates worldwide nuclear power production and includes many steps.
Uranium, which contains about 0.7 percent uranium-235, is obtained from either
surface or underground mines. The ore is concentrated by milling and then
shipped to a conversion plant, where its elemental form is changed to uranium
hexafluoride gas (UF6). At an isotope enrichment plant, the gas is forced
against a porous barrier that permits the lighter uranium-235 to penetrate more
readily than uranium-238. This process enriches uranium to about 3 percent
uranium-235. The depleted uranium—the tailings—contain about 0.3 percent
uranium-235. The enriched product is sent to a fuel fabrication plant, where
the UF6 gas is converted to uranium oxide powder, then into ceramic pellets
that are loaded into corrosion-resistant fuel rods. These are assembled into
fuel elements and are shipped to the reactor power plant. The world’s supply of
enriched uranium fuel for powering commercial nuclear power plants is produced
by five consortiums located in the United States, Western Europe, Russia, and
Japan. The United States consortium—the federally owned United States
Enrichment Corporation—produces 40 percent of this enriched uranium.
A typical 1,000-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. At the end of its life in the reactor, the fuel is tremendously
radioactive because of the fission products it contains and hence is still
producing a considerable amount of energy. 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. At a reprocessing plant, the unused uranium and
the plutonium-239 produced in the reactor are recovered and the radioactive
wastes concentrated. (In the late 1990s neither such facility was yet available
in the United States for power plant fuel, and temporary storage was used.)
The spent fuel still contains almost all the original
uranium-238, about one-third of the uranium-235, and some of the plutonium-239
produced in the reactor. In cases where the spent fuel is sent to permanent
storage, none of this potential energy content is used. In cases where the fuel
is reprocessed, the uranium is recycled through the diffusion plant, and the
recovered plutonium-239 may be used in place of some uranium-235 in new fuel
elements. At the end of the 20th century, no reprocessing of fuel occurred in
the United States because of environmental, health, and safety concerns, and
the concern that plutonium-239 could be used illegally for the manufacture of
weapons.
In the fuel cycle for the LMFBR, plutonium bred in the
reactor is always recycled for use in new fuel. The feed to the fuel-element
fabrication plant consists of recycled uranium-238, depleted uranium from the
isotope separation plant stockpile, and part of the recovered plutonium-239. No
additional uranium needs to be mined, as the existing stockpile could support
many breeder reactors for centuries. Because the breeder produces more
plutonium-239 than it requires for its own refueling, about 20 percent of the
recovered plutonium is stored for later use in starting up new breeders.
Because new fuel is bred from the uranium-238, instead of using only the
natural uranium-235 content, about 75 percent of the potential energy of
uranium is made available with the breeder cycle.
The final step in any of the fuel cycles is the long-term
storage of the highly radioactive wastes, which remain biologically hazardous
for thousands of years. Fuel elements may be stored in shielded, guarded
repositories for later disposition or may be converted to very stable
compounds, fixed in ceramics or glass, encapsulated in stainless steel
canisters, and buried far underground in very stable geologic formations.
However, the safety of such repositories is the subject of
public controversy, especially in the geographic region in which the repository
is located or is proposed to be built. For example, environmentalists plan to
file a lawsuit to close a repository built near Carlsbad, New Mexico. In 1999,
this repository began receiving shipments of radioactive waste from the
manufacture of nuclear weapons in United States during the Cold War. Another
controversy centers around a proposed repository at Yucca Mountain, Nevada.
Opposition from state residents and questions about the geologic stability of
this site have helped prolong government studies. Even if opened, the site will
not receive shipments of radioactive waste until at least 2010 (see Nuclear
Fuels and Wastes, Waste Management section below).
B -Nuclear Safety
Public concern about the acceptability of nuclear power from
fission arises from two basic features of the system. The first is the high
level of radioactivity present at various stages of the nuclear cycle,
including disposal. The second is the fact that the nuclear fuels uranium-235
and plutonium-239 are the materials from which nuclear weapons are made. See
Nuclear Weapons; Radioactive Fallout.
U.S. President Dwight D. Eisenhower announced the U.S. Atoms
for Peace program in 1953. It was perceived as offering a future of cheap,
plentiful energy. The utility industry hoped that nuclear power would replace
increasingly scarce fossil fuels and lower the cost of electricity. Groups
concerned with conserving natural resources foresaw a reduction in air
pollution and strip mining. The public in general looked favorably on this new
energy source, seeing the program as a realization of hopes for the transition
of nuclear power from wartime to peaceful uses.
Nevertheless, after this initial euphoria, reservations
about nuclear energy grew as greater scrutiny was given to issues of nuclear
safety and weapons proliferation. In the United States and other countries many
groups oppose nuclear power. In addition, high construction costs, strict
building and operating regulations, and high costs for waste disposal make
nuclear power plants much more expensive to build and operate than plants that
burn fossil fuels. In some industrialized countries, the nuclear power industry
has come under growing pressure to cut operating expenses and become more
cost-competitive. Other countries have begun or planned to phase out
nuclear power
completely.
At the end of the 20th century, many experts viewed Asia as
the only possible growth area for nuclear power. In the late 1990s, China,
Japan, South Korea, and Taiwan had nuclear power plants under construction.
However, many European nations were reducing or reversing their commitments to
nuclear power. For example, Sweden committed to phasing out nuclear power by
2010. France canceled several planned reactors and was considering the
replacement of aging nuclear plants with environmentally safer fossil-fuel
plants. Germany announced plans in 1998 to phase out nuclear energy. In the
United States, no new reactors had been ordered since 1978. In 1996, 21.9
percent of the electricity generated in the United States was produced by
nuclear power. By 1998 that amount had decreased to 20 percent. Because no
orders for nuclear plants have been placed since 1978, this share should
continue to decline as existing nuclear plants are eventually closed. In 1998
Commonwealth Edison, the largest private owner and operator of nuclear plants
in the United States, had only four of 12 nuclear power plants online. Industry
experts cite economic, safety, and labor problems as reasons for these
shutdowns.
B1 -Radiological Hazards
Radioactive materials emit penetrating, ionizing radiation
that can injure living tissues. The commonly used unit of radiation dose
equivalent in humans is the sievert. (In the United States, rems are still used
as a measure of dose equivalent. One rem equals 0.01 sievert.) Each individual
in the United States and Canada is exposed to about 0.003 sievert per year from
natural background radiation sources. An exposure to an individual of five
sieverts is likely to be fatal. A large population exposed to low levels of
radiation will experience about one additional cancer for each 10 sieverts
total dose equivalent. See Radiation Effects, Biological.
Radiological hazards can arise in most steps of the nuclear
fuel cycle. Radioactive radon gas is a colorless gas produced from the decay of
uranium. As a result, radon is a common air pollutant in underground uranium
mines. The mining and ore-milling operations leave large amounts of waste
material on the ground that still contain small concentrations of uranium. To
prevent the release of radioactive radon gas into the air from this uranium
waste, these wastes must be stored in waterproof basins and covered with a
thick layer of soil.
Uranium enrichment and fuel fabrication plants contain large
quantities of three-percent uranium-235, in the form of corrosive gas, uranium
hexafluoride, UF6. The radiological hazard, however, is low, and the usual care
taken with a valuable material posing a typical chemical hazard suffices to
ensure safety.
B2 -Reactor Safety Systems
The safety of the power reactor itself has received the
greatest attention. In an operating reactor, the fuel elements contain by far
the largest fraction of the total radioactive inventory. A number of barriers
prevent fission products from leaking into the air during normal operation. The
fuel is clad in corrosion-resistant tubing. The heavy steel walls of the
primary coolant system of the PWR form a second barrier. The water coolant
itself absorbs some of the biologically important radioactive isotopes such as
iodine. The steel and concrete building is a third barrier.
During the operation of a power reactor, some radioactive
compounds are unavoidably released. The total exposure to people living nearby
is usually only a few percent of the natural background radiation. Major
concerns arise, however, from radioactive releases caused by accidents in which
fuel damage occurs and safety devices fail. The major danger to the integrity
of the fuel is a loss-of-coolant accident in which the fuel is damaged or even
melts. Fission products are released into the coolant, and if the coolant
system is breached, fission products enter the reactor building.
Reactor systems rely on elaborate instrumentation to monitor
their condition and to control the safety systems used to shut down the reactor
under abnormal circumstances. Backup safety systems that inject boron into the
coolant to absorb neutrons and stop the chain reaction to further assure
shutdown are part of the PWR design. Light-water reactor plants operate at high
coolant pressure. In the event of a large pipe break, much of the coolant would
flash into steam and core cooling could be lost. To prevent a total loss of
core cooling, reactors are provided with emergency core cooling systems that
begin to operate automatically on the loss of primary coolant pressure. In the
event of a steam leak into the containment building from a broken primary
coolant line, spray coolers are actuated to condense the steam and prevent a
hazardous pressure rise in the building.
B3 -Three Mile Island and Chernobyl'
Despite the many safety features described above, an
accident did occur in 1979 at the Three Mile Island PWR near Harrisburg,
Pennsylvania. A maintenance error and a defective valve led to a
loss-of-coolant accident. The reactor itself was shut down by its safety system
when the accident began, and the emergency core cooling system began operating
as required a short time into the accident. Then, however, as a result of human
error, the emergency cooling system was shut off, causing severe core damage
and the release of volatile fission products from the reactor vessel. Although
only a small amount of radioactive gas escaped from the containment building,
causing a slight rise in individual human exposure levels, the financial damage
to the utility was very large, $1 billion or more, and the psychological stress
on the public, especially those people who live in the area near the nuclear
power plant, was in some instances severe. The official investigation of the
accident named operational error and inadequate control room design, rather
than simple equipment failure, as the principal causes of the accident. It led
to enactment of legislation requiring the Nuclear Regulatory Commission to
adopt far more stringent standards for the design and construction of nuclear
power plants. The legislation also required utility companies to assume
responsibility for helping state and county governments prepare emergency
response plans to protect the public health in the event of other such
accidents.
Since 1981, the financial burdens imposed by these
requirements have made it difficult to build and operate new nuclear power
plants. Combined with other factors, such as high capital costs and long
construction periods (which means builders must borrow more money and wait
longer periods before earning a return on their investment), safety regulations
have forced utility companies in the states of Washington, Ohio, Indiana, and
New York to abandon partly completed plants after spending billions of dollars
on them. On April 26, 1986, another serious incident alarmed the world. One of
four nuclear reactors at Chernobyl', near Pripyat’, about 130 km (about 80 mi)
north of Kyiv (now in Ukraine) in the USSR, exploded and burned. Radioactive
material spread over Scandinavia and northern Europe, as discovered by Swedish
observers on April 28. According to the official report issued in August, the
accident was caused by unauthorized testing of the reactor by its operators.
The reactor went out of control; there were two explosions, the top of the
reactor blew off, and the core was ignited, burning at temperatures of 1500° C
(2800° F). Radiation about 50 times higher than that at Three Mile Island
exposed people nearest the reactor, and a cloud of radioactive fallout spread
westward. Unlike most reactors in western countries, including the United
States, the reactor at Chernobyl' did not have a containment building. Such a
structure could have prevented material from leaving the reactor site. About
135,000 people were evacuated, and more than 30 died. The plant was encased in
concrete. By 1988, however, the other three Chernobyl' reactors were back in
operation. One of the three remaining reactors was shut down in 1991 because of
a fire in the reactor building. In 1994 Western nations developed a financial
aid package to help close the entire plant, and a year later the Ukrainian
government finally agreed to a plan that would shut down the remaining reactors
by the year 2000.
C -Fuel Reprocessing
The fuel reprocessing step poses a combination of
radiological hazards. One is the accidental release of fission products if a
leak should occur in chemical equipment or the cells and building housing it.
Another may be the routine release of low levels of inert radioactive gases
such as xenon and krypton. In 1966 a commercial reprocessing plant opened in
West Valley, New York. But in 1972 this reprocessing plant was closed after
generating more than 600,000 gallons of high-level radioactive waste. After the
plant was closed, a portion of this radioactive waste was partially treated and
cemented into nearly 20,000 steel drums. In 1996, the United States Department
of Energy began to solidify the remaining liquid radioactive wastes into glass
cylinders. At the end of the 20th century, no reprocessing plants were licensed
in the United States.
Of major concern in chemical reprocessing is the separation
of plutonium-239, a material that can be used to make nuclear weapons. The
hazards of theft of plutonium-239, or its use for intentional but hidden
production for weapons purposes, can best be controlled by political rather
than technical means. Improved security measures at sensitive points in the
fuel cycle and expanded international inspection by the International Atomic
Energy Agency (IAEA) offer the best prospects for controlling the hazards of
plutonium diversion.
D -Waste Management
The last step in the nuclear fuel cycle, waste management,
remains one of the most controversial. The principal issue here is not so much
the present danger as the danger to generations far in the future. Many nuclear
wastes remain radioactive for thousands of years, beyond the span of any human
institution. The technology for packaging the wastes so that they pose no
current hazard is relatively straightforward. The difficulty lies both in being
adequately confident that future generations are well protected and in making
the political decision on how and where to proceed with waste storage.
Permanent but potentially retrievable storage in deep stable geologic
formations seems the best solution. In 1988 the U.S. government chose Yucca
Mountain, a Nevada desert site with a thick section of porous volcanic rocks,
as the nation's first permanent underground repository for more than 36,290
metric tons of nuclear waste. However, opposition from state residents and
uncertainty that Yucca Mountain may not be completely insulated from
earthquakes and other hazards has prolonged government studies. For example, a
geological study by the U.S. Department of Energy detected water in several
mineral samples taken at the Yucca Mountain site. The presence of water in
these samples suggests that water may have once risen up through the mountain
and later subsided. Because such an event could jeopardize the safety of a
nuclear waste repository, the Department of Energy has funded more study of
these fluid intrusions.
A $2 billion repository built in underground salt caverns
near Carlsbad, New Mexico, is designed to store radioactive waste from the
manufacture of nuclear weapons during the Cold War. This repository, located
655 meters (2,150 feet) underground, is designed to slowly collapse and
encapsulate the plutonium-contaminated waste in the salt beds. Although the
repository began receiving radioactive waste shipments in April 1999,
environmentalists planned to file a lawsuit to close the Carlsbad repository.
VI -NUCLEAR FUSION
The release of nuclear energy can occur at the low end of
the binding energy curve (see accompanying chart) through the fusion of two
light nuclei into a heavier one. The energy radiated by stars, including the
Sun, arises from such fusion reactions deep in their interiors. At the enormous
pressure and at temperatures above 15 million ° C (27 million ° F) existing
there, hydrogen nuclei combine according to equation (1) and give rise to most
of the energy released by the Sun.
Nuclear fusion was first achieved on earth in the early
1930s by bombarding a target containing deuterium, the mass-2 isotope of
hydrogen, with high-energy deuterons in a cyclotron (see Particle
Accelerators). To accelerate the deuteron beam a great deal of energy is
required, most of which appeared as heat in the target. As a result, no net
useful energy was produced. In the 1950s the first large-scale but uncontrolled
release of fusion energy was demonstrated in the tests of thermonuclear weapons
by the United States, the USSR, the United Kingdom, and France. This was such a
brief and uncontrolled release that it could not be used for the production of
electric power.
In the fission reactions discussed earlier, the neutron,
which has no electric charge, can easily approach and react with a fissionable
nucleus—for example, uranium-235. In the typical fusion reaction, however, 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 (90 to 180 million ° F). In a gas of the heavy hydrogen
isotopes deuterium and tritium at such temperature, the fusion reaction occurs,
releasing about 17.6 MeV per fusion event. The energy appears first as kinetic
energy of the helium-4 nucleus and the neutron, but is soon transformed into
heat in the gas and surrounding materials.
If the density of the gas is sufficient—and at these
temperatures the density need be only 10-5 atm, or almost a vacuum—the
energetic helium-4 nucleus can transfer its energy to the surrounding hydrogen
gas, thereby maintaining the high temperature and allowing subsequent fusion
reactions, or a fusion chain reaction, to take place. Under these conditions,
“nuclear ignition” is said to have occurred.
The basic problems in attaining useful nuclear fusion
conditions are (1) to heat the gas to these very high temperatures and (2) to
confine a sufficient quantity of the reacting nuclei for a long enough time to
permit the release of more energy than is needed to heat and confine the gas. A
subsequent major problem is the capture of this energy and its conversion to
electricity.
At temperatures of even 100,000° C (180,000° F), all the
hydrogen atoms are fully ionized. The gas consists of an electrically neutral
assemblage of positively charged nuclei and negatively charged free electrons.
This state of matter is called a plasma.
A plasma hot enough for fusion cannot be contained by
ordinary materials. The plasma would cool very rapidly, and the vessel walls
would be destroyed by the extreme heat. However, since the plasma consists of
charged nuclei and electrons, which move in tight spirals around the lines of
force of strong magnetic fields, the plasma can be contained in a properly
shaped magnetic field region without reacting with material walls.
In any useful fusion device, the energy output must exceed
the energy required to confine and heat the plasma. This condition can be met
when the product of confinement time t and plasma density n exceeds about 1014.
The relationship tn≥ 1014 is called the Lawson criterion. Numerous schemes for
the magnetic confinement of plasma have been tried since 1950 in the United
States, Russia, the United Kingdom, Japan, and elsewhere. Thermonuclear
reactions have been observed, but the Lawson number rarely exceeded 1012. One
device, however—the tokamak, originally suggested in the USSR by Igor Tamm and
Andrey Sakharov—began to give encouraging results in the early 1960s.
The confinement chamber of a tokamak has the shape of a
torus, with a minor diameter of about 1 m (about 3.3 ft) and a major diameter
of about 3 m (about 9.8 ft). A toroidal (donut-shaped) 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.
Based on the successful operation of small tokamaks at
several laboratories, two large devices were built in the early 1980s, one at
Princeton University in the United States and one in the USSR. The enormous
magnetic fields in a tokamak subject the plasma to extremely high temperatures
and pressures, forcing the atomic nuclei to fuse. As the atomic nuclei are
fused together, an extraordinary amount of energy is released. During this
fusion process, the temperature in the tokamak reaches three times that of the
Sun’s core.
Another possible route to fusion energy is that of inertial
confinement. In this concept, the fuel—tritium or deuterium—is contained within
a tiny glass sphere that is then bombarded on several sides by a pulsed laser
or heavy ion beam. This causes an implosion of the glass sphere, setting off a
thermonuclear reaction that ignites the fuel. Several laboratories in the
United States and elsewhere are currently pursuing this possibility. In the
late 1990s, many researchers concentrated on the use of beams of heavy ions,
such as barium ions, rather than lasers to trigger inertial-confinement fusion.
Researchers chose heavy ion beams because heavy ion accelerators can produce
intense ion pulses at high repetition rates and because heavy ion accelerators
are extremely efficient at converting electric power into ion beam energy, thus
reducing the amount of input power. Also in comparison to laser beams, ion
beams can penetrate the glass sphere and fuel more effectively to heat the
fuel.
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. However, some progress was made in the early 1990s. In
1991, for the first time ever, a significant amount of energy—about 1.7 million
watts—was produced from controlled nuclear fusion at the Joint European Torus
(JET) Laboratory in England. In December 1993, researchers at Princeton
University used the Tokamak Fusion Test Reactor to produce a controlled fusion
reaction that output 5.6 million watts of power. However, both the JET and the
Tokamak Fusion Test Reactor consumed more
energy than they produced during
their operation.
If fusion energy does become practical, it offers the
following advantages:
(1) a limitless source of fuel, deuterium from the ocean;
(2) no possibility of a reactor accident, as the amount of fuel in the system
is very small; and
(3) waste products much less radioactive and simpler to
handle than those from fission systems.
