Nuclear Power
By David E. Newton, Paula Anne Ford-Martin | Oct 25, 2005 | 2407 words, 0 images
Nuclear power
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 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. 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%.
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 is 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
and boron 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 is 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.
Neutrons have to be moving quickly to produce plutonium , and sodium does not moderate their speed as
much as water does. Another design, the CANDU reactor, (acronym for CANada Deuterium Uranium) uses
deuterium oxide (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 which 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 back-up systems for increased safety which are
themselves supported by other back-up systems. But the components of these systems age, and human
errors can and do occur; safety measures do not always function the way there 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 50 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. No new plants have been ordered in the United States since this accident, and 65
plants on order at that time were cancelled. The explosion at the Chernobyl reactor near Kiev, Ukraine,
dealt a second blow to the industry.
Even without these accidents, another problem with nuclear power would remain. This is the problem of
spent radioactive wastes. About a 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 fashion, but 50 years
after the first controlled reaction, no method has yet been discovered to address this issue. Today, 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.
The International Nuclear Safety Center (INSC), which operates under the guidance of the Director of
International Nuclear Safety and Cooperation in the U.S. Department of Energy (DOE), 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 U.S. and its international partners.
Safety issues are addressed at several levels, including: 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 growing 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 11 evaluations performed in 2000 and 2001 resulted in the "attackers" being able to penetrate
security and simulate damage to reactor equipment.
In the post-September 11 terrorist attack environment, the vulnerability of America's 104 nuclear power
facilities are a critical national security concern. All nuclear facilities were placed on high alert
immediately following the attacks, and in early 2002 the Nuclear Regulatory Commission (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 web site, 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.
The future role of nuclear power in energy production throughout the world is uncertain. But the current
absence of nuclear power plant development in the United States should not be taken as indicative of
future trends, as well as trends in the rest of world. France, for example, obtains more than half of its
electricity from nuclear power, despite the safety problems. And many believe that nuclear power should
be an important part of energy production in the United States as well. Proponents of nuclear power
argue that its dangers have been greatly exaggerated in this country. The risks, they argue, must be
compared with the health and environmental hazards of other fuels, particularly fossil fuels .
In 1999, the U.S. Department of Energy announced a new initiative called Generation IV. Designed to
generate interest and scientific research in nuclear power advances, the program set out the following
goals for the next generation (i.e., generation IV) of nuclear power plants: 1) highly economical; 2)
enhanced safety; 3) sustainable with minimal waste; and 4) commercially viable by 2025. Ten countries,
including the United States, United Kingdom, Canada, South Africa, and Japan, were participating in the
Generation IV International Forum (GIF) as of early 2002. One of the most promising prototypes reactor
designers have come up with is the gas-cooled pebble-bed reactor, which uses a non-reactive helium
coolant and operates more efficiently and at less cost than current water-cooled systems.
For some, hope for the future of nuclear power rests with the development of nuclear fusion . A nuclear
power plant based on a fusion reaction would amount to a controlled version of a hydrogen bomb, just as
conventional nuclear plants are equivalent to a controlled version of an atomic bomb. But the problem of
managing the reaction is far more difficult with fusion than it is with fission, and scientists have been
working on this issue unsuccessfully for more than 40 years. Some believe that a fusion power plant could
become a reality in the next century, but many now doubt that such a plant will ever be feasible.