Nuclear energy is energy in the
nucleus (core) of an atom. Atoms are tiny particles that make up every
object in the universe. There is enormous energy in the bonds that hold
atoms together. Nuclear energy can be used to make electricity. But first
the energy must be released. It can be released from atoms in two ways:
nuclear fusion and nuclear fission.
In nuclear fusion, energy is
released when atoms are combined or fused together to form a larger atom. This
is how the sun produces energy.
In nuclear fission, atoms are
split apart to form smaller atoms, releasing energy. Nuclear power plants use
nuclear fission to produce electricity.
Nuclear power plants are very
clean and efficient to operate. However, nuclear power plants have some
major environmental risks. Nuclear power plants produce radioactive gases.
These gases are to be contained in the operation of the plant. If these
gases are released into the air, major health risks can occur. Nuclear
plants use uranium as a fuel to produce power. The mining and handling of
uranium is very risky and radiation leaks can occur. The third concern of
nuclear power is the permanent storage of spent radioactive fuel. This fuel
is toxic for centuries, handling and disposal is an ongoing environmental
An atom consists of an extremely
small, positively charged nucleus surrounded by a cloud of negatively
charged electrons. Although typically the nucleus is less than one
ten-thousandth the size of the atom, the nucleus contains more than 99.9% of
the mass of the atom! Nuclei consist of positively charged protons and
electrically neutral neutrons held together by the so-called strong or
nuclear force. This force is much stronger than the familiar electrostatic
force that binds the electrons to the nucleus, but its range is limited to
distances on the order of a few x10-15 meters.
number of protons in the nucleus, Z, is called the atomic number. This
determines what chemical element the atom is. The number of neutrons in the
nucleus is denoted by N. Theatomic mass of the nucleus, A,
is equal to Z + N. A given element can have many different
isotopes, which differ from one another by the number of neutrons contained
in the nuclei. In a neutral atom, the number of electrons orbiting the
nucleus equals the number of protons in the nucleus. Since the electric
charges of the proton and the electron are +1 and -1 respectively (in units
of the proton charge), the net charge of the atom is zero. At present, there
are 112 known elements which range from the lightest, hydrogen, to the
recently discovered and yet to-be-named element 112. All of the elements
heavier than uranium are man made. Among the elements are approximately 270
stable isotopes, and more than 2000 unstable isotopes.
The Nuclear Fuel Cycle
The nuclear fuel cycle consists
of "front end" steps that lead to the preparation of uranium for
use as fuel for reactor operation and "back end" steps that are
necessary to safely manage, prepare, and dispose of the highly radioactive
spent nuclear fuel.
Exploration. A deposit of
uranium, discovered by geophysical techniques, is evaluated and sampled to
determine the amounts of uranium materials that are extractable at specified
costs from the deposit. Uranium reserves are the amounts of ore that are
estimated to be recoverable at stated costs.
Mining. Uranium ore can be
extracted through conventional mining in open pit and underground methods
similar to those used for mining other metals. In situ leach mining methods also
are used to mine uranium in the United States. In this technology, uranium is
leached from the in-place ore through an array of regularly spaced wells and is
then recovered from the leach solution at a surface plant. Uranium ores in the
United States typically range from about 0.05 to 0.3 percent uranium oxide
(U3O8). Some uranium deposits developed in other countries are of higher grade
and are also larger than deposits mined in the United States. Uranium is also
present in very low grade amounts (50 to 200 parts per million) in some domestic
phosphate-bearing deposits of marine origin. Because very large quantities of
phosphate-bearing rock are mined for the production of wet-process phosphoric
acid used in high analysis fertilizers and other phosphate chemicals, at some
phosphate processing plants the uranium, although present in very low
concentrations, can be economically recovered from the process stream.
Milling. Mined uranium
ores normally are processed by grinding the ore materials to a uniform particle
size and then treating the ore to extract the uranium by chemical leaching. The
milling process commonly yields dry powder-form material consisting of natural
uranium, "yellowcake," which is sold on the uranium market as U3O8.
Uranium conversion. Milled
uranium oxide, U3O8, must be converted to uranium hexafluoride, UF6, which is
the form required by most commercial uranium enrichment facilities currently in
use. A solid at room temperature, UF6 can be changed to a gaseous form at
moderately higher temperatures. The UF6 conversion product contains only
natural, not enriched, uranium.
concentration of the fissionable isotope, 235U (0.71 percent in natural uranium)
is less than that required to sustain a nuclear chain reaction in light water
reactor cores. Natural UF6 thus must be "enriched" in the fissionable
isotope for it to be used as nuclear fuel. The different levels of enrichment
required for a particular nuclear fuel application are specified by the
customer: light-water reactor fuel normally is enriched up to about 4 percent
235U, but uranium enriched to lower concentrations also is required. Gaseous
diffusion and gas centrifuge are the commonly used uranium enrichment
technologies. The gaseous diffusion process consists of passing the natural UF6
gas feed under high pressure through a series of diffusion barriers (semiporous
membranes) that permit passage of the lighter 235UF6 atoms at a faster rate than
the heavier 238UF6 atoms. This differential treatment, applied across a large
number of diffusion "stages," progressively raises the product stream
concentration of 235U relative to 238U. In the gaseous diffusion technology, the
separation achieved per diffusion stage is relatively low, and a large number of
stages is required to achieve the desired level of isotope enrichment. Because
this technology requires a large capital outlay for facilities and it consumes
large amounts of electrical energy, it is relatively cost intensive. In the gas
centrifuge process, the natural UF6 gas is spun at high speed in a series of
cylinders. This acts to separate the 235UF6 and 238UF6 atoms based on their
slightly different atomic masses. Gas centrifuge technology involves relatively
high capital costs for the specialized equipment required, but its power costs
are below those for the gaseous diffusion technology. New enrichment
technologies currently being developed are the atomic vapor laser isotope
separation (AVLIS) and the molecular laser isotope separation (MLIS). Each
laser-based enrichment process can achieve higher initial enrichment (isotope
separation) factors than the diffusion or centrifuge processes can achieve. Both
AVLIS and MLIS will be capable of operating at high material throughput rates.
Fabrication. For use as
nuclear fuel, enriched UF6 is converted into uranium dioxide (UO2) powder which
is then processed into pellet form. The pellets are then fired in a high
temperature sintering furnace to create hard, ceramic pellets of enriched
uranium. The cylindrical pellets then undergo a grinding process to achieve a
uniform pellet size. The pellets are stacked, according to each nuclear core's
design specifications, into tubes of corrosion-resistant metal alloy. The tubes
are sealed to contain the fuel pellets: these tubes are called fuel rods. The
finished fuel rods are grouped in special fuel assemblies that are then used to
build up the nuclear fuel core of a power reactor.
The back end of
the cycle is divided into the following steps:
Interim Storage. After its
operating cycle, the reactor is shut down for refueling. The fuel discharged at
that time (spent fuel) is stored either at the reactor site or, potentially, in
a common facility away from reactor sites. If on-site pool storage capacity is
exceeded, it may be desirable to store aged fuel in modular dry storage
facilities known as Independent Spent Fuel Storage Installations (ISFSI) at the
reactor site or at a facility away from the site. The spent fuel rods are
usually stored in water, which provides both cooling (the spent fuel continues
to generate heat as a result of residual radioactive decay) and shielding (to
protect the environment from residual ionizing radiation).
Reprocessing. Spent fuel
discharged from light-water reactors contains appreciable quantities of fissile
(U-235, Pu-239), fertile (U-238), and other radioactive materials. These fissile
and fertile materials can be chemically separated and recovered from the spent
fuel. The recovered uranium and plutonium can, if economic and institutional
conditions permit, be recycled for use as nuclear fuel. Currently, plants in
Europe are reprocessing spent fuel from utilities in Europe and Japan.
Waste Disposal. A current
concern in the nuclear power field is the safe disposal and isolation of either
spent fuel from reactors or, if the reprocessing option is used, wastes from
reprocessing plants. These materials must be isolated from the biosphere until
the radioactivity contained in them has diminished to a safe level. Under the
Nuclear Waste Policy Act of 1982, as amended, the Department of Energy has
responsibility for the development of the waste disposal system for spent
nuclear fuel and high-level radioactive waste. Current plans call for the
ultimate disposal of the wastes in solid form in licensed deep, stable geologic
Inside the reactor of an atomic
power plant, uranium atoms are split apart in a controlled chain reaction.
In a chain reaction, particles released by the splitting of the atom go off
and strike other uranium atoms splitting those. Those particles given off
split still other atoms in a chain reaction. In nuclear power plants,
control rods are used to keep the splitting regulated so it doesn't go too
fast. If the reaction is not controlled, you could have an atomic bomb. But
in atomic bombs, almost pure pieces of the element Uranium-235 or Plutonium,
of a precise mass and shape, must be brought together and held together,
with great force. These conditions are not present in a nuclear reactor. The
reaction also creates radioactive material. This material could hurt people
if released, so it is kept in a solid form.
This view looks down on the fuel
rods at Penn State's Breazeale Reactor. The reactor is a TRIGA model
manufactured by General Atomics. The blue light surrounding the fuel is
known as Cherenkov radiation, produced when charged particles travel through
matter (in this case, water) at speeds greater than light. Penn State
University is the site of the first licensed reactor. Sources: the Penn
State Radiation Science and Engineering Center
This chain reaction gives off
heat energy. This heat energy is used to boil water in the core of the
reactor. So, instead of burning a fuel, nuclear power plants use the chain
reaction of atoms splitting to change the energy of atoms into heat energy.
This water from around the nuclear core is sent to another section of the
power plant. Here it heats another set of pipes filled with water to make
steam. The steam in this second set of pipes powers a turbine to generate
Fission is a nuclear process in
which a heavy nucleus splits into two smaller nuclei. An example of a
fission reaction that was used in the first atomic bomb and is still used in
nuclear reactors is.
An atom's nucleus can be split
apart. When this is done, a tremendous amount of energy is released. The
energy is both heat and light energy. This energy, when let out slowly, can
be harnessed to generate electricity. When it is let out all at once, it
makes a tremendous explosion in an atomic bomb. The word fission means to
+ n ----> 134Xe + 100Sr
products shown in the above equation are only one set of many possible product
nuclei. Fission reactions can produce any combination of lighter nuclei so long
as the number of protons and neutrons in the products sum up to those in the
initial fissioning nucleus. As with fusion, a great amount of energy can be
released in fission because for heavy nuclei, the summed masses of the lighter
product nuclei is less than the mass of the fissioning nucleus.
Fission occurs because of the electrostatic
repulsion created by the large number of positively charged protons contained in
a heavy nucleus. Two smaller nuclei have less internal electrostatic repulsion
than one larger nucleus. So, once the larger nucleus can overcome the strong
nuclear force which holds it together, it can fission. Fission can be seen as a
"tug-of-war" between the strong attractive nuclear force and the
repulsive electrostatic force. In fission reactions, electrostatic repulsion
Fission is a process that has been occurring in
the universe for billions of years. As mentioned above, we have not only used
fission to produce energy for nuclear bombs, but we also use fission peacefully
everyday to produce energy in nuclear power plants.
The following are essential
components/systems of a thermal nuclear fission reactor:
- The fuel - the
fissile material (U-235), either as found in natural uranium or enriched. In
some cases plutonium is added. The fuel is produced in the form of metal or
- Fuel cladding - a
metal shell in which the fuel pellets are contained. It protects the fuel
from corrosion and prevents fission products from escaping.
- A moderator - made of
light elements, it slows down the fission neutrons to thermal levels without
unduly absorbing them.
- A coolant - to
transport the heat generated from the core to the steam generator for
driving the turbine.
- Control rods - made
of neutron absorbing material, these can be moved in or out of the core to
control the reaction and maintain it at a critical level or to stop the
reaction during shutdown.
- A pressure vessel -
to prevent radioactive material from escaping in case of excessive internal
- A containment structure or
neutron shield - (concrete or other material) to protect operators and
the public from radiation.
University of California,
Another form of nuclear energy
is called fusion. Fusion means joining smaller nuclei (the plural of
nucleus) to make a larger nucleus.
Fusion is a nuclear process in
which two light nuclei combine to form a single heavier nucleus. An example of a
fusion reaction important in thermonuclear weapons and in future nuclear
reactors is the reaction between two different hydrogen isotopes to form an
isotope of helium:
+ 3H ----> 4He
reaction liberates an amount of energy more than a million times greater than
one gets from a typical chemical reaction. Such a large amount of energy is
released in fusion reactions because when two light nuclei fuse, the sum of the
masses of the product nuclei is less than the sum of the masses of the initial
fusing nuclei. Once again, Einstein's equation, E=mc2, explains that
the mass that is lost it converted into energy carried away by the fusion
Even though fusion n is an energetically favorable
reaction for light nuclei, it does not occur under standard conditions here on
Earth because of the large energy investment that is required. Because the
reacting nuclei are both positively charged, there is a large electrostatic
repulsion between them as they come together. Only when they are squeezed very
close to one another do they feel the strong nuclear force, which can overcome
the electrostatic repulsion and cause them to fuse.
Fusion reactions have been going on for billions
of years in our universe. In fact, nuclear fusion reactions are responsible for
the energy output of most stars, including our own Sun. Scientists on Earth have
been able to produce fusion reactions for only about the last sixty years. At
first, there were small scale studies in which only a few fusion reactions
actually occurred. However, these first experiments later lead to the
development of thermonuclear fusion weapons (hydrogen bombs).
Fusion is the process that takes place in stars
like our Sun. Whenever we feel the warmth of the Sun and see by its light, we
are observing the products of fusion. We know that all life on Earth exists
because the light generated by the Sun produces food and warms our planet.
Therefore, we can say that fusion is the basis for our life.
When a star is formed, it
initially consists of hydrogen and helium created in the Big Bang, the process
that created our universe. Hydrogen isotopes collide in a star and fuse forming
a helium nucleus. Later, the helium nuclei collide and form heavier elements.
Fusion is a nuclear reaction in which nuclei combine to form a heavier nucleus.
It is the basic reaction which drives the Sun. Lighter elements fuse and form
heavier elements. These reactions continue until the nuclei reach iron (around
mass sixty), the nucleus with the most binding energy. When a nucleus reaches
mass sixty, no more fusion occurs in a star because it is energetically
unfavorable to produce higher masses. Once a star has converted a large fraction
of its core's mass to iron, it has almost reached the end of its life.
fusion chain cannot continue so its fuel is reduced. Some stars keep shrinking
until they become a cooling ember made up of iron. However, if a star is
sufficiently massive, a tremendous, violent, brilliant explosion can happen. A
star will suddenly expand and produce, in a very short time, more energy than
our Sun will produce in a lifetime. When this happens, we say that a star has
become a supernova.
star is in the supernova phase, many important reactions occur. The nuclei are
accelerated to much higher velocities than can occur in a fusing star. With the
added energy caused by their speed, nuclei can fuse and produce elements higher
in mass than iron. The extra energy in the explosion is necessary to over come
the energy barrier of a higher mass element. Elements such as lead, gold, and
silver found on Earth were once the debris of a supernova explosion. The element
iron that we find all through the Earth and in its center is directly derived
from both super novae and dead stars.
are Two Types of Reactors in the United States
Pressurized Water Reactor (PWR)
keep water under pressure so that it heats, but does not boil. Water from the
reactor and the water in the steam generator that is turned into steam never
mix. In this way, most of the radioactivity stays in the reactor area.
Boiling Water Reactor (BWR)
Regulatory Commission Graphic
actually boil the water. In both types, water is converted to steam, and then
recycled back into water by a part called the condenser, to be used again in the
heat process. Since radioactive materials can be dangerous, nuclear power plants
have many safety systems to protect workers, the public, and the environment.
These safety systems include shutting the reactor down quickly and stopping the
fission process, systems to cool the reactor down and carry heat away from it,
and barriers to contain the radioactivity and prevent it from escaping into the
environment. Radioactive materials, if not used properly, can damage human cells
or even cause cancer over long periods of time.
Nuclear power (% of total
primary energy supply)
IEA (International Energy
- Glossary of Nuclear Science
- Any material that stops
ionizing radiation. Lead, concrete, and steel attenuate gamma rays. A thin
sheet of paper or metal will stop or absorb alpha particles and most beta
- Alpha particle (alpha
radiation, alpha ray)
- A positively charged particle
(a Helium-4 nucleus) made up of two neutrons and two protons. It is the
least penetrating of the three common forms of radiation, being stopped by a
sheet of paper. It is not dangerous to living things unless the
alpha-emitting substance is inhaled or ingested or comes into contact with
the lens of the eye.
- A particle of matter
indivisible by chemical means. It is the fundamental building block of
- Atomic number
- The number assigned to each
element on the basis of the number of protons found in the element's
- Atomic weight (atomic mass)
- Approximately the sum of the
number of protons and neutrons found in the nucleus of an atom.
- B -
- Background radiation
- The radiation of man's natural
environment originating primarily from the naturally radioactive elements of
the earth and from the cosmic rays. The term may also mean radiation
extraneous to an experiment.
- Beta particle (beta
radiation, beta ray)
- An electron of either positive
charge (+) or negative charge (-), which has been emitted by an atomic
nucleus or neutron in the process of a transformation. Beta particles are
more penetrating than alpha particles but less than gamma rays or x-rays.
- C -
- Radioactive material deposited
or dispersed in materials or places where it is not wanted.
- A radioisotope generator
- Curie (Ci)
- The basic unit used to
describe the intensity of radioactivity in a sample of material. One curie
equals thirty-seven billion disintegrations per second, or approximately the
radioactivity of one gram of radium.
- D -
- A nucleus formed by the
radioactive decay of a different (parent) nuclide.
- Decay (radioactive)
- The change of one radioactive
nuclide into a different nuclide by the spontaneous emission of alpha, beta,
or gamma rays, or by electron capture. The end product is a less energetic,
more stable nucleus. Each decay process has a definite half-life.
- The removal of radioactive
contaminants by cleaning and washing with chemicals.
- That property of a substance
which is expressed by the ratio of its mass to its volume.
- A general term denoting the
quantity of radiation or energy absorbed in a specific mass.
- E -
- Electromagnetic radiation
- Radiation consisting of
electric and magnetic waves that travel at the speed of light. Examples:
light, radio waves, gamma rays, x-rays.
- An elementary particle with a
unit electrical charge and a mass 1/1837 that of the proton. Electrons
surround the atom's positively charged nucleus and determine the atom's
- Electron capture
- A radioactive decay process in
which an orbital electron is captured by and merges with the nucleus. The
mass number is unchanged, but the atomic number is decreased by one.
- Washing solution (The solution
that is introduced into the cow).
- The washings obtained by
elution (the solution that comes out of the cow).
- To separate by washing (to
- Excited state
- The state of an atom or
nucleus when it possesses more than its normal energy. The excess energy is
usually released eventually as a gamma ray.
- F -
- The splitting of a heavy
nucleus into two roughly equal parts (which are nuclei of lighter elements),
accompanied by the release of a relatively large amount of energy in the
form of kinetic energy of the two parts and in the form of emission of
neutrons and gamma rays.
- Fission products
- Nuclei formed by the fission
of heavy elements. They are of medium atomic weight and almost all are
radioactive. Examples: strontium-90, cesium-137.
- G -
- Gamma ray
- A highly penetrating type of
nuclear radiation, similar to x-radiation, except that it comes from within
the nucleus of an atom, and, in general, has a shorter wavelength.
- Geiger counter
- A Geiger-Müller detector and
measuring instrument. It contains a gas-filled tube which discharges
electrically when ionizing radiation passes through it and a device that
records the events.
- A cow-a system containing a
parent-daughter set of radioisotopes in which the parent decays through a
daughter to a stable isotope. The daughter is a different element from that
of the parent, and, hence, can be separated from the parent by elution
- H -
- The time in which half the
atoms of a particular radioactive nuclide disintegrate. The half-life is a
characteristic property of each radioactive isotope.
- Health physics
- That science devoted to
recognition, evaluation, and control of all health hazards from ionizing
- I -
- Induced radioactivity
- Radioactivity that is created
by bombarding a substance with neutrons in a reactor or with charged
particles produced by particle accelerators.
- An atomic particle that is
electrically charged, either negative or positive.
- Ionizing radiation
- Radiation that is capable of
producing ions either directly or indirectly.
- To expose to some form of
- One of several nuclides with
the same number of neutrons and protons capable of existing for a measurable
time in different nuclear energy states.
- Isometric transition
- A mode of radioactive decay
where a nucleus goes from a higher to a lower energy state. The mass number
and the atomic number are unchanged.
- Isotopes of a given element
have the same atomic number (same number of protons in their nuclei) but
different atomic weights (different number of neutrons in their nuclei).
Uranium-238 and uranium-235 are isotopes of uranium.
- K -
- The capture by an atom's
nucleus of an orbital electron from the first K-shell surrounding the
- One thousand electron volts.
- M -
- One million electron volts.
- Microcurie (Ci)
- One millionth of a curie (3.7
x 104 disintegrations per second).
- To elute a cow.
- A trademark of Union Carbide
Corporation that is used to identify radioisotope generator systems for
- N -
- An electrically neutral
particle with negligible mass. It is produced in many nuclear reactions such
as in beta decay.
- One of the basic particles
which make up an atom. A neutron and a proton have about the same weight,
but the neutron has no electrical charge.
- Nuclear reactor
- A device in which a fission
chain reaction can be initiated, maintained, and controlled. Its essential
components are fissionable fuel, moderator, shielding, control rods, and
- A constituent of the nucleus;
that is, a proton or a neutron.
- The science, technology, and
application of nuclear energy.
- The core of the atom, where
most of its mass and all of its positive charge is concentrated. Except for
hydrogen, it consists of protons and neutrons.
- Any species of atom that
exists for a measurable length of time. A nuclide can be distinguished by
its atomic weight, atomic number, and energy state.
- P -
- A radionuclide that decays to
another nuclide which may be either radioactive or stable.
- A quantity of electromagnetic
energy. Photons have momentum but no mass or electrical charge.
- One of the basic particles
which makes up an atom. The proton is found in the nucleus and has a
positive electrical charge equivalent to the negative charge of an electron
and a mass similar to that of a neutron: a hydrogen nucleus.
- R -
- Radiation Absorbed Dose. The
basic unit of an absorbed dose of ionizing radiation. One rad is equal to
the absorption of 100 ergs of radiation energy per gram of matter.
- Radioactive dating
- A technique for estimating the
age of an object by measuring the amounts of various radioisotopes in it.
- Radioactive waste
- Materials which are
radioactive and for which there is no further use.
- The spontaneous decay of
disintegration of an unstable atomic nucleus accompanied by the emission of
- A radioactive isotope. A
common term for a radionuclide.
- A radioactive nuclide. An
unstable isotope of an element that decays or disintegrates spontaneously,
- Rate meter
- An electronic instrument that
indicates, on a meter, the number of radiation induced pulses per minute
from radiation detectors such as a Geiger-Muller tube.
- S -
- An electronic instrument for
counting radiation induced pulses from radiation detectors such as a
- Scintillation counter
- An instrument that detects and
measures gamma radiation by counting the light flashes (scintillations)
induced by the radiation.
- Secular equilibrium
- A state of parent-daughter
equilibrium which is achieved when the half-life of the parent is much
longer than the half-life of the daughter. In this case, if the two are not
separated, the daughter will eventually be decaying at the same rate at
which it is being produced. At this point, both parent and daughter will
decay at the same rate until the parent is essentially exhausted.
- A protective barrier, usually
a dense material, which reduces the passage of radiation from radioactive
materials to the surroundings.
- A radioactive material that
produces radiation for experimental or industrial use.
- The accidental release of
- T -
- A small amount of radioactive
isotope introduced into a system in order to follow the behavior of some
component of that system.
- The transformation of one
element into another by a nuclear reaction
U.S. Department of Energy, International Atomic Energy Agency, U.S. Nuclear
Regulatory Commission, Nuclear Science Division ---- Lawrence Berkeley National
Laboratory, Penn State Radiation Science and Engineering Center, American
Nuclear Society, European Commision