Nuclear technology is technology that involves the
reactions of
atomic nuclei. It has found applications from
smoke detectors to
nuclear reactors, and from
gun sights to
nuclear
weapons.
History and scientific background
Discovery
The vast majority of common, natural phenomena do not involve
nuclear reactions. Most everyday phenomena only involve
gravity and
electromagnetism. (The two other
fundamental forces of nature act only on
the
atomic scale.) Atomic nuclei are generally
kept apart because they contain positive electrical charges and
therefore repel each other.
In 1896,
Henri Becquerel was
investigating
phosphorescence in
uranium salts when he discovered a new
phenomenon which came to be called
radioactivity. He,
Pierre Curie and
Marie
Curie began investigating the phenomenon. In the process, they
isolated the element
radium, which is highly
radioactive. They discovered that radioactive materials produce
intense, penetrating rays of three distinct sorts, which they
labeled alpha, beta, and gamma after the
Greek letters. Some of these kinds of
radiation could pass through ordinary matter, and all of them could
be harmful in large amounts. All the early researchers received
various
radiation burns, much like
sunburn, and thought little of it.
The new phenomenon of radioactivity was seized upon by the
manufacturers of
quack medicine (as
had the discoveries of
electricity and
magnetism, earlier), and a number of
patent medicines and treatments
involving radioactivity were put forward.Gradually it was realized
that the radiation produced by radioactive decay was
ionizing radiation, and that even
quantities too small to burn presented a
severe long-term hazard. Many of the
scientists working on radioactivity died of
cancer as a result of their exposure.Radioactive
patent medicines mostly disappeared, but other applications of
radioactive materials persisted, such as the use of radium salts to
produce
glowing dials on meters.
As the atom came to be better understood, the nature of
radioactivity became clearer. Some larger atomic nuclei are
unstable, and so
decay (release
matter or energy) after a random interval. The three forms of
radiation that Becquerel and the
Curies discovered are also more fully understood.
Alpha decay is when a nucleus releases an
alpha particle, which is two
protons and two
neutrons,
equivalent to a
helium nucleus.
Beta decay is the release of a
beta particle, a high-energy
electron.
Gamma decay
releases
gamma rays, which unlike alpha
and beta radiation are not matter but
electromagnetic radiation of very
high
frequency, and therefore
energy. This type of radiation is the most dangerous,
and most difficult to block. All three types of radiation occur
naturally in
certain
elements.
Fission
In natural nuclear radiation, the byproducts are very small
compared to the nuclei from which they originate. Nuclear fission
is the process of splitting a nucleus into roughly equal parts, and
releasing energy and neutrons in the process. If these neutrons are
captured by another unstable nucleus, they can fission as well,
leading to a
chain reaction. The
average number of neutrons released per nucleus that go on to
fission another nucleus is referred to as
k. Values of
k larger than 1 mean that the fission reaction is
releasing more neutrons than it absorbs, and therefore is referred
to as a self-sustaining chain reaction. A mass of fissile material
large enough (and in a suitable configuration) to induce a
self-sustaining chain reaction is called a
critical mass.
When a neutron is captured by a suitable nucleus, fission may occur
immediately, or the nucleus may persist in an unstable state for a
short time. If there are enough immediate decays to carry on the
chain reaction, the mass is said to be
prompt critical, and the energy release will
grow rapidly and uncontrollably, usually leading to an
explosion.
When discovered on the eve of
World War
II, this insight led multiple countries to begin programs
investigating the possibility of constructing an
atomic bomb — a weapon which utilized fission
reactions to generate far more energy than could be created with
chemical explosives.
The Manhattan
Project, run by the United States
with the help of the United Kingdom
and Canada
, developed
multiple fission weapons which were used against Japan
in
1945. During the project, the first
fission reactors were developed as well,
though they were primarily for weapons manufacture and did not
generate electricity.
However, if the mass is critical only when the delayed neutrons are
included, then the reaction can be controlled, for example by the
introduction or removal of
neutron
absorbers. This is what allows
nuclear reactors to be built. Fast neutrons
are not easily captured by nuclei; they must be slowed (
slow neutrons), generally by collision with the
nuclei of a
neutron moderator,
before they can be easily captured. Today, this type of fission is
commonly used to generate electricity.
Fusion
If nuclei are forced to collide, they can undergo
nuclear fusion. This process may release or
absorb energy. When the resulting nucleus is lighter than that of
iron, energy is normally released; when the
nucleus is heavier than that of iron, energy is generally absorbed.
This process of fusion occurs in
stars, which
derive their energy from
hydrogen and
helium. They form, through
stellar nucleosynthesis, the light
elements (
lithium to
calcium) as well as some of the heavy elements
(beyond iron and
nickel, via the
S-process). The remaining abundance of heavy
elements, from nickel to uranium and beyond, is due to
supernova nucleosynthesis, the
R-process.
Of course, these natural processes of astrophysics are not examples
of nuclear "technology". Because of the very strong repulsion of
nuclei, fusion is difficult to achieve in a controlled fashion.
Hydrogen bombs obtain their enormous
destructive power from fusion, but their energy cannot be
controlled. Controlled fusion is achieved in
particle accelerators; this is how many
synthetic elements are produced. A
fusor can also produce controlled fusion and
is a useful
neutron source. However,
both of these devices operate at a net energy loss. Controlled,
viable
fusion power has proven elusive,
despite the occasional
hoax. Technical
and theoretical difficulties have hindered the development of
working civilian fusion technology, though research continues to
this day around the world.
Nuclear fusion was initially pursued only in theoretical stages
during World War II, when scientists on the Manhattan Project (led
by
Edward Teller) investigated it as a
method to build a bomb. The project abandoned fusion after
concluding that it would require a fission reaction to detonate. It
took until 1952 for the first full
hydrogen
bomb to be detonated, so-called because it used reactions between
deuterium and
tritium. Fusion reactions are much more energetic
per unit mass of
fuel than fission
reactions, but starting the fusion chain reaction is much more
difficult.
Nuclear Weapons
A nuclear weapon is an explosive device that derives its
destructive force from
nuclear
reactions, either
fission or a
combination of fission and
fusion.
Both reactions release vast quantities of energy from relatively
small amounts of matter. Even small nuclear devices can devastate a
city by blast, fire and radiation. Nuclear weapons are considered
weapons of mass
destruction, and their use and control has been a major aspect
of international policy since their debut.
The
design of a nuclear weapon
is more complicated than it might seem. Such a weapon must hold one
or more subcritical fissile masses stable for deployment, than
induce criticality (create a critical mass) for detonation. It also
is quite difficult to ensure that such a chain reaction consumes a
significant fraction of the fuel before the device flies apart. The
procurement of a
nuclear fuel is also
more difficult than it might seem, as no naturally occurring
substance is sufficiently unstable for this process to occur.
One
isotope of
uranium, namely uranium-235, is naturally occurring
and sufficiently unstable, but it is always found mixed with the
more stable isotope uranium-238. The latter accounts for more than
99% of the weight of natural uranium. Therefore some method of
isotope separation based on the
weight of three
neutrons must be performed
to
enrich (isolate)
uranium-235.
Alternatively, the element
plutonium
possesses an isotope that is sufficiently unstable for this process
to be usable. Plutonium does not occur naturally, so it must be
manufactured in a
nuclear
reactor.
Ultimately, the
Manhattan Project
manufactured nuclear weapons based on each of these elements.
They
detonated the first nuclear weapon in a test code-named "Trinity
", near Alamogordo
, New
Mexico
, on July 16, 1945. The test was conducted to
ensure that the
implosion method
of detonation would work, which it did.
A uranium bomb,
Little
Boy
, was dropped on the Japanese city
Hiroshima on August 6, 1945, followed
three days later by the plutonium-based Fat Man
on Nagasaki. In the wake of
unprecedented devastation and casualties from a single weapon, the
Japanese government soon surrendered, ending
World war II.
Since
these
bombings, no nuclear weapons have been deployed offensively.
Nevertheless, they prompted an
arms race
to develop increasingly destructive bombs to provide a
nuclear deterrent.
Just over four years
later, on August 29, 1949, the Soviet Union
detonated its first fission
weapon
. The
United Kingdom
followed on October 2, 1952;
France, on February
13, 1960; and
China
on October 16, 1964. These five powers are permitted to possess
nuclear weapons under the
Nuclear Non-Proliferation
Treaty.
Only four recognized sovereign states are not parties to the
treaty: India
, Israel
, Pakistan
and North
Korea
. India, Pakistan and North Korea have openly
tested and declared that they possess nuclear weapons. Israel has
had a policy of
opacity regarding
its own nuclear weapons program.
North
Korea acceded to the treaty, violated it, and withdrew it in
2003.
Unlike convention weapons, the intense light, heat, and explosive
force is not the only deadly component to a nuclear weapon.
Approximately half of the deaths from
Hiroshima and
Nagasaki died two to five years afterward from radiation
exposure. A
radiological
weapons is a type of nuclear weapon designed to distribute
hazardous nuclear material in enemy areas. Such a weapon would not
have the explosive capability of a fission or fusion bomb, but
would kill many people and contaminate a large area. A radiological
weapon has never been deployed. While considered useless by a
conventional military, such a weapon raises concerns over
nuclear terrorism.
There have been
over 2,000 nuclear
tests conducted since 1945. In 1963, all nuclear and many
non-nuclear states signed the
Limited Test Ban Treaty, pledging to
refrain from
testing nuclear
weapons in the atmosphere, underwater, or in
outer space. The treaty permitted
underground nuclear testing.
France continued atmospheric testing until 1974, while China
continued up until 1980. The last underground test by the United
States was in 1992, the Soviet Union in 1990, the United Kingdom in
1991, and both France and China continued testing until 1996. After
adopting the
Comprehensive
Test Ban Treaty in 1996, all of these states have pledged to
discontinue all nuclear testing. Non-signatories
India and
Pakistan last
tested nuclear weapons in 1998.
Nuclear weapons are the most destructive weapons known - the
archetypal
weapons of mass
destruction. Throughout the
Cold War,
the opposing powers had huge nuclear arsenals, sufficient to kill
hundreds of millions of people. Generations of people grew up under
the shadow of nuclear devastation, portrayed in films such as
Dr. Strangelove and
The Atomic Cafe.
However, the tremendous energy release in the detonation of a
nuclear weapon also suggested the possibility of a new energy
source.
Civilian uses
Nuclear power
Nuclear power is a type of nuclear technology involving the
controlled use of nuclear fission to release energy for work
including propulsion, heat, and the generation of electricity.
Nuclear energy is produced by a controlled nuclear chain reaction
which creates heat—and which is used to boil water, produce steam,
and drive a steam turbine. The turbine is used to generate
electricity and/or to do mechanical work.
Currently nuclear power provides approximately 15.7% of the world's
electricity (in 2004) and is used to propel
aircraft carriers,
icebreakers and
submarines (so far economics and fears in some
ports have prevented the use of nuclear power in transport ships).
All nuclear power plants use fission. Despite years of effort and
the
occasional hoax, no man-made fusion
reaction has produced more energy than it consumed and been a
viable source of electricity.
Medical applications
The medical applications of nuclear technology are divided into
diagnostics and radiation treatment.
Imaging - medical and dental x-ray imagers use of Cobalt-60 or
other x-ray sources.
Technetium-99m
is used, attached to organic molecules, as radioactive tracer in
the human body, before being excreted by the kidneys. Positron
emitting nucleotides are used for high resolution, short time span
imaging in applications known as
Positron emission
tomography.
Radiation therapy is an effective treatment for cancer.
Industrial applications
Oil and Gas Exploration- Nuclear
well logging is used to help predict the
commercial viability of new or existing wells. The technology
involves the use of a neutron or gamma-ray source and a radiation
detector which are lowered into boreholes to determine the
properties of the surrounding rock such as porosity and
lithography.
[14743]
Road Construction - Nuclear moisture/density
gauges are used to determine the density of soils, asphalt, and
concrete. Typically a Cesium-137 source is used.
Commercial applications
An ionization
smoke detector includes
a tiny mass of radioactive
americium-241,
which is a source of
alpha
radiation.
Tritium is used with
phosphor in rifle sights to increase nighttime
firing accuracy. Luminescent exit signs use the same
technology.
Food processing and agriculture
Food irradiation is the process of
exposing food to
ionizing
radiation in order to destroy
microorganisms,
bacteria,
viruses, or
insects that might be present in the food. The
radiation sources used include radioisotope gamma ray sources,
X-ray generators and electron accelerators. Further applications
include sprout inhibition, delay of ripening, increase of juice
yield, and improvement of re-hydration.
Irradiation is a more general term of deliberate
exposure of materials to radiation to achieve a technical goal (in
this context 'ionizing radiation' is implied). As such it is also
used on non-food items, such as medical hardware, plastics, tubes
for gas-pipelines, hoses for floor-heating, shrink-foils for food
packaging, automobile parts, wires and cables (isolation), tires,
and even gemstones. Compared to the amount of food irradiated, the
volume of those every-day applications is huge but not noticed by
the consumer.
The genuine effect of processing food by ionizing radiation relates
to damages to the
DNA, the basic genetic
information for life. Microorganisms can no longer proliferate and
continue their malignant or pathogen activities. Spoilage causing
micro-organisms cannot continue their activities. Insects do not
survive or become incapable of procreation. Plants cannot continue
the natural ripening or aging process. All these effects are
beneficial to the consumer and the food industry, likewise.
It should be noted that the amount of energy imparted for effective
food irradiation is low compared to cooking the same; even at a
typical dose of 10 kGy most food, which is (with regard to warming)
physically equivalent to water, would warm by only about .
The specialty of processing food by ionizing radiation is the fact,
that the energy density per atomic transition is very high, it can
cleave molecules and induce ionization (hence the name) which
cannot be achieved by mere heating. This is the reason for new
beneficial effects, however at the same time, for new concerns. The
treatment of solid food by ionizing radiation can provide an effect
similar to heat pasteurization of liquids, such as milk. However,
the use of the term, cold pasteurization, to describe irradiated
foods is controversial, because pasteurization and irradiation are
fundamentally different processes, although the intended end
results can in some cases be similar.
Food irradiation is currently permitted by over 40 countries and
volumes are estimated to exceed annually world wide.
It should be noted that food irradiation is essentially a
non-nuclear technology; it relies on the use of ionizing radiation
which may be generated by accelerators for electrons and conversion
into bremsstrahlung, but which may use also gamma-rays from nuclear
decay. There is a world-wide industry for processing by ionizing
radiation, the majority by number and by processing power using
accelerators. Food irradiation is only a niche application compared
to medical supplies, plastic materials, raw materials, gemstones,
cables and wires, etc.
Accidents
Nuclear accidents, because of the powerful forces involved, are
often very dangerous. Historically, the first incidents involved
fatal
radiation exposure.
Marie Curie died from
aplastic anemia which resulted from her high
levels of exposure. Two American scientists,
Harry Daghlian and
Louis Slotin, died after mishandling the
same plutonium mass. Unlike convention
weapons, the intense light, heat, and explosive force is not the
only deadly component to a nuclear weapon. Approximately half of
the deaths from
Hiroshima and
Nagasaki died two to five years afterward from radiation
exposure.
Civilian
nuclear
and
radiological accidents
primarily involve nuclear power plants. Most common are nuclear
leaks that exposure workers to hazardous material. A
nuclear meltdown refers to the more serious
hazard of releasing nuclear material into the surrounding
environment.
The most significant meltdowns occurred at
Three Mile
Island
in Pennsylvania
and Chernobyl
in the Soviet
Ukraine
.
Military
reactors that experienced similar accidents were Windscale
in the United Kingdom and SL-1
in the
United States.
Military
accidents usually involve the loss or unexpected detonation of
nuclear weapons.
The Castle Bravo
test in 1954 produced a larger yield than expected,
which contaminated nearby islands, a Japanese fishing boat (with
one fatality), and raised concerns about contaminated fish in Japan. In the 1950s through 1970s,
several nuclear bombs were lost from submarines and aircraft, some
of which have never been recovered. The last twenty years have seen
a marked decline in such accidents.
See also
References
- Henri Becquerel
- The somatic effects of exposure to atomic
radiation: The Japanese experience, 1947–1997
- Nuclear-powered Ships
- Tritium Information
- anon., Food Irradiation - A technique for preserving and
improving the safety of food, WHO, Geneva, 1991
- NUCLEUS - Food Irradiation Clearances
- Food irradiation, Position of ADA, J Am Diet Assoc.
2000;100:246-253.
http://www.mindfully.org/Food/Irradiation-Position-ADA.htm
retrieved 2007-11-15
- C.M. Deeley, M. Gao, R. Hunter, D.A.E. Ehlermann, The
development of food irradiation in the Asia Pacific, the Americas
and Europe; tutorial presented to the International Meeting on
Radiation Processing, Kuala Lumpur, 2006.
http://www.doubleia.org/index.php?sectionid=43&parentid=13&contentid=494
last visited 2007-11-16
- The somatic effects of exposure to atomic
radiation: The Japanese experience, 1947–1997
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