|
|
|
|
Nuclear technology
|
| |
|
| |
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.
896, Henri Becquerel was investigating phosphorescence in uranium salts when he discovered a new phenomenon which came to be called radioactivity.
Discussion
Ask a question about 'Nuclear technology' Start a new discussion about 'Nuclear technology' Answer questions from other users |
Encyclopedia
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
Discovery
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 several distinct sorts, which they called alpha rays, beta rays and gamma rays.
Some of these kinds of radiation could pass through ordinary matter, and all of them could cause damage 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 any number of patent medicines and treatments involving radioactivity were put forward.
Gradually it came to be realized that the radiation produced by radioactive decay was ionizing radiation, and that 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 atomic nuclei are unstable, and can decay releasing energy in the form of gamma rays (high-energy photons), alpha particles (a pair of protons and a pair of neutrons) and beta particles, high-energy electrons.
Nuclear fission
Radioactivity is generally a slow and difficult process to control, and is unsuited to building a weapon. However, other nuclear reactions are possible. In particular, a sufficiently unstable nucleus can undergo nuclear fission, breaking into two smaller nuclei and releasing energy and some fast neutrons. This neutron could, if captured by another nucleus, cause that nucleus to undergo fission as well. The process could then continue in a nuclear chain reaction. Such a chain reaction could release a vast amount of energy in a short amount of time. When discovered on the eve of World War II, it 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 power.
Nuclear fusion
Nuclear fusion technology was initially pursued only in theoretical stages during World War II, when scientists on the Manhattan Project (led by Edward Teller) investigated the possibility of using the great power of a fission reaction to ignite fusion reactions. It took until 1952 for the first full detonation of a hydrogen bomb to take place, so-called because it utilized reactions between deuterium and tritium, isotopes of hydrogen. Fusion reactions are much more energetic per unit mass of fusion material, but it is much more difficult to ignite a chain reaction than is fission.
Research into the possibilities of using nuclear fusion for civilian power generation was begun during the 1940s as well. Technical and theoretical difficulties have hindered the development of working civilian fusion technology, though research continues to this day around the world.
Nuclear Weapons
The design of a nuclear weapon is more complicated than it might seem; it is quite difficult to ensure that such a chain reaction consumes a significant fraction of the fuel before the device flies apart. The construction of a nuclear weapon 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. Thus a complicated and difficult process of isotope separation must be performed to obtain 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.
The first atomic bomb was detonated in a test code-named "Trinity", near Alamogordo on July 16, 1945. After much debate on the morality of using such a horrifying weapon, two bombs were dropped on the Japanese cities Hiroshima and Nagasaki, and the Japanese surrender followed shortly.
Several nations began nuclear weapons programs, developing ever more destructive bombs in an arms race to obtain what many called a nuclear deterrent. 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.
However, the tremendous energy release in the detonation of a nuclear weapon also suggested the possibility of a new energy source.
Nuclear Power
Types of nuclear reaction
Most natural nuclear reactions fall under the heading of radioactive decay, where a nucleus is unstable and decays after a random interval. The most common processes by which this can occur are alpha decay, beta decay, and gamma decay. Under suitable circumstances, a large unstable nucleus can break into two smaller nuclei, undergoing nuclear fission.
If these neutrons are captured by a suitable nucleus, they can trigger fission as well, leading to a chain reaction. A mass of radioactive material large enough (and in a suitable configuration) 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. However, if the mass is critical only when the delayed neutrons are included, 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.
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, and results in the formation, in stellar nucleosynthesis, of the light elements, from lithium to calcium, as well as some formation of the heavy elements, beyond Iron and Nickel, which cannot be created by nuclear fusion, via neutron capture - 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 obtaining controlled fusion power has so far proved elusive. Controlled fusion can be achieved in particle accelerators; this is how many synthetic elements were produced. The Farnsworth-Hirsch Fusor is a device which can produce controlled fusion (and which can be built as a high-school science project), albeit at a net energy loss. It is sold commercially as a neutron source.
The vast majority of everyday phenomena do not involve nuclear reactions. Most everyday phenomena only involve gravity and electromagnetism. Of the fundamental forces of nature, they are not the strongest, but the other two, the strong nuclear force and the weak nuclear force are essentially short-range forces so they do not play a role outside the atomic nucleus. Atomic nuclei are generally kept apart because they contain positive electrical charges and therefore repel each other, so in ordinary circumstances they cannot meet.
Nuclear Accidents
Examples of Nuclear Technology
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).
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.
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 2.5 °C.
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 500 000 metric tons 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.
External links
|
| |
|
|
