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. There is a great deal of public concern about its possible implications, and every application of nuclear technology is reviewed with care.

History

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 they can decay, releasing energy (in the form of gamma rays, high-energy photons) and nuclear fragments (alpha particles, a pair of protons and a pair of neutrons, and beta particles, high-energy electrons).

During World War II, nuclear reactions were sufficiently well understood that all the factions began to see the possibility of constructing a nuclear weapon. Nuclear reactions release far more energy per reaction than chemical reactions, so if large numbers of reactions could be induced to occur at once, tremendous amounts of energy could be released. The British and the Americans set up the Manhattan Project under the direction of Robert Oppenheimer to build such a device.

Radioactivity is generally a slow and difficult to control process, 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, case 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. 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 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", at 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.

After the end of World War II, the nations that could afford to began nuclear weapons programs, developing ever more destructive bombs in an arms race to obtain what they called a nuclear deterrent. 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 plants were built to generate household electric power. Nuclear submarines were built, able to travel at speed while submerged for months at a time. Nuclear ships were built, primarily aircraft carriers, although a few icebreakers were built. Research projects were started into the possibility of nuclear-powered aircraft and nuclear thermal rockets.

The first generations of nuclear reactors were built to produce power; safety was a secondary consideration. However, as more nuclear reactors were built, it became clear that they were complex devices in which failures were extremely dangerous. Early safety features were primarily concerned with the exposure of operators to intense radiation. However, it was gradually realized that the release of radioactive material into the environment, called radioactive contamination, was also potentially serious. Radioactive isotopes of common elements are chemically very similar to non-radioactive isotopes, so the human body may take up the radioactive materials and deposit them in the bones, thyroid, lungs, or elsewhere. The radioactive materials then decay in place, often leading to cancer.

As the appreciation for the dangers of radioactive contamination increased, people became more and more concerned about accidents at nuclear plants - and there were many, as the technologies matured (see this list of nuclear accidents).

One severe early accident, the Three Mile Island incident, coincided with the release of a disaster film "The China Syndrome", and the news reports caught the public imagination. People began to view nuclear power with alarm. The nuclear power industry began to improve its safety measures.

In 1986, the Chernobyl accident shocked the world when an old, poorly-designed nuclear reactor exploded. It burned for days, killed many people, and contaminated a large area, rendering it unusable for centuries. This more or less cemented the public view of nuclear technology as dangerous.

Types of nuclear reaction

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, these are the weakest, but 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.

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.

Nuclear fission normally releases fast neutrons. 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 is the way all elements heavier than helium were produced. Because of the very strong repulsion of nuclei, fusion is difficult to achieve in a controlled fashion. Fusion 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 artificial 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.

Major current applications

• Nuclear weapons of various designs can release tremendous destructive power. Some have been designed to level cities (up to the largest, Tsar Bomba), while other designed have looked at smaller nuclear weapons, for nuclear artillery, nuclear land mines, and nuclear bunker-busting missiles. International agreements attempt to regulate nuclear testing and limit nuclear proliferation.
• Nuclear medicine is the application of nuclear technology to medicine. This includes the use of radiation to obtain images of the inside of a living body, as well as to destroy cancer. Radioactive tracers are used to probe the motion of elements on the body.
• Nuclear power is the application of nuclear technology to generate power. This includes both large nuclear power plants and smaller, safer, radioisotope thermoelectric generators. The latter have been used on a number of spacecraft as compact, light, long-term power sources.
• Ionizing radiation is readily generated by radioactive decay. Nuclear technology is often used to construct gamma ray or neutron sources. This ionizing radiation can be useful in killing cancerous cells, or in sterilizing food and water (this process is generally known as irradiation). During the 2001 anthrax attacks, the US Postal Service irradiated mail in an effort to kill potential anthrax spores (although they did not in fact use a radioactive source).
• A number of consumer items use nuclear technology:
  • Smoke detectors often contain americium; they detect smoke because it reduces the ability of alpha radiation to ionize air in the detector's ionization chamber.
  • Glowing watch dials and gunsights (now) often contain tritium, whose decay triggers phosphorescence in a pigment on the dial. During World War II they often contained radium instead.