nuclear reactor Device that can initiate and control a self-sustaining series of nuclear-fission reactions. Neutrons released in one fission reac¬ tion may strike other heavy nuclei, causing them to fission. The rate of this chain reaction is controlled by introducing materials, usually in the form of rods, that readily absorb neutrons. Typically, control rods made of cadmium or boron are gradually inserted into the core if the series of fissions begins to proceed at too great a rate, which could lead to melt¬ down of the core. The heat released by fission is removed from the reac¬ tor core by a coolant circulated through the core. Some of the thermal energy in the coolant is used to heat water and convert it to high-pressure steam. This steam drives a turbine, and the turbine’s mechanical energy is then converted into electricity by means of a generator. Besides pro¬ viding a valuable source of electric power for commercial use, nuclear reactors also serve to propel certain types of military surface vessels, sub¬ marines, and some unmanned spacecraft. Another major application of reactors is the production of radioactive isotopes that are used extensively in scientific research, medical therapy, and industry.
Nuclear Regulatory Commission (NRC) U.S. independent regu¬ latory agency that oversees the civilian use of nuclear energy. Established in 1974 to replace the Atomic Energy Commission, the NRC licenses the
construction and operation of nuclear reactors and other facilities and the ownership and use of nuclear materials. It issues standards, rules, and regulations for the maintenance of licenses, and it regularly inspects nuclear facilities to ensure compliance with public health and safety, envi¬ ronmental quality, national security, and antitrust laws. The NRC also investigates nuclear accidents, conducts public hearings, and reviews power-plant operations. Its commissioners are appointed by the president of the U.S.
nuclear species See nuclide
Nuclear Test-Ban Treaty officially Treaty Banning Nuclear Weapons Tests in the Atmosphere, in Outer Space and Under Water Treaty that prohibits all tests of nuclear weapons except those conducted underground. U.S.-Soviet test-ban talks began after con¬ cerns arose in the 1940s and ’50s about the dangers of radioactive fallout from above-ground nuclear tests. These talks made little progress until the Cuban missile crisis of 1962. In the following year, Britain, the U.S., and the Soviet Union signed the treaty, and more than 100 other govern¬ ments soon followed. France and China were notable nonsignatories. In 1996 the treaty was replaced by the Comprehensive Test-Ban Treaty, which will not take effect until it is signed by all 44 countries with nuclear power plants. India refuses to do so on the ground that the treaty lacks disarmament provisions and permits nonexplosive testing. See also Nuclear Non-proliferation Treaty.
nuclear weapon or atomic weapon or thermonuclear weapon Bomb or other warhead that derives its force from nuclear fis¬ sion, nuclear fusion, or both and is delivered by an aircraft, missile, or other system. Fission weapons, commonly known as atomic bombs, release energy by splitting the nuclei of uranium or plutonium atoms; fusion weapons, known as hydrogen bombs or thermonuclear bombs, fuse nuclei of the hydrogen isotopes tritium or deuterium. Most modern nuclear weapons actually combine both processes. Nuclear weapons are the most potent explosive devices ever invented. Their destructive effects include not only a blast equivalent to thousands of tons of TNT but also blinding light, searing heat, and lethal radioactive fallout. The number of nuclear weapons reached its peak in the 1980s, when the U.S. had some 33,000 and the Soviet Union 38,000. Since the end of the Cold War both coun¬ tries have decommissioned or dismantled thousands of warheads. Other declared nuclear powers are the United Kingdom, France, China, India, and Pakistan. Israel is widely assumed to possess nuclear weapons, and North Korea, Iran, and Iraq often have been accused of seeking to build them. Some countries, such as South Africa, Brazil, and Argentina, have acknowledged pursuing nuclear weapons in the past but have abandoned their programs. See also Nuclear Non-proliferation Treaty; Nuclear Test- Ban Treaty.
nuclear winter Environmental devastation that some scientists con¬ tend would result from a nuclear war. The basic cause, as hypothesized, would be huge fireballs created by exploding nuclear warheads, which would ignite great fires (firestorms). Smoke, soot, and dust would be lifted to high altitudes and driven by winds to form a uniform belt encircling the Northern Hemisphere. The clouds could block out all but a fraction of the Sun’s light, and surface temperatures would plunge for as much as several weeks. The semidarkness, killing frosts, and subfreezing tempera¬ tures, combined with high doses of radiation, would interrupt plant pho¬ tosynthesis and could thus destroy much of the Earth’s vegetation and animal life. Other scientists dispute the results of the original calculations, and, though such a nuclear war would undoubtedly be devastating, the degree of damage to life on the Earth remains controversial.
nucleic \nu-'kle-ik, nu-‘kla-ik\ acid Any of the organic compounds making up the genetic material of living cells. Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Their transmission from one generation to the next is the basis of heredity. The two main types, DNA and RNA, are composed of similar materials but differ in structure and function. Both are long chains of repeating nucle¬ otides. The sequence of purines and pyrimidines (bases)— adenine (A), gua¬ nine (G), cytosine (C), and either thymine (T; in DNA) or uracil (U; in RNA)—in the nucleotides, in groups of three (triplets, or codons), con¬ stitutes the genetic code.
nucleolus See nucleus
nucleophile \ , nu-kle-3- 1 fil\ Atom or molecule that contains an electron pair available for bonding and in chemical reactions therefore seeks a posi-
© 2006 Encyclopaedia Britannica, Inc.
nudeoprotein ► Nullarbor Plain I 1387
tive centre, such as the nucleus of an atom or the positive end of a polar molecule (see covalent bond; electric dipole). In the Lewis electron theory (see acid-base theory), advanced by the U.S. chemist Gilbert Lewis ( 1 875— 1946) in 1923, nucleophiles are by definition Lewis bases. Examples include the hydroxide ion (OH - ), the ions of the halogens chlorine, bro¬ mine, and iodine (Cl - , Br - , and I - , respectively), ammonia (NH 3 ), and water (H 2 0). See also base; electrophile.
nudeoprotein V.nii-kle-o-'pro-.tenX Macromolecular complex consist¬ ing of a protein linked to a nucleic acid, either DNA or RNA. The proteins that combine with DNA are generally of characteristic types called his¬ tones and protamines. The resulting nucleoproteins (deoxynucleoproteins) make up the chromosomes of living cells. Many viRUSes are little more than organized agglomerations of deoxynucleoproteins. Many specific RNA nucleoproteins are also known; they have diverse cellular functions.
nucleoside \ , nu-kle-a- l sld\ Any of a class of organic compounds, including structural subunits of nucleic acids. Each consists of a molecule of a five-carbon sugar (ribose in RNA, deoxyribose in DNA) and a nitrogen-containing base, either a purine or a pyrimidine. The base uracil occurs in RNA, thymine in DNA, and adenine, guanine, and cytosine in both, as part of the nucleosides uridine, deoxythymidine, adenosine or deoxy- adenosine, guanosine or deoxyguanosine, and cytidine or deoxycytidine. Nucleosides usually have a phosphate group attached, forming nucleotides. Usually obtained by decomposition of nucleic acids, nucleosides are important in physiological and medical research. Those that are not part of nucleic acids include puromycin and certain other antibiotics produced by fungi.