Natural nuclear reactor

Although other technologies are now coming into use for this purpose, gaseous diffusion has played an important role in the enrichment of uranium for use in nuclear reactors. Natural uranium is mostly 2, which cannot be fissioned to prodnce energy. It contains only about 0.7% of the fissionable nuclide IfU. For uranium to be useful as a nuclear fuel, the relative amount of IfU must be increased to abont 3%. In the gas diffusion enrichment process, the natnral nraninm (containing IfU and a small amount of 9iU) reacts with fluorine to form a mixtnre of and UFg. Because these... 

Uranium is a silvery metal. As it is chemically reactive, it tarnishes in air. It is attacked by steam and acids but not by aUcabs. Due to its radioactivity and accessibibty, the element is the main fuel in nuclear reactors. Natural uraruum consists of 99.3% of the isotope and 0.7% of the fissile isotope... 

Nuclear reactors use radioactive fuels with atoms that can be easily split when struck by the proper energy neutrons. and Pu are the two most popular fuel materials, and the simplest to use within a nuclear reactor. Naturally occurring uranium contains only a small percentage of typically about 0.7 % of the total mass of natural uranium is Even though this means that IP is relatively rare in ordinary uranium samples, it is possible to produce a reactor that uses uranium in its natural state as the fuel source. For example, Fermi s Chicago Pile 1 used naturally occurring uranium as its fuel. However, most of the world s power reactors use enriched uranium in their core. Enriched fuels contain IP at concentrations of between 3.5 and 5 % (US Nuclear Regulatory Commission, n.d.a). 

In 1934, scientists discovered that when they bombarded natural bismuth (209Bi) with neutrons, 210Bi, the parent of polonium, was obtained. Milligram amounts of polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors. 

By far of greatest importance is the isotope Pu2sy with a half-life of 24,100 years, produced in extensive quantities in nuclear reactors from natural uranium 23su(n, gamma) —> 239U—(beta) —> 239Np—(beta) —> 239pu. Fifteen isotopes of plutonium are known. 

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

The high cost of isotope separation has limited, the use of separated isotopes in nuclear reactors to specific cases where substitutes that do not involve separated isotopes are not available. The most important example is that of uranium-235 [15117-96-17, the most abundant naturally occurring... 

Boron-10 has a natural abundance of 19.61 atomic % and a thermal neutron cross section of 3.837 x 10 m (3837 bams) as compared to the cross section of 5 x 10 m (0.005 bams). Boron-10 is used at 40—95 atomic % in safety devices and control rods of nuclear reactors. Its use is also intended for breeder-reactor control rods. 

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

Uranium-235 Enrichment. The enrichment of uranium is expressed as the weight percent of in uranium. For natural uranium the enrichment level is 0.72%. Many appHcations of uranium requite enrichment levels above 0.72%, such as nuclear reactor fuel (56,57). Normally for lightwater nuclear reactors (LWR), the 0.72% natural abundance of is enriched to 2—5% (9,58). There are special cases such as materials-testing reactors, high flux isotope reactors, compact naval reactors, or nuclear weapons where enrichment of 96—97% is used. 

Uranium hexafluoride [7783-81-5], UF, is an extremely corrosive, colorless, crystalline soHd, which sublimes with ease at room temperature and atmospheric pressure. The complex can be obtained by multiple routes, ie, fluorination of UF [10049-14-6] with F2, oxidation of UF with O2, or fluorination of UO [1344-58-7] by F2. The hexafluoride is monomeric in nature having an octahedral geometry. UF is soluble in H2O, CCl and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The importance of UF in isotopic enrichment and the subsequent apphcations of uranium metal cannot be overstated. The U.S. government has approximately 500,000 t of UF stockpiled for enrichment or quick conversion into nuclear weapons had the need arisen (57). With the change in pohtical tides and the downsizing of the nation s nuclear arsenal, debates over releasing the stockpiles for use in the production of fuel for civiUan nuclear reactors continue. 

Separation of Hafnium. Zirconium and hafnium always occur together in natural minerals and therefore all zirconium compounds contain hafnium, usually about 2 wt % Hf/Hf + Zr. However, the only appHcations that require hafnium-free material are zirconium components of water-cooled nuclear reactors. 

Beryllium has a high x-ray permeabiUty approximately seventeen times greater than that of aluminum. Natural beryUium contains 100% of the Be isotope. The principal isotopes and respective half-life are Be, 0.4 s Be, 53 d Be, 10 5 Be, stable Be, 2.5 x 10 yr. Beryllium can serve as a neutron source through either the (Oi,n) or (n,2n) reactions. Beryllium has alow (9 x 10 ° m°) absorption cross-section and a high (6 x 10 ° m°) scatter cross-section for thermal neutrons making it useful as a moderator and reflector in nuclear reactors (qv). Such appHcation has been limited, however, because of gas-producing reactions and the reactivity of beryUium toward high temperature water. 

The recognition in 1940 that deuterium as heavy water [7789-20-0] has nuclear properties that make it a highly desirable moderator and coolant for nuclear reactors (qv) (8,9) fueled by uranium (qv) of natural isotopic composition stimulated the development of industrial processes for the manufacture of heavy water. Between 1940 and 1945 four heavy water production plants were operated by the United States Government, one in Canada at Trail,... 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystalline material, the development of neutron diffraction as a useful analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailability of neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. 

Polonium, because of its very low abundance and very short half-life, is not obtained from natural sources. Virtually all our knowledge of the physical and chemical properties of the element come from studies on Po which is best made by neutron irradiation of in a nuclear reactor ... 

Phenomene d Oklo, Proceedings of a Symposium on the Oklo Phenomenon, International Atomic Energy Agency, Vienna, Proceedings Series, 1975. Natural Fission Reactions, IAEA, Vienna, Panel Proceedings Series STI/PUB/475, 1978, 754 pp. R. West, Natural nuclear reactors. J. Chem. Ed. 53, 336-40 (1976). 

Proponents of the laboratories counter that, despite these shortcomings, the laboratories seiwe a vital mission of undertaking the high risk and expensive investments that the private sector would never agree to invest in. Although natural gas research and development was minimal, DOE support accelerated technological advances on natural gas-fired turbines. Much of the research and development at the laboratories has provided a net social benefit to the nation and economy, work such as safe nuclear reactors and the development of sophisticated defense weapons. 

Nuclear fission is also involved in nuclear weapons. To create a bomb, the concentration of the isotope uranium-235 must be increased to at least 85 percent from its natural concenti ation of only 0.7 percent. This increase ot concentration is difficult and expensive. In a typical nuclear reactor the uranium-235 concentration in the fuel is only 3 to 4 percent, and hence a nuclear reactor cannot explode like a bomb. In a nuclear bomb... 

A nuclear power plant generates electricity in a manner similar to a fossil fuel plant. The fundamental difference is the source of heat to create the steam that turns the turbine-generator. A fossil plant relies on the combustion of natural resources (coal, oil) to create steam. A nuclear reactor creates steam with the heat produced from a controlled chain reaction of nuclear fission (the splitting of atoms). 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

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