Uranium, natural

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 23su while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 

Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. 

Planet pluto) Plutonium was the second transuranium element of the actinide series to be discovered. The isotope 238pu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley, California. Plutonium also exists in trace quantities in naturally occurring uranium ores. It is formed in much the same manner as neptunium, by irradiation of natural uranium with the neutrons which are present. 

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. 

It is not necessary that there be two isotopes in both the sample and the spike. One isotope in the sample needs to be measured, but the spike can have one isotope of the same element that has been produced artificially. The latter is often a long-lived radioisotope. For example, and are radioactive and all occur naturally. The radioactive isotope does not occur naturally but is made artificially by irradiation of Th with neutrons. Since it is commercially available, this last isotope is often used as a spike for isotope-dilution analysis of natural uranium materials by comparison with the most abundant isotope ( U). 

Uranium hexafluoride is used in the gaseous diffusion process for the separation and enrichment of uranium-235, which exists in low concentration in natural uranium. The enriched UF is converted back into an oxide and used as fuel for the nuclear power industry. 

World annual uranium requirements in 1993 were estimated at about 58,382 t natural uranium equivalent. Reactor-related requirements are expected to rise about 1015 t/yr on the average, reaching 75,700 t U total requirements in the year 2010. The cumulative aggregate world uranium requirements for the period 1993—2010 are estimated to be about 1.185 X 10 t U metal (29). 

De-enrichment of HEU from approximately 93% to 3% can be accompHshed using the depleted tails from the original enrichment process. These tails contain on the average 0.20% U. The de-enrichment of 11 of HEU uses 32 t of tads, yielding approximately 33 t of fuel having an enrichment of 3% U. Producing the same amount of 3% enriched uranium from natural sources would requite approximately 180 t of natural uranium metal. Therefore, 1 t of HEU is equivalent to 180 t of natural uranium. 

The amount of HEU that becomes avadable for civdian use through the 1990s and into the twenty-first century depends on the number of warheads removed from nuclear arsenals and the amount of HEU in the weapons complex that is already outside of the warheads, ie, materials stockpdes and spent naval reactor fuels. An illustrative example of the potential amounts of weapons-grade materials released from dismanded nuclear weapons is presented in Table 7 (36). Using the data in Table 7, a reduction in the number of warheads in nuclear arsenals of the United States and Russia to 5000 warheads for each country results in a surplus of 1140 t of HEU. This inventory of HEU is equivalent to 205,200 t of natural uranium metal, or approximately 3.5 times the 1993 annual demand for natural uranium equivalent. 

Canadian reactors (CANDU) are fueled using natural uranium. The discharged fuel contains small amounts of plutonium, but the fissile uranium content is below that of natural uranium. Therefore, the irradiated fuel is not normally considered a candidate for economic reprocessing. 

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. 

Over the years, a variety of fuel types were employed. Originally, natural uranium slugs canned in aluminum were the source of plutonium, while lithium—aluminum alloy target rods provided control and a source of tritium. Later, to permit increased production of tritium, reactivity was recovered by the use of enriched uranium fuel, ranging from 5—93%. 

The GANDU Reactors. The Canadian deuterium uranium (CANDU) reactors are unique among power reactors in several respects. Heavy water is used as moderator natural uranium having 235u... 

The determination of critical si2e or mass of nuclear fuel is important for safety reasons. In the design of the atom bombs at Los Alamos, it was cmcial to know the critical mass, ie, that amount of highly enriched uranium or plutonium that would permit a chain reaction. A variety of assembhes were constmcted. Eor example, a bare metal sphere was found to have a critical mass of approximately 50 kg, whereas a natural uranium reflected 235u sphere had a critical mass of only 16 kg. 

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. 

The electromagnetic separation plant built during World War 11 at Oak Ridge, involved two types of calutrons, alpha and beta. The larger alpha calutrons were used for the enrichment of natural uranium, and the beta calutrons were used for the final separation of U from the pre-enriched alpha product. For the electromagnetic separation process, UO was converted into UCl [10026-10-5] with CCl. The UCl was fed into the calutron for separation. The calutron technique has been used to separate pure samples of and stable isotopes of many other elements. The Y-12 calutron... 

Production in Fission of Heavy Elements. Tritium is produced as a minor product of nuclear fission (47). The yield of tritium is one to two atoms in 10,000 fissions of natural uranium, enriched uranium, or a mixture of transuranium nucHdes (see Actinides and transactinides Uranium). 

Natural uranium consists mostly of and 0.711 wt % plus an inconsequential amount of The United States was the first country to employ the gaseous diffusion process for the enrichment of the fissionable natural uranium isotope. During the 1940s and 1950s, this enrichment appHcation led to the investment of several bUHon dollars in process faciHties. The original plants were built in 1943—1945 in Oak Ridge, Teimessee, as part of the Manhattan Project of World War II. 

The Canadian Deuterium Uranium reactor fissions with natural uranium, hence, no dependence on national or international fuel enrichment facilities that are needed to enrich uranium to about 3% U-235 to achieve criticality with light water moderation. 

Natural uranium consists almost entirely of the oi emitters " U and As " - U decays more than six times fastet than (Fig. 31.1) the proportion of is very slowly hut inexorably decreasing with time. Prioi to 1972, all analyses of naturally occurring uranium had shown this proportion to be notably constant at 0.7202 O.OOObfr. In that year, however, workers at the French Atomic F.ncrgy laboratories in Pierrelatte [terformiiig routine mass speetroinctiie analyses recorded a value of 0.7171%. The difference was small but significant. 

Fuels. Although the concentration of in natural uranium is sufficient to sustain a chain reaction, its effective dilution by the fuel... 

Some nuclear fission reactors are designed to use natural uranium having 0.7 percent and 99.3 percent CANDU reactors, manufactured in... 

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