235U-enrichment

Depleted uranium (DU)—Uranium having a percentage of 235U smaller than the 0.7% found in natural uranium. It is obtained as a by-product from 235U enrichment. 

Photolysis is carried out on IR laser irradiation and is isotope selective reaction, as the molecules containing 238U turn to be more sensitive to its action and the samples become finally 235U-enriched. 

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

U enrichment is described in In the reactor core, shown in Fig. 21.15, uranium that has been enriched... 

Uranium-plutonium mixed oxides Uranium-plutonium mixed oxides (MOX) are becoming increasingly important, since plutonium is produced during the reprocessing of spent fuel elements. In these mixed oxide fuel elements a mixture of uranium(IV) and plutonium(IV) oxides with a plutonium content of 3 to 4% is utilized instead of ca. 4% 235u-enriched uranium(IV) oxide. Such fuel elements have similar nuclear physical properties to the standard elements with and can therefore be used in their place. 

The raw material for nuclear reactor fuel, uranium, exits the mining—milling sequence as uranium oxide. Because of its color, it is called yellow cake. The yellow cake is converted to uranium hexafluoride and enriched in 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. 

One of the early popular low power research and training reactors was the AGN-201, suppHed by Aerojet General Nuclear. This is a homogeneous sohd fuel reactor, consisting of a mixture of polyethylene and uranium at 20% enrichment in The core 235U... 

Cost and Value of Plutonium. The cost of building all U.S. nuclear weapons has been estimated as 378 biUion in 1995 dollars (24). If half of this sum is attributed to U.S. weapons-grade plutonium production (- lOOt), the cost is 1.9 x 10 /kg of weapons-grade Pu. Some nuclear weapons materials (Be, enriched U, Pu) also have value as a clandestine or terrorist commodity. The economic value of reactor-grade plutonium as a fuel for electric power-producing reactors has depended in the past on the economic value of pure 235u... 

Enriched Uranium—Uranium in which the abundance of the 235U isotope is increased above normal. 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

When a lies close to 1 the minimum reflux ratio is large, but since Xj varies with stage number so does [n(i+1)/P]MiN- At the feed point in a 235U plant enriching to 90% 235U, (nf/P)MiN is 29,100, but at the product end of the cascade it approaches zero. 

For the 90% 235U example, taking h = 1 s, tp 30 days. For squared-off cascades tp is longer because more enriched material is required in the upper stages. 

In any uranium separation process the work of enrichment increases rapidly with 235U content in the product. Because the price of natural uranium varies widely with time and location (and fluctuating government subsidies) it is useful to distinguish between the price of the feed and the value added by the separative process. For example, the purchaser himself might provide the feed and then pay only for the separative work required to make the desired product. Separative work is defined in Equation 8.7. 

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