Isotope separation plants

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

The licensing process consists of two steps construction and operating license that must be completed before fuel loading. Licensing covers radiological safety, environmental protection, and antitru,st considerations. Activities not defined as production or utilization of special nuclear material (SNM), use simple one-step. Materials Licenses, for the possession of radioactive materials. Examples are uranium mills, solution recovery plants, UO fabrication plants, interim spent fuel storage, and isotopic separation plants. 

A practical isotope separation plant can operate at neither minimum reflux (where the separation is zero, but the rate of production is high), nor at minimum number of stages (where the rate of production is zero, but the separation is high). A compromise is required. Since optimum reflux varies with stage number it is customary to employ tapered cascades for isotope separation. This results in marked savings in material hold-up, and in plant size and investment. 

Our brief discussion of cascade principles serves to demonstrate the critical dependence of the size and operating costs of isotope separation plants on the elementary separation factor c. The size and initial cost are proportional to c 2. The operating cost is less sensitive to c, but varies at least as c The economic importance of these factors is readily seen in context with the separation of In 1960 the USAEC had three gaseous diffusion plants in operation. The cost of each plant was approximately 1 billion dollars the power consumption in each plant was 1,800,000 kw. If the plants were to be built with processes or equipment giving separation factors one half the one used, the additional construction cost to the U.S. taxpayers would be nine billion dollars. The increase in the annual operating costs of the plants can be conservatively estimated from the increase in the reflux ratio or power consumption to be 100,000,000/yr. This is a realistic demonstration of the economic benefits and importance of fundamental research and development to society. 

Uranium (ca. 20% is used as the fuel, but mainly with 239pu in the form of a UO2/PUO2 mixture. The breeding blanket consists of depleted uranium from isotope separation plants or from reprocessing plants for spent nuclear fuels. Axially movable boron carbide absorbers are distributed in the fuel zone for shutting down purposes. The uranium utilized can be ca. 100 times better utilized than e.g. in light-water reactors. 

Manhattan Project highly resistant materials for isotope separation plants, lubricants for gas centrifuges, coolants... 

A second Welsh chemical warfare establishment was at Rhy-dymwyn, near Mold in Clwyd. Here, the Ministry of Supply built a gas factory which was joined, in 1942., by an even more secret installation an isotope-separation plant, part of the British project to create an atom bomb. The atomic plant employed over one hundred people, supervised by twenty Oxford scientists from the Clarendon Laboratory. Employees from one site were not allowed into the other, but as workers at both had to carry gas masks it was assumed by the local inhabitants that they were all engaged on the same project this, it was rumoured, was a scheme to manufacture synthetic rubber. 

Under some conditions it is economically attractive or environmentally preferable to reprocess spent fuel in order to (1) recover uranium to be recycled to provide part of the enriched uranium used in subsequent lots of fuel, (2) recover plutonium, and (3) reduce radioactive wastes to more compact form. In part II of Fig. 1.11 the recovered 0.83 percent enriched uranium is recycled and the 244 kg of plutonium recovered per year is stored for later use in either a light-water reactor or a fast-breeder reactor. This recycle of uranium to the isotope separation plant reduces the annual UaOg feed rate to 220 short tons, still appreciably greater than for the heavy-water reactor. 

The smallest element of an isotope separation plant that effects some separation of the process material is called a separating unit. Examples of a single separating unit are one stage of a mixer-settler, one plate of a distillation column, one gas centrifuge, one calutron, or one electrolytic cell. 

Some relations for isotope separation plants are simpler when expressed as weight, mole, or atom ratios, defined as the ratio of the fraction of one component to the fraction of a second. These ratios are denoted by Greek letters f,, or r) for feed, depleted, or enriched stream, corresponding to z, x, or y. In a two-component mixture, these ratios are defined as the ratio of the fraction of the desired component to that of the other component. For example, in a tails stream, the weight, mole of atom ratio for a two-component mixture is... 

The minimum number of stages increases as the overall separation increases and as the separation factor approaches unity. Because both these conditions hold in a typical isotope separation plant, the minimum number of stages is often very large. For example, in a gaseous diffusion plant (a = 1.00429) making product containing 90 percent and tails 0.3 percent. 

One type of tapered plant that is easy to treat theoretically, which has minimum interstage flow for a specified separation, and which is approximated by all isotope separation plants designed for minimum cost, is the so-called ideal cascade. An ideal cascade is one in which... 

This result is of great importance for isotope separation plants. It states that the total flow in the plant is the product of two factors, the first a function only of the heads separation factor p, and the second a function only of the flow rates and composition of feed, product, and tails. 

These formulas are extraordinarily useful in roughing out the characteristics of an isotope separation plant without the necessity of designing every one of its stages, which often number in the thousands. As an illustration, the total heads flow rate in the uranium isotope separation example considered in Fig. 12.17 is... 

The separative capacity is analogous to the heat duty of an evaporator or other process equipment. The separation potential is analogous to the enthalpy per mole of the streams entering or leaving an evaporator. Calculations of material balances and separative capacity in an isotope separation plant are made in similar fashion to conventional material and heat balances. A form for such calculations is illustrated in Table 12.8, which illustrates the calculation of the separative capacity of an isotope separation cascade producing 1 mol/day of at 0.80 mole... 

Most large isotope separation plants have so much flexibility that their separative capacity can be kept very nearly constant under moderately changed conditions. 

In many isotope separation plants the initial cost of the plant is proportional to the separative capacity of the plant and the annual operating costs are proportional to the amount of separative work done per year. In such cases the annual charges for plant investment plus aimual operating costs exclusive of feed, in dollars per year, equal Dcs, where Z) is the annual separative capacity in kilograms of uranium per year and cs is the unit cost of separative work, in dollars per kilogram of uranium of separative work units ( /kg SWU). If kg of feed is charged per year at a unit cost of cp /kg, the total annual cost c is... 

Figure 12.23 shows the nomenclature to be used in describing the operation of an isotope separation plant during the transient period in which it is approaching steady-state performance. Figure 12.24 represents qualitatively the way tails and product flow rates and compositions will change with time during this transient period. Compositions are represented by a scale linear in In [x/(l -x)]. 

In some isotope separation plants, notably those using distillation or exchange processes, it is more economic to use a constant interstage flow rate over a considerable composition interval rather than a flow rate that decreases steadily from the feed point to the product ends, as is characteristic of an ideal cascade. Cohen [C3] has called such cascades squared-off cascades and has derived equations for their separation performance. This section summarizes the derivation for a close-separation, squared-off cascade. 

A uranium isotope separation plant has been operating as an ideal cascade to produce 200 kg of U/day in product containing 3.2 w/o while stripping tails to 0.2 w/o, from natural uranium feed containing 0.711 w/o U. 

Distillation-cascade design principles. Some of the principles involved in designing an isotope separation plant for minimum cost will be iUustiated by roughing out optimum conditions for a water distillation plant incorporating the two improvements noted above. 

Uranium isotope separation plants may be fed either with natural uranium, which contains only the isotopes and in nearly fixed proportions or uranium discharged from a... 

Because of the possibility of natural depletion of U and because of the availability of tails from isotope separation plants that might become mixed with natural uranium, it is important that natural uranium feed for an isotope separation plant be analyzed for its U content. 

Table 14.27. Estimated characteristics of uranium metal laser isotope separation plants...

The flrst U.K. uranium isotope separation plant, built in the early 1950 s, was based on gaseous difiusion. This phenomenon depends on the observation that the rate of passage of molecules through a membrane is inversely proportional to the square root of the molecular weight. The application of this principle in a cascade stage requires a compressor, membrane, control valve, and cooler. 

Third, the transition to nuclear economy will be possible only if vast amounts of fissionable material will be available when needed. Some such fissionable material will come from isotope separation plants. However, the large inventories necessary for the huge nuclear fueled power industry of the future can be conveniently obtained only from breeders which produce fissionable material well ahead of the time when these materials will be needed for power production. In fact, several generations of breeders fueled by materials produced by earlier breeders will be necessary to produce the inventories for the critical period. 

Eq. 9 Is of major Importance for estimating the size and cost of an Isotope separation plant. It Indicates that the total flow Is a product of two factors the first of these, proportional to l/(a-l) Is a function only of the elementary separation factor which Is determined by the separation process used. The second factor [In square brackets], which Is usually called the separative duty or separative work units (S.W.U.) Is a function only of quantities and concentrations of feed, product, emd waste. It has the same dimensions as those used for the quantities of material, and Its value Is Independent of the process used to accomplish the separation task. The significance of the magnitude of the elementary factor Is Immediately apparent a two-fold reduction In (a-1) requires an Increase In the total flow by a factor of 4. Since for a gaseous diffusion process, the total flow rate Is closely related to the total area of porous barriers, the total pumping capacity and the total power consumption required, all the associated costs vary proportionately. 

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