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Appendix D Plutonium Because both plutonium and uranium are fissionable by slow neutrons and are used in nuclear weapons, there is ~ tendency to think that they have similar physical and chemical properties, but this is not the case. Both are silvery metals, with freshly exposed surfaces resembling iron or nickel in appearance, and both have densities approximately 50 percent greater than lead. Beyond these similarities, the two elements differ widely in their properties. Plutonium is harder and more brittle than uranium, and has a melting point some 500°C (900°F) lower. Although both are relatively easily oxidized, plutonium is much more reactive chemically, and in air it is readily oxidized to plutonium dioxide, PuO2, the most common fond of plutonium in the environment Two useful reference works covering the properties and chemist of plutonium are Cleveland (1976) and Comar et al. (1976). Uranium is less reactive, but it, too, is oxidized, producing a variety of oxides. In contrast to plutonium, which does not exist in nature to a significant degree, uranium occurs naturally in a number of chemical and mineral forms. Plutonium dissolves more readily in acids, and once dissolved particularly in nitric acid its chemistry is so different from that of uranium that the two elements can be chemically separated from each other. Simply stated, the chemical differences between the two elements in solution result primarily from the differences in electrical charges on their ions. Because their ions behave differently, they may be separated from each other by a process known as solvent extraction. Plutonium is produced in nuclear reactors by the irradiation of uranium-238 with neurons emitted in the fission of uranium-235. (Natural uranium contains 99.3 percent uranium-238, O.7 percent uranium-235.) After discharge from the 118
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APPENDIX D 119 reactor and storage for several months to allow the short half-life fission products (produced by the fission of uranium-235) to decay, the uranium must tee processed to remove the few hundred parts per million of plutonium product. The irradiated uranium is dissolved in concentrated nitric acid After suitable adjustments, this nitric acid solution, containing uranium, plutonium, and fission products, is contacted with an immiscible organic solution of ~ibutyl phosphate (TBP) in a diluent that is essentially a highly purified kerosene fraction. The uranium and plutonium are extracted into the organic phase, leaving the fission products in the nitric acid solution. After additional extraction to remove residual uranium and plutonium, this solution is sent to waste treatment and storage; it is the primary source of high-level waste in the DOE weapons complex. The organic solution containing the plutonium and uranium is first contacted with a more dilute nitric acid solution containing a"reducing agent" to decrease the electric charge on the plutonium ions, so that they are extracted, leaving only the uranium in the organic phase. The uranium is then extracted into a very dilute nitric acid solution. The nitric acid solution of plutonium is further purified and concentrated by ion exchange, a process in which the plutonium is selectively sorbed onto beds of organic resin while impurities remain in solution and pass through the bed. The plutonium is then removed from the resin (eluted) with dilute nitric acid. This solvent extraction procedure is known as the PUREX process, and it is used with minor modifications at Hanford, SRS, and ~EL. The process can achieve separation factors of uranium from plutonium of greater than 10 million, and of plutonium from uranium of 1 million. Decontamination of fission products from plutonium exceeds 100 million. Recovery of both plutonium and uranium is about 99.9 percent An additional advantage of the PUREX process is the solid waste minimization: because the primary chemical used in the process is nitric acid, a volatile liquid, it can be removed by evaporation, leaving only a small volume of solid waste. The organic solution of TBP in kerosene, after a simple cleanup step, can be reused. Preparation of plutonium metal from the nitric acid solution is accomplished by one of several conversion processes which are based on similar chemistry. All three involve precipitation reactions and all require the use of hydrogen fluoride (HF), either as a gas or in aqueous solution. Plutonium is precipitated from the nitric acid solution as the oxalate, peroxide, or trifluoride (the latter only at SRS, using an aqueous solution of HF). After drying, the oxalate or peroxide is converted to PuO2 by heating in a stream of air. [The trifluoride is converted into a mixture of PuO2 and plutonium te~fluoride (E>uF) by heating in air.] The PuO2 is then heard in a stream of gaseous HE to convert it to PUF4, which can be reduced to plutonium metal by reaction with metallic calcium in a pressure vessel. (The PuF4-PuO2 mixture produced from the tnfluoride precipitate is reduced directly to metal without reaction with gaseous HF.) Reduction yields average 97 to 98 percent for PuF and about 95 Dercent for the PuF.-PuO. The calcium —4 4 ~
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120 APPENDIX D fluoride reduction slags are dissolved and reprocessed to recover the residual plutonium. Plutonium metal scrap, calcium fluoride induction slags, reduction crucibles, and plutonium-contain~ng incinerator ash are dissolved in concentrated nitric acid and purified usually by ion exchange. The purified solution is then treated by one of the conversion processes described above to produce the metal. The choice of PuF,, as the plutonium compound for reduction is based on several favorable factors. The large amount of heat released in the reaction of PUF4 with calcium, combined with the relatively low melting point of the resulting calcium fluoride slag, results in a low viscosity medium that allows plutonium aggregation and thus enhanced yield. In addition, Puff, unlike plutonium mchloride, another possible reduction candidate, does not absorb appreciable moisture from the air. (Reduction of compounds with a high moisture content results in excessive PuO2 formation and lower yield of metallic plutonium.) The principal disadvantages of using PllF4 are the high neutron fluxes it produces as a result of alpha reactions with fluoride ions, the corrosiveness and toxicity of the aqueous or gaseous HF used to produce it, and the need to use an aluminum salt (typically the nitrate) in dissolving the calcium fluoride slag, thus increasing the volume of solid wastes. Alternative conversion processes have been studied with varying degrees of success. The nitric acid solution of plutonium may be evaporated and the solid plutonium nitrate converted directly to PuO2 by heating in air. This procedure, known as direct denigration, is not promising: it tends to produce gummy residues, and the product PuO2 is inert toward either reaction with HE or direct reduction with calcium. It appears likely that the existing processes involving precipitation and calculation to produce PuO2 as an intermediate will be retained for the foreseeable future. It is in the subsequent trea~nent of PuO2 that viable alternatives exist. Calcium can reduce PuO2 directly to metal, but there are problems because the heat evolved is lower than for PUF4 reduction and the calcium oxide has a higher melting point. The slag is not melted by the heat of reaction, and as a result finely dispersed metal is produced. This problem has been overcome, however, by the use of a molten calcium chloride flux to dissolve the calcium oxide slag and allow the product plutonium to coalesce. The process has found production application at LANL. Impurities can sometimes be removed from plutonium metal without resort to aqueous processing. Often americium~he impurity of most concern can be removed froth plutonium in recycled weapons by molten-salt extraction using, for example, a stadium chlonde-potassium chloride salt containing a few percent plutonium bichloride: the americium, being more reactive, goes into the salt phase and is replaced in the metal phase by more plutonium. Impure metal also may be purified by molten-salt elec~oref~ning procedures using similar salt mixtures. Judicious use of these nonaqueous procedures can, In many cases, simplify processes and increase efficiency, and safety.
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APPENDIX D 121 Because plutonium reacts with the air win the evolution of heat and because it is a poor conductor of heat, it can be pyrophoric, that is, it can spontaneously ignite in air, particularly when in the fonn of lathe turnings, which have relatively high surface area and poor contact between individual turnings. Such conditions can promote the build-up of a "hot spot" in a small area that can exceed the ignition temperate of the metal. Several serious fires in the weapons complex have started in this manner. To prevent their recurrence, current practice calls for handling potentially ignitable plutonium in enclosures with a low-oxygen atmosphere. Since plutonium reacts so readily with the air, it is rarely, if ever, found in the metallic form in the environment. Thus the properties of PuO2, the common environmental form, are most relevant when attempting to assess the behavior of plutonium. Plutonium dioxide can vary in color from tan to olive green to black, depending on purity and conditions of formation; it should be noted, however, that it is not observed in the environment in quantities anywhere near large enough for its color to be perceived by the eye. Typically, when it is present in soils, for example, it is in the form of a relatively small number of microscopic particles. The density of PuO2 is high compared to that of most chemical compounds, but only slightly more than half that of the metal. Nevertheless, individual particles, depending on how they were formed, can vary considerably in density and in aerodynamic properties. Panicles are frequently very small and can be subject to short-range atmospheric dispersion under suitable climatic conditions. The dispersion will be spatially nonunifonn, but even a small isolated panicle can emit appreciable radiation. These factors combine to cause high variability in soil contamination analyses: whether a given soil sample contains high radioactivity or no detectable activity whatever may depend on whether it contains a single "hot particle." Plutonium dioxide is normally quite insoluble in water and in body fluids (with a few exceptions as noted below); it is even less soluble when formed at high temperature, as in a fire. Hence its dispersion in soil is primarily by mechanical means. It can also be blown along the surface by the wind ("saltation"~. It can be washed downward into the soil column by natural factors, and it can be spread both horizontally and vertically by plants and animals. Some limited dissolution of PuO2 can occur in ocean water and in ground-waters with chemical compositions that enhance plutonium volubility, but this does not generally occur in domestic groundwaters because of Heir low chemical contents. The low solubility of PuO2 in body fluids has several rarniD~caiions. Uptake through the gastrointestinal system is small, since PuO2 is poorly absorbed through the intestinal walls. The most serious modes of entry are inhalation and the contamination of wounds. Once in the body, plutonium can be difficult to remove. Inhaled PuO2 can be lodged in the lungs for considerable periods of time, and ultimately it works its way into the lymph nodes. Plutonium entering the blood stream through a contaminated wound ultimately deposits in the liver or the bone marrow: in the latter site it can be especially harmful to the blood-forming
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122 APPENDIX D process. Some success has been achieved in the removal of plutonium Mom body systems by flee use of chemicals lmown as chelating agents that can dissolve it and allow it to be excreted from the body. Such treatments are more effective when administered soon afte, contamination, before the plutonium has been `'fixed" in the body. The comparable uranium compound, UO2 is similar in density to PuO2, but it is considerably more soluble. Because the common fonn~uranium-23X and uranium-23Ware much less radioactive than plutonium, the radiotoxicity of uranium is lower. In fact, the primary hazard of uranium ingestion it tends to concentrate in We kidneys~s chemical ("heavy metal poisoning') rather than radiological.
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