9.2 Difficulties

Production of uranium metal sufficiently pure for use in nuclear reactors is difficult. Uranium forms very stable compounds with oxygen, nitrogen, and carbon, and it reduces the oxides of many common refractories. Methods that yield uranium metal at temperatures below its melting point result in a fine powder that oxidizes rapidly in air and is difficult to consolidate into massive metal. Uranium cannot be deposited electrolytically from aqueous solution. It is not practical to purify uranium by distillation because of its very high boiling point, 3900° C. Any nonvolatile impurities introduced into uranium during production will remain in it during subsequent operations and contaminate the final product.

9.3 Alternative Methods

Four methods that have been used to produce uranium metal are

1. Electrolysis of fused salts

2. Reduction of UOj

3. Reduction of UF4

4. Reduction of UCI4

Electrolysis. Electrolysis of KUF$ or UF4 dissolved in a molten mixture of 80 percent Cad2 and 20 percent NaG was the method used by the Westinghouse Electric Company to produce the first pure uranium metal for the Manhattan Project [S4]. Because electrolysis was carried out below the melting point of uranium metal, 1130°C, the crude metal contained salt that had to be leached with water and had to be remelted before acceptably pure metal could be obtained. By 1943 this method was superseded by the less costly reduction of UF4 by magnesium, to be described later.

Possible reductants. Elements that might be considered for reducing U02, UF4, or UQ4 to metallic uranium are hydrogen, sodium, magnesium, or calcium. Carbon is impractical because of formation of uranium carbide, and aluminum is undesirable because it forms an intermetallic compound with uranium. Sodium, magnesium, and calcium do not do this.

To show which combinations of uranium compound and reductant are thermodynamically favorable, the ftee-energy change in reducing U02, UF4, or UQ4 by hydrogen, sodium, calcium, or magnesium has been evaluated in Table 5.29. These data are for a temperature of

1500 К (1227°C), which is high enough for rapid reaction and is above the melting point of uranium, 1130°C, so that the uranium product would be consolidated rather than a fme powder. Part 1 of Table 5.29 lists the free energy of formation of 1 mol of each of the uranium compounds from its elements and the free energy of formation of the number of gram-moles of the oxide, fluoride, or chloride of the four possible reductants needed to produce 1 g-mol of uranium. Part 2 of Table 5.29 lists the free-energy change in reaction of each possible combination of uranium compound and reductant. For example, the free-energy change in the reaction

UF4 + 2Mg -*• U + 2MgF2

is —407,219 — (—351,544) =—55,675 cal/g-mol uranium (5.16)

For reduction of the uranium compound to be complete without requiring a large excess of reductant, the free-energy change at 1500 К should be more negative than 10,000 cal/g-mol. Table 5.29 shows that hydrogen is completely impractical and that the only feasible reductant for U02 is calcium. For UF4 or UC14, sodium, magnesium, or calcium meet the free-energy criterion.

Reduction of U02. Production of metallic uranium by reacting U02 with calcium metal is thermodynamically possible and was practiced in Germany in 1942 [SI]. However, the melting point of CaO is so high, 2615°C, and the heat of reaction is so small, 44 kcal/g-mol uranium, that it is impossible to melt the lime to make a clean separation between it and uranium metal. The result is that the uranium product is in the form of small particles and the recovery of clean uranium is usually no more than 35 to 40 percent.

Reduction of halides. Table 5.30 lists pertinent properties of substances that might take part in the reduction of the uranium halides UF4 or UC14. As this table shows, the heat available per mole of uranium produced is much higher than in the reduction of U02. In addition, the melting point of the halide by-product is much lower than that of CaO and is near that of uranium metal. Consequently, the reaction temperature can be raised enough to melt both the uranium metal and the halide by-product, so that a clean separation between metal and slag can be obtained.

Production of reactive metals by reduction of the tetrachlorides is the basis of the well-known Kroll process for titanium or zirconium (Chap. 7). Although metallothermic reduction of uranium tetrachloride is thermodynamically possible for uranium also, its use in practice is made difficult by the hygroscopic character of UC14. This salt picks up water from moist air, which would contaminate uranium metal with uranium oxide after reduction. Moreover, the low boiling point of UC14 (789°C) relative to the higher melting point of uranium (1130°C) means that the UC14 would have to be fed into the reaction zone as vapor, a complication avoided with UF4, whose boiling point is higher (1457°C). For these reasons, UF4 is generally used.

Of possible reductants for UF4, sodium is less desirable than magnesium or calcium because, like UC14, its boiling point is far below that of uranium metal. Use of sodium would require either feed of sodium vapor from an external source or operation of the reactor at very high pressure. Choice of reductant for UF4 thus is essentially limited to calcium or magnesium. In practice magnesium is used for production of large batches of uranium, with calcium being used for smaller quantities, as when criticality considerations limit batch size. For example, at the French refinery at Malvesi [B5], calcium is used to produce uranium in small batches, under 100 kg, with magnesium used for larger batches. The same general practice is followed in the United States. Magnesium reduction has been used in the United States since 1943, when F. H. Spedding and co-workers at Iowa State College developed the process for the Manhattan Project [S4] to supersede fused-salt electrolysis.

The advantages of calcium include the following:

1. The reaction can be carried out at atmospheric pressure, because the melting point of calcium fluoride is below the boiling point of calcium metal.

2. The heat of reaction is sufficient to melt both uranium metal and CaF2 slag, with reactants initially at room temperature, so that preheating is unnecessary.

The advantages of magnesium include the following:

1. Magnesium costs much less than calcium, and only 60 percent as much mass of reductant is needed.

2. It is easier to obtain magnesium of the requisite purity than calcium, and magnesium does not pick up oxygen from air or moisture.

The problems in using magnesium are these:

1. The reaction must be carried out in a sealed reactor to contain the superatmospheric vapor

pressure of magnesium developed when the reactants are heated to the melting point of MgF3.

2. It is necessary to preheat the charge because the heat of reaction is too small to melt the reaction products when the reactants are initially at room temperature.