Plutonium oxides. The phase diagram of the plutonium-oxygen system is shown in Fig. 9.2. The observed compounds are the stoichiometric Pu203 and Pu02 and the nonstoichiometric Pu01S2 and Pu161. PuO has also been shown to exist, but only under extreme conditions. No oxide of higher oxidation state than Pu02 has been formed.

Plutonium dioxide is the form of plutonium most commonly specified for fuel for power reactors. It has the same general features already described for pure U02 fuel, such as high melting point, irradiation stability, compatability with metals and with reactor coolants, and ease of preparation. In most designs of plutonium-fueled power reactors the fuel is a mixture of uranium and plutonium oxides.

Pu02 is formed when plutonium or its compounds, except the phosphates, are ignited in air. The most common starting materials are the nitrate or oxalate. Heating Pu(III) or Pu(IV) oxalate at 1000°С in air results in pure crystalline Pu02. The physical appearance of the dioxide depends on its origin, ranging from yellow-black to green and from powder to shiny particles. The Pu02 crystalline density is 11.46 g/cm3. The melting point varies from 2280°C in helium to about 2400°C in air [Cl].

The only other binary oxide of plutonium of practical importance is the peroxide, which is the basis of a process for the purification of plutonium and its conversion to the metal. Addition of H202 to an aqueous plutonium solution first converts plutonium ions to the tetravalent state.

Further addition of peroxide precipitates the plutonium peroxide complex, a nonstoichiometric compound whose composition and crystalline form depend on precipitating conditions. Manage­able hexagonal precipitates are promoted by adding sulfate ions or by precipitating at acidities as high as 4.7 M. The dry peroxide is unstable, decomposing rapidly and sometimes explosively, especially when iron is present. Plutonium peroxide is a stable solid in acid of concentrations up to 5 N. Dry sulfate-free plutonium peroxide can be fluorinated directly at 600°C in HF containing small quantities of oxygen, yielding PuF4 which can be readily reduced to the metal [C1,C2,M1].

Most of the designs for power-reactor fuel utilizing recycled plutonium involve the use of the mixed oxides of plutonium and natural or depleted uranium. The mixed-oxide fuel is formed either from mechanically mixed powders of the individual Pu02 and U02 binary oxides or by calcining a coprecipitated uranium-plutonium compound. A portion of the phase diagram of the uranium-plutonium-oxygen system at 20°C, in the region of U02-U3 08-Pu2 03-Pu02, is shown in Fig. 93 [II, K2]. The phase boundaries deduced for the same region of the uranium-plutonium — oxygen system at 400,600, and 800°C [II] are shown in Fig. 9.4. The mixed uranium-plutonium oxides with the stoichiometric dioxide composition form a continuous solid solution from U02 to Pu02, with the fee fluorite structure, which is stable also at high temperature.

Oxidation of mixed oxides to overall oxygen-to-metal ratios greater than 2, and subsequent cooling to 20° C, results in a two-phase region M02+x + M409 up to the oxygen-to-metal ratio of 2.20 and up to a Pu/(U + Pu) ratio of 0.30. For overall oxygen-to-metal ratios of 2.20 to 2.27, a single phase M409 exists that is stable up to 1000°C. Oxidation of mixed uranium-plutonium oxides containing more than 39 percent plutonium results in the oxidation of uranium from U(IV) to U(V). An equimolar uranium-plutonium oxide forms a single phase of overall composition M409 when all uranium has been oxidized to U(V). At 1400°C a single fluorite phase exists for all plutonium concentrations and for oxygen-to-metal ratios up to 2.27.

The complete miscibility of the stoichiometric uranium-plutonium dioxide results in the simple liquidus-solidus melting-point curves of Fig. 9.5. The curves are consistent with ideal-
solution theory for heats of fusion of 112 ± 13 kcal/mol for U02 and 16.8 ± 13 kcal/mol for Pu02 [II].

The preferred industrial process for manufacturing mixed-oxide uranium-plutonium fuel involves mechanical mixing of U02 and Pu02 powder, followed by compaction and sintering above 1200°C. At the temperatures normally used in commercial sintering of the mixed oxides only a small portion of the sintered material contains the solid solution of U02 — Pu02. Even at temperatures as high as 1400 to 1750°C long sintering times are required for complete homo­genization of the binary oxides.

It is important that the size of the remaining discrete particles of Pu02 be small enough so that fission heat generated in the particles, particularly during rapid power transients, is not suf­ficient to locally overheat the Pu02 particles. This requirement, which is most stringent for mixed-oxide fuel for fast-breeder reactors, is fulfilled by using Pu02 powder with particle sizes of less than about 0.01 cm [Bl].

Another important consideration is the problem of dissolving the mixed-oxide fuel for subse­quent reprocessing and plutonium recovery after the irradiated mixed-oxide fuel has been dis­charged from the reactor. When plutonium dioxide is in solid solution with uranium dioxide at low concentrations, as in the case of plutonium created during the irradiation of uranium dioxide

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fuel, the contained plutonium is soluble in the nitric acid normally used in fuel dissolution. How­ever, pure Pu02 is very difficult to dissolve in nitric acid, unless a fluoride catalyst is used, so problems with the dissolution of fuel containing crystallites of pure plutonia can be expected.

The fabrication and irradiation histories of uranium-plutonium mixed-oxide fuels strongly affect their solubility in nitric acid [L3, G3]. Fuel containing up to 28 percent Pu02 can be dissolved in a few hours in boiling 6 to 10 M HN03, provided that all of the plutonium is in solid solution with the uranium. However, poorly fabricated fuels consist of very small islands of essentially pure plutonia in a matrix of U02-Pu02 solid solution. Consequently, most of the fuel dissolves readily, leaving a refractory residue containing 1 to 10 percent of the plutonium. This residue is readily dissolved in 8 M HNO3-O. O5 M HF. Irradiation improves the solubility of poorly fabricated fuels, but it cannot be depended on to erase the solubility defect [01, L3]. Fuels completely soluble in HN03 have been prepared successfully by methods of coprecipitation, sol-gel, and mechanical mixing of the separate oxides followed by sintering [G3].

Plutonium carbides. Carbides of plutonium and uranium are of interest as high-performance fuel for advanced breeder reactors. As compared with oxide fuel, the higher density of the mono­carbide (U, Pu)C results in higher breeding ratio. Although the carbides cannot operate at as high a temperature as can the oxides, the much greater thermal conductivity of the carbides allows greater heat generation rates in the fuel. However, the technology for carbide fuel is not as far advanced as that for oxide fuel.

Plutonium carbides that have been prepared are PuC, PuC2, and Pu2C3, formed by the reac­tion of graphite with metallic plutonium or PuH3 at high temperature. The products are usually mixtures of PuC and Pu2 C3. Plutonium oxide can also be reduced by carbon, but the stability of Pu(0, C) prevents the formation of plutonium carbide of high purity. PuC2 exists only at tempera­tures above 1750°C. Plutonium monocarbide exists only as a substoichiometric compound, with a crystalline density of 13.58 g/cm3, in the presence of excess carbon. It undergoes peritectic decomposition to the metallic liquid and Pu2C3 at 1654°C [K2].

For reactor fuel, the ternary uranium-plutonium-carbon monocarbide is prepared by reduc­tion of (U, Pu)02 with graphite [FI], by melting a uranium-plutonium alloy with graphite, or by melting separately prepared individual carbides in an electric arc [K2]. Even though at low temperatures UC exists in the stoichiometric composition, the need for excess carbon for the existence of PuC limits the region of PuC-UC miscibility to a maximum of 35 a/о (atom percent) plutonium at room temperature. At higher plutonium concentrations the excess carbon is pre­cipitated as Pu2C3.

The unirradiated mixed carbide (U, Pu)C readily hydrolyzes in water or acid, but neutron irradiation profoundly reduces the tendency toward hydrolysis.

Plutonium nitride. Unlike the corresponding uranium-nitrogen system, only the one plutonium nitride PuN exists. It is prepared by heating plutonium hydride in nitrogen at 250 to 400°C, by reacting plutonium metal with a hydrogen-ammonia mixture at 600° C, or by direction reaction of molten plutonium with nitrogen at 1000°C. Plutonium nitride forms solid solutions with UN. However, because of the appreciable volatility and dissociation of PuN at temperatures at about 1600°C and above, the ternary (U, Pu)N is less attractive as a nuclear fuel than pure UN [K2, S4].

Plutonium hydrides. Plutonium hydrides are made by reacting plutonium metal with hydrogen at atmospheric pressure and at temperatures between 50 and 300°C, forming a series of hydrides up to PuH3. Plutonium hydride is a useful intermediary in the formation of other plutonium compounds.

Plutonium halides. Table 9.18 lists plutonium halides together with some of their more significant properties.

PuF3 and PuF4 are important intermediates in the production of plutonium metal. The trifluoride is made by reacting Pu02 with a mixture of HF and H2 at 600° C:

Pu02 + 3HF + jH2 -+• PuF3 + 2H2 О (9.37)

The tetrafluoride is made by reacting oxide or oxalate in HF at 550°C, in the presence of oxygen to prevent reduction of tetravalent plutonium:

Pu02 + 4HF -*■ PuF4 + 2H2 О (938)

The volatile plutonium hexafluoride can be prepared by fluorination of the tetrafluoride at 550°C:

PuF4 + F2 -+ PuF6 (939)

or by direct fluorination of the oxide. In contrast to UF6, PuF6 does not sublime when heated at atmospheric pressure. It first melts, at 51.59°С, and then boils at 62.16°C. Its triple point is 51.59°C and 533 Ton. The vapor pressure of PuF6 above liquid PuF6) in the range from 51.59 to 77.17°C, is given by [Cl]

і 007 5

logiop(Torr) = — — 1.5340log, о T+ 12.14545 (9.40)

Unlike the stable uranium hexafluoride, which has a negative free energy of formation, plutonium hexafluoride is thermodynamically unstable. It dissociates to F2 and the relatively nonvolatile PuF4 , although the rate of thermal decomposition is very low at room temperature. If the specific alpha activity of plutonium is equivalent to that of239 Pu, the rate of decomposition of solid PuF6 at room temperature is controlled by radiolytic decomposition, amounting to 1.5 percent per day [W2].

Plutonium also forms the ternary oxyhalides PuOF, Pu02 F2, PuOCl, PuOBr, and PuOI.