Metal Plutonium, alloys

Plutonium is by far one of the most toxic radioactive poisons known. The metal, its alloys, and its compounds must be handled in a shielded and enclosed glove box that contains an inert argon atmosphere. It is a carcinogen that can cause radiation poisoning leading to death. 

Misch metal, an alloy of cerium with other lanthanides is a pyrophoric substance and is used to make gas lighters and ignition devices. Some other applications of the metal or its alloys are in solid state devices rocket propellant compositions as getter in vacuum tubes and as a diluent for plutonium in nuclear fuel. 

The metallurgical properties of metallic plutonium are even more unfavourable than those of uranium. The melting point of Pu is 639 °C and six solid phases are known. Furthermore, the critical mass of a reactor operating with pure Pu as fuel is below 10 kg, and it would be very difficult to take away the heat from such a small amount of material. A great number of plutonium alloys have been investigated with respect to their possible use as nuclear fuel, but they have not found practical application. 

The solubilities of uranium, plutonium, and thorium in magnesium at 650°C are 0.002 wt %, 55 wt %, and 44 wt %, respectively. Thus, assuming no solute interaction, uranium is essentially insoluble in magnesium, while plutonium is quite soluble and good separation may be effected. While precipitation of an insoluble phase from solution would appear to be a straightforward process, the behavior of a solute in a given metal or alloy may differ from its behavior when influenced by the inclusion of other solutes. One element may increase or suppress the solubility of another through coprecipitation or intermetallic compound formation. Such effects must be determined experimentally. 

The main objective of this phase is to convert plutonium feed materials into plutonium oxide for immobilization in Phase 2. The feed materials include clean and impure oxides, clean and impure metals and alloys, and various fresh fuels. The basic steps include ... 

Farkas, M.S., Storhok, V.W, Pardue, W.M., Smith, R.A., Veigel, N.D., Miller,N.E., Wright, T.R., Bames, R.H., Chubb, W., Lemmon, A. W., Berry, W.E., Rough, F.A, Fuel and Fertile Materials -Uranium Metal and Alloys - Plutonium - Thorium - Metal-Ceramic Fuels - Coated-Paiticle Fuel Materials - Uranium and Thorium Oxides - Uranium Carbides, Nitrides, Phosphides, Sulfides and Arsenides - Fuel-Water Reactions , Reactor Mater., 9(3), 151-165 (1966) (Assessment, Electr. Prop., Meehan. Prop., Phys. Prop., Transport Phenomena, 77)... 

For example, as early as in 1957 for the pulse reactor IBR-30, a core fi om metal fuel-plutonium alloy was manufactured. In 1957-1965 a fuel was produced in the form of plutonium dioxide for the BR-5 and IBR-2 reactors. 

Plutonium forms refractory compounds with A-subgroup metals and metalloids, but only the 5 and phases exhibit an affinity for solid solutions. Most 5-phase solid solutions can be retained at room temperature by rapid quenching. Significant solid solutions in the other Pu phases are rare Np and Pu are mutually soluble in the a phase, and Th and U mix with Pu in both the P and y phases. Alpha-phase plutonium is highly reactive with oxygen, while 8-phase plutonium alloys are not as reactive. 

Plutonium alloys can be prepared by melting and mixing the constituent metals however, it is possible to introduce oxides or halides of plutonium to a melt of the alloying element if it is sufficiently reducing, and may remove the need for an inert atmosphere while producing the alloy. Important 5-phase weapon alloys are made by adding Pup3 to molten Ga or Al. 

The thermodynamic data, Gibbs energies, enthalpies and entropies of formation of intermetallic compounds have been obtained from a literature search. We have also consulted the handbook Selected values of thermodynamic properties of binary alloys by Hultgren et al. (1973a) and a compilation of thermodynamic data on transition metal based alloys done by de Boer et al. in 1988. For the actinide-based alloys a literature search and a critical analysis of the data was done by Rand and Kubaschewski (1963) for uranium compounds, by Rand et al. (1966) for plutonium alloys, by Rand et al. (1975) for thorium alloys, and more recently by Chiotti et al. (1981) for binary actinide alloys. We have included in our review the data obtained from the original publications and also the assessed data of Chiotti et al. (1981) when they were different. 

A first look at figs. 1-7 indicates that rare earths and actinides behave like early transition metals. When alloyed with elements of columns IB to VB the phase diagrams exhibit many intermetallic compounds. The exceptions are of interest uranium possesses a miscibility gap in the liquid state when alloyed with Cu, Ag, Au, Zn, Cd, Pb and Bi even if intermetallic compounds are found in the solid state (except in the Ag-U system) plutonium also has a miscibility gap in the liquid state when alloyed with Ag and Cd. 

The last chapter (134) in this volume is an extensive review by Colinet and Pasturel of the thermodynamic properties of landianide and actinide metallic systems. In addition to compiling useful theiTnodynamic data, such as enthalpies, entropies, and free eneigies of formation and of mixing, the authors have made an extensive comparative analysis of the thermodynamic behavior of the rare earths and actinides when alloyed with metallic elements. They note that when alloyed with non-transition metals, the enthalpies of formation of uranium alloys are less negative than those of the rare earths while those of thorium and plutonium are about the same as the latter. For transition metal alloys the formation enthalpies of thorium and uranium are more negative than diose of the rare earths and plutonium (the latter two are about the same). The anomalous behaviors of cerium, europium and ytterbium in various compounds and alloys are also discussed along with the effect of valence state changes found in uranium and plutonium alloys. 

Other alloys of plutonium which are more dilute in fuel and have not too unreasonable melting temperatures are the magnesium-plutonium and bismuth-plutonium alloys. The spatial dilution of fuel atoms alleviates the high power density problem but, unfortunately, these alloys have melting temperatures significantly higher than the transition metal alloys. 

Container materials. A material capable of being fabricated into various shapes and resistant to high-temperature corrosion by the fuel alloy is a necessity if practical use is to be made of the low melting temperature plutonium alloys. Since the transition metals readily form low melting point alloys with plutonium, the normal constructional materials, steels and nickel alloys, are eliminated. 

Full advantage of the neutron production by plutonium requires a fast reactor, in which neutrons remain at high energy. Cooling is provided by a hquid metal such as molten sodium or NaK, an alloy of sodium and potassium. The need for pressurization is avoided, but special care is required to prevent leaks that might result in a fire. A commonly used terminology is Hquid-metal fast-breeder reactor (LMFBR). 

Preparation of Plutonium Metal from Fluorides. Plutonium fluoride, PuF or PuF, is reduced to the metal with calcium (31). Although the reactions of Ca with both fluorides are exothermic, iodine is added to provide additional heat. The thermodynamics of the process have been described (133). The purity of production-grade Pu metal by this method is ca 99.87 wt % (134). Metal of greater than 99.99 wt % purity can be produced by electrorefining, which is appHcable for Pu alloys as well as to purify Pu metal. The electrorefining has been conducted at 740°C in a NaCl—KCl electrolyte containing PuCl [13569-62-5], PuF, or PuF. Processing was done routinely on a 4-kg Pu batch basis (135). 

Within the general trend in the behavior across the actinide series, their alloys, and their metallic compounds from superconductors to local moment magnets, the only serious irregularity occurs in some plutonium compounds. These compounds should be magnetic but turn out to be temperature independent paramagnets. 

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. 

Other Pyrochemical Processes. The chemistry of pyrochemi-cal separation processes is another fertile area of research e.g., new molten salt systems, scrub alloys, etc. and the behavior of plutonium in these systems. Studies of liquid plutonium metal processes should also be explored, such as filtration methods to remove impurities. Since Rocky Flats uses plutonium in the metal form, methods to convert plutonium compounds to metal and purify the metal directly are high-priority research projects. 

PUCI3, and MgCl2 to form a 50/50 mole % NaCl-CaCl salt phase and a molten Am-Pu-Mg-Ca alloy which is immiscible in the above salt(lO). After cooling, the metal phase is cleaved away from the salt phase and the salt phase is analyzed. Little, if any, Am or Pu remains in the salt phase and the salt residues can be discarded to waste. Metal recovery begins by evaporating magnesium and calcium from the residual metal button at about 800°C in vacuum. The americium can then be distilled away from the plutonium in a vacuum still operated at 1200°C, using yttria ceramic vessels to contain the molten metal fraction. The bottoms fraction contains the plutonium which is recycled back into the main plutonium stream. 

The anode residues must be chemically processed to recover the plutonium remaining in the residues. This may amount to about 10% of the feed mass if delta alloy is the feed metal. Either aqueous or pyrochemical processes may be used for anode recovery. One pyrochemical process used for recovery utilizes oxidation of the plutonium with zinc chloride to form plutonium chloride salt, followed by calcium reduction of the PUCI3 contained in the salt phase to produce pure plutonium metal (the impurities follow the zinc metal obtained from the oxidation reaction and are discarded to waste). Impurities more stable than calcium chloride remain in the salt phase and are also... 

CSC atomization was developed by AEA Harwell Laboratories in the UK in the early 1970 s. Initially, the CSC process was used for the atomization of refractory and oxide materials such as alumina, plutonium oxides, and uranium monocarbide in nuclear fuel applications. Since it is well-suited to the atomization of reactive metals/alloys or those subject to segregation, the CSC process has been applied to a variety of materials such as iron, cobalt, nickel, and titanium alloys and many refractory metals. The process also has potential to scale up to a continuous process. 

A kind of summary of the similarities which, albeit with some uncertainties, may be evidenced between the single lanthanide and actinide metals is reported, according to Ferro et al. (2001a) in Fig. 5.13. According to this scheme the alloying behaviour of plutonium could be simulated by cerium whereas a set of similarities may especially be considered between the block of elements from praseodymium to samarium with those from americium to californium. 

Plutonium can form compounds with many nonmetals such as oxygen, the halogens, and nitrogen. It can also be used as an alloy with other metals. A few examples of plutonium compounds exhibiting the oxidation states of +3 and +4 follow ... 

Magnesium—nickel hydride, 4458 Plutonium(III) hydride, 4504 Poly(germanium dihydride), 4409 Poly(germanium monohydride), 4407 Potassium hydride, 4421 Rubidium hydride, 4444 Sodium hydride, 4438 f Stibine, 4505 Thorium dihydride, 4483 Thorium hydride, 4535 Titanium dihydride, 4484 Titanium—zirconium hydride, 4485 Trigermane, 4415 Uranium(III) hydride, 4506 Uranium(IV) hydride, 4536 Zinc hydride, 4486 Zirconium hydride , 4487 See COMPLEX HYDRIDES, PYROPHORIC MATERIALS See entry LANTHANIDE—TRANSITION METAL ALLOY HYDRIDES... 

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