Plutonium alloys

The Preparation of Plutonium Aluminum and other Plutonium Alloys. 

Another important group of alloys involved in martensitic transformation is represented by several plutonium alloys. The martensitic nature, for instance, of the 6 to a transformation has been clearly established in Pu-Ga and Pu-Al alloys and a behaviour similar to that shown in Fig. 5.30 has been observed. For a systematic description of plutonium alloys, the stability of the different phases and their transformations see Hecker (2000). 

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 plutonium alloy is in the shape of an ordinary table napkin ring in order to make it possible to remove the heat developed in the nuclear fission reaction by a water-cooling process. 

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... 

Jan. 22, 1957 Plutonium Alloy and Method of Separating it from Uranium F.H. Spedding T.A. Butler... 

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. 

R. Kraft, C. J. Wensrich and A. L. Langhorst, "Chemical Analysis of Plutonium and Plutonium Alloys Methods and Techniques," Lawrence Radiation Laboratory, University of California, Livermore, Calif. UCRL-6873, 1962. 

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. 

The uranium-neptunium and uranium-plutonium alloys show a total miscibility in the liquid phase and in the A2-solid solution, moreover they exhibit phases with structures different from those of the pure components in a rather large range of composition in the middle of the phase diagram. 

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. 

Moreover physics concerning plutonium and its alloys involves predicting its properties under long-term aging in both weapons and storage environnement. The knowledge of all plutonium properties is a major challenge and first-principles studies of pure plutonium, alloys, and finite temperature simulations are needed. 

As discussed in the introduction, the understanding of equilibrium between Ga and PurGa or A1 and PusAl with temperature is one of the main goal in the study of plutonium alloys. Given the crystal structure of an ordered compound, one can calculate its equilibrium properties and the more stable structure between a few possible different crystal forms at 0 K. However, it is necessary to treat solid solutions and to know the evolution of the structural stability as a function of temperature. In this study, the basic tool to determine such properties is based on a generalized three-dimensional Ising model. 

Current capabilities on DOE reservations SRS— powder metallurgy processing and extrusion LANL—plutonium alloy casting None... 

The thermal analysis of the pin-type reference fuel element was repeated, using an assumed thermal conductivity of 11 Btu/(hr)(sq ft)(F/ft) for the uranium-plutonium alloys and a maximum alloy temperature of 1200F. 

As would be expected, lower performance resulted. The Argonne Metallurgy Division is preparing samples of uraniupi-plutonium alloy and will measure the thermal conductivity. 

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. 

The limitations of motallurgical knowledge at present lead to the conclusion that tantalum will be one of the be.st container materials for those plutonium alloys. The high-temperature strength properties and the heat-transfer properties of tantalum are excellent moreover, it is weldable. The parasitic capture cross section of tantalum would be intolerable in an epithermal or thermal power breeder reactor and, although relatively large in a fast spectrum, its effect on neutron economy in a fast reactor can be made small, if not minor, by careful design. 

---