Uranium Alloy

any of the uranium-base alloys, which are used as nuclear fuel in metal fuel elements. The use of pure uranium, which has three allotropic modifications, is limited because of the element’s poor mechanical properties. Uranium alloys subjected to heat treatment have higher ultimate strengths and creep strengths than pure uranium; they also exhibit a higher resistance to corrosion and a lower tendency to undergo deformation when exposed to radiation and variations in temperature. The significant improvement in the properties of uranium obtained by introducing other elements results from the formation of solid solutions or intermetallic compounds, which usually strengthen the metal through precipitation hardening when the additives are present in low concentrations.

The elements used in the alloys should have minimal neutron-capture cross sections so that the charge of enriched uranium in the reactor can be reduced. Special attention is devoted to the compatibility of alloys with the material of the protective layer at working temperatures and to the ability of the alloys to undergo processing.

Uranium alloys are by convention divided into two groups. Alloys with elements having low solubility in the α, β, and γ phases of uranium, such as Al, Be, Fe, Si, Ta, and Cr, constitute one group. The other group comprises the alloys made with elements having high solubility in the γ phase. These elements include Nb, Zr, Ti, Pu, and Hf (miscible in all proportions) and Mo, V, and Re (solubility exceeding 10 percent on the basis of individual atoms).

In alloys based on natural or slightly concentrated uranium and having a low concentration of additives, a martensite structure of the supersaturated solid solution of the α phase is obtained upon hardening. The structure of the γ phase is obtained by hardening uranium alloys having a relatively high content of additives. These alloys retain their mechanical strength well at elevated temperatures and have marked corrosion resistance when exposed to water at high pressures and temperatures. Items made of these alloys do not alter their shape and dimensions upon irradiation. Binary and ternary uranium alloys, mainly with Mo, Zr, Al, Nb, and Cr, have the greatest practical importance. The introduction of approximately 3 percent Mo (by weight) entirely precludes the formation of the β phase. In alloys containing more than 7 percent Mo (by weight) the γ phase, which is metastable at room temperature and which has a body-centered cubic lattice and isotropic properties, is easily stabilized. The addition of 1–2 percent Zr (by weight) significantly strengthens uranium and reduces the rate of creep, while 1.5–2.0 percent Nb (by weight) increases the radiation resistance of U-Zr alloys.

Uranium-aluminum alloys (based on highly concentrated uranium) are used in making fuel elements of the dispersion type. Alloys containing less than 35 percent U (by weight) are of special interest. The structure of such alloys comprises UAl₃ species surrounded by a UAl₄ shell. Up to 3 percent Si (by weight) is introduced to stabilize the UAl₃ phase. Such alloys retain gaseous fission products well and have good radiation resistance.

Uranium alloys having a low concentration of additives are produced by the combined reduction of fluorides and oxides of uranium and the other components of the alloy by metallic calcium or magnesium. For higher concentrations, melting and casting are employed, as are the methods of powder metallurgy.