Alloying of uranium is done to improve the mechanical properties, dimensional stability and corrosion resistance of uranium. However, selection of alloying ele­ments should not adversely affect the neutron economy; hence, a lot of emphasis was placed on the alloying elements like Al, Be, and Zr. The alloying elements like Ti, Zr, Nb, and Mo have extensive solid solubility in uranium at higher tempera­tures, V and Cr have moderate solubility, and Ta and W are further less soluble in C-U. Figure 7.5 shows the equilibrium-phase diagram of U-Mo system. U-Zr and U-Pu-Zr fuels in EBR-II were used as the alloying raised the alloy solidus

temperature, enhanced dimensional stability under irradiation, and reduced fuel­cladding material chemical interaction. Furthermore, uranium-fissium/fizzium (U-Fs or U-Fz) alloys are being utilized in LMFBRs. U-Fs alloys can be developed during the reprocessing of spent fuels in which part of the fission products such as Mo, Nb, Zr, Rh, Ru etc. are left in the uranium matrix. These types of alloys (e. g., U-15 wt% Pu-10 wt% Fs or U-5 wt% Fs) show better irradiation stability.

Addition of alloying elements to small concentrations in uranium can improve the high-temperature strength of the alloy. This is beneficial since the strength of uranium falls drastically at elevated temperatures. For example, addition of Cr to the tune of 0.5 wt% or Zr to 2.0 wt% can increase the yield strength by four-five times. Addition of Si and Al may also improve strength when added in small amounts. However, addition of larger amounts may result in the formation of brit­tle intermetallics, adversely affecting the ductility and fabricability of the alloy. Mar­tensitic transformation is another way of hardening the uranium alloys. The addition of Zr to the tune of about 5-10 wt% can be water quenched from the gamma-phase regime to produce supersaturated metastable alpha-phase (alpha — prime) regime. The as-quenched U-5 wt% Zr alloy (900 °C at 1 h and quenched) is very hard (~535 VHN). Upon tempering at 650 °C for 2h, it loses its hardness (~315 HVN). However, a range of microstructure can be developed by manipulat­ing the tempering parameters. Similar martensitic transformations also occur in U-Mo, U-Ti, and U-Nb alloy systems.

Uranium alloys exhibit better corrosion resistance by forming and retaining a protective oxide film up to 350 °C if the alloy is in the form of (i) metastable gamma — phase, (ii) supersaturated alpha-phase, and (iii) intermetallic compounds.

The first type of alloys contains 7 wt% or more Mo or Nb. The alloying elements remain dissolved in the gamma-matrix (BCC) by cooling at moderate or rapid rates from the gamma-phase regime. As long as the gamma phase is retained in the alloy, the corrosion rate remains low. It can be noted here that U-Mo alloys are being developed under the ‘Reduced Enrichment for Research and Test Reactors (RERTR)’ program. The RERTR program was started by the US Department of Energy in 1978 to develop technologies essential for enabling the conversion of civilian nuclear facilities using high enriched uranium (HEU; > 20 wt.% U235) to low enriched uranium (LEU; <20 wt.% U235). By the end of 2011, over 40 research reactors have been converted from HEU to LEU.

Supersaturated alpha-phase alloy is formed by adding a small amount of niobium (up to 3 wt%) and letting it cool rapidly leading to martensitic transformation. As long as the martensitic structure is maintained, the corrosion resistance property is retained. Further improvement in corrosion resistance can be achieved by adding zir­conium. For example, a ternary uranium alloy with 1.5 wt% Nb and 5 wt% Zr has good corrosion resistance. However, the alloy is susceptible to embrittling hydrogen attack.

Uranium-based intermetallic compounds may provide better corrosion resist­ance as typified by uranium silicide (U3Si). This class of uranium-based materials includes a range of intermetallics such as UAl2, UAl3, U6Ni, U6Fe, and so on. The main advantage of these compounds is that they provide corrosion resistance at elevated temperatures at which the first two types of alloys (metastable gamma and supersaturated alpha) cannot.