Fabrication

Uranium dioxide production can follow the same methods described in Section 7.1.1.1, except the steps involved to produce metallic uranium. UO2 can also be processed into bulk shapes such as pellets, tubes, rods, etc. by usual ceramic proc­essing methods, including powder metallurgy. Sintering of UO2 must be done

under an atmosphere either inert or reducing since sintering in air has conse­quences. The UO2 can exist in the form of a wide range of variable compounds depending on temperature and environment. U3O7 (2UO2 + UO3) in an unstable state of mixture forms at ~150-190 °C and U3O8 at about 375 °C. Figure 7.18 shows thermal analysis curves of UO2 as a function of temperature, showing the evolution of U3O8 is unstable above ~500 °C and converts back to UO2 at higher temperatures (1100-1300 °C).

UO2 + 2UO3 ! U3O8 ! 3UO2 + O2 (7.13)

The change in density with all the phase changes causes disruption during sin­tering in air. That is why sintering in hydrogen at 1700-1725 °C for 8-10h pro­duces bulk UO2 with 93-95% of the theoretical density. Minor additions of titanium dioxide (TiO2) or cerium dioxide (CeO2) can act as sintering additives and help to reduce the sintering temperature. However, there are now different kinds of sintering process that may allow better processing characteristics.

If the oxygen to uranium atom ratio is 2.0, the UO2 is stoichiometric. If an oxy­gen-deficient or excessive uranium exists (i. e., O/U < 2.0), the fuel is called super­stoichiometric fuel (UO2_x). If O/U > 2.0, UO2+x is called hypostoichiometric fuel (x is a small fraction). The departures from stoichiometry influence self-diffusion behavior in fuel itself and interdiffusion between fuel and cladding materials to form hyperstoichiometric or hypostoichiometric fuel during the reactor operation complicating the chemical composition of UO2. It can also affect fuel density, melt­ing point, and other physical and temperature-dependent properties.