Polycrystalline uranium undergoes a kind of dimensional instability when sub­jected to repeated heating and cooling (i. e., thermal cycling) in the alpha-phase regime. This phenomenon is known as thermal cycling growth and results in (i) growth (change in length, that is, increase or decrease) and (ii) surface roughen­ing arising out of wrinkling effect. The growth effect comes from a thermal ratchet­ing mechanism involving (i) relative movement between two neighboring grains with different thermal expansion coefficient due to the basic anisotropy of alpha — uranium, and (ii) stress relaxation in one of the grains by plastic deformation or creep. An interesting example of thermal cycling growth is shown in Figure 7.6, where an alpha-uranium rod has elongated over several times due to thermal cycling between 50 and 500 °C. In superplasticity literature, it is known as thermal cycling superplasticity! Thermal cycling growth coefficient (Gt) is expressed by

As thermal cycling is an inherent feature of nuclear reactor kinetics, it can thus have important influence on the thermal stability of the fuel. The gamma phase of

uranium does not show thermal cycling growth phenomenon and may thus be desirable. Thus, suitable amounts of gamma stabilizing alloying additions (Al, Mo, and Mg) help in avoiding thermal cycling growth effect as illustrated with an exam­ple from U-Mo system (Figure 7.7). Note that U-Mo alloys generally contain at least 6 wt% Mo in order to avoid thermal cycling growth.