The thermal stability of Al-USiAl dispersion fuels and Al-U alloys

The Al-U alloys showed no dimensional or phase changes after being heated at 4000C for as long as 93 days. These alloys are thermally stable because the fuel phase (UAl4) is thermodynamically stable with aluminum. Thus the irradiation behavior of the Al-37 wt%U alloy should be as good as that of Al-21 wt%U or Al-28 wt%U alloys at the same burnup of 235U atoms. After 30 days at 2000C, Al-USiAl dispersion fuels showed no significant dimensional or metallurgical changes. However, between 250 and 4000C, USiAl particles did react with the aluminum matrix to form UAl3 or UAl4 and cause fuel swelling. The amount of reaction and fuel swelling increased with increases in temperature and time. The aluminum appears to diffuse into the USiAl particles along unstable pathways such as grain boundaries or subparticle boundaries to form the new Al-U compounds. The in-reactor behavior of Al-USiAl dispersion fuels is expected to be satisfactory under normal operation since fuel temperatures will generally be less than or equal to2000C and external restraint will be greater than in these tests