The stability of Al-USiAl dispersion fuels and Al-U alloys was examined at temperatures between 200 and 400°C for times up to 93 days. The Al-U alloys, which contained 21 to 37 wt% uranium, did not show any dimensional or metallurgical changes after 93 days at 400°C. After being heated at 200°C, the Al-USiAl fuel showed no dimensional or metallurgical changes. However, between 250 and 400°C, the USiAl particles reacted with the aluminum matrix to form UAl3 and UAl4. The amount of reaction increased with temperature and time. The aluminum diffuses into the USiAl particles along grain boundaries to form the new Al-U compounds. Although the Al-USiAl fuel showed thermal instability at 250°C and above, the in-reactor behavior under normal operation is expected to be satisfactory since fuel temperatures will generally be <200°C.