Experimental Study Of The Thermal Stability Of Dispersion-type Fuel Elements

China Advanced Research Reactor uses Low Enrichment Uranium dispersion fuels (20%235U), in which dispersants, matrix and clad are U3Si2, Al and 6061 aluminum alloy respectively. The fuel meat has a uranium density of 4.3g/cm3, and may exceed 200 癈 during operation. The thermal stability of fuel elements is one of the factors that decide lifetime of fuel elements. Deformation of fuel elements will change temperature field and the microstructures of material, which in turn will produce larger deformation and swelling than before, even damage fuel elements. So it is very important for safety evaluation of in-pile fuel elements that we explore the thermal stability of fuel plates.The paper describes the heat treatment at the temperature of 160℃, 250℃, 350℃, 400℃, 450℃ and 500℃, the dimension change and microstructural evolution of miniature fuel element plates after heat treatment. The result reveals that: The fuel plates did not deform after 60 days at 160℃ and 250℃, slightly curved after 60 days at 350℃ and 400℃, and heavily curved after 20 days at 450℃ and so did after 1 day at 500℃.After 60 days at 160℃, 250℃, 350℃ and 400℃, U3Si2 particles are compact with Al-matrix, the meat did not crack and there were no phase change. Fuel meat and clad binded tightly, no crack was found at the boundary of the meat and clad. No U3Si2 particles invaded to clad and the clad did not become thinner (>0.3mm). . The fuel meat exposing to air at 160℃ and 250℃ did not affect the thermal performance considerably, but at 350℃ and 400℃, U3Si2 particles fragmented, the meat cracked, U3Si2 particles invaded to clad and the clad became thinner (<0.2mm). After 20 days at 450℃ or 1 day at 500℃, the U3Si2 particles cracked and fragmented, there were a lot of cracks in Al-matrix and reacted areas , there were also a lot of holes in the meat. The meat lost its continuity and perfection, and departed from clad. Above 450℃ the volume fraction of the Al-matrix and the U3Si2 decreased and produced U(Al,Si)3 in the meat, U3Si2 particles invaded to clad .The initial cracks in the U3Si2 particles and gas absorption in original miniature fuelplates were due to the fuel plate fabrication procedure. Both the initial cracks'propagation and the gas release had large effect on the meat swelling. So thefabrication quality of fuel element plate should be controlled strictly.Above 450, the formation of low density U(Al,Si)3 phase, together with the releaseof the impurity gas and fragmentation of U3Si2 particles, will cause fuel meat toswell , Al matrix to crack and U3Si2 particles to invade to clad. As a result, thethickness of the meat increased, the thickness of clad decreased and the deformationof fuel plates increased, and the leakage probability of the radioactive massincreased reasonably.The effect of meat exposure on fuel element plates was small under normal conditions,but could not be neglected under off-normal conditions.Fuel element plates should be thermally stable and could be used safely if operationtemperature was below 250 .