Experiments on transient combustion behaviors of cerium metal slice and droplets in oxygen with high-speed microscopic imaging

Uncontrolled uranium combustion and the release of radioactive aerosols under fire accidents remain a significant risk to the wide application of uranium fuel. This study investigates the burning characteristics of cerium metal slice, utilized as a surrogate for uranium, in an oxygen environment. A combustion diagnosis setup was conducted, including 25 kHz high-speed microscopic and 1 kHz color imaging. Observations revealed that the cerium slice formed a liquid pool upon melting, generating multi-scaled burning droplets ranging from a few to several hundred microns. The results indicate that the droplets exhibit transient combustion behaviors, especially a rapid transition from the initial surface combustion to gaseous combustion within several hundred microseconds. This transition was marked by a shift in the oxidation condition of the droplets from the kinetic to the diffusion regime, which was accompanied by the emergence of an envelope flame. Analysis of the droplet and flame diameter changes revealed a competing mechanism determining droplet size evolution dominated by evaporation and thermal expansion. During surface combustion, droplets underwent thermal expansion due to temperature rise, whereas the expansion gradually slowed down after the transition point with evaporation occurred and strengthened. During gaseous combustion, steady evaporation replaced thermal expansion as the dominant factor controlling droplet size, leading to observable shrinkage. This study gives a comprehensive insight into the droplet staged combustion process, which would be helpful for the further modeling of the intrinsic driving mechanism of cerium combustion behaviors. •The combustion characteristics of cerium metal slice and droplets are discussed.•Burning cerium droplets experience a transition from surface to gaseous combustion.•A competing thermal mechanism leads to cerium droplet size evolution is analyzed.