Optical observation of the ballooning and burst of E110 and E110G cladding tubes

During the last decades several experiments revealed interesting details of the ballooning and burst of nuclear cladding tubes in accident conditions involving high temperature and high pressure. To further investigate this phenomenon, a new set of experiments was conducted at MTA EK. A large tube furnace was fitted with two optical telescopes on opposite sides to observe the ballooning and burst of Russian E110 and E110G (sponge based E110) fuel cladding samples. The experiments were recorded using regular and high-speed cameras. Based on the captured images we developed a method to separate the contours of the cladding tubes during the ballooning to measure the change in diameter. Two successive modes of ballooning were observed, the uniform growth of the samples was followed by an asymmetrical, local ballooning (bulge formation) 2 s before the burst. It was found that the samples have bent by approximately 5° relative to their original axis in the latter phase and this was not caused by the jet effect of the high pressure argon gas escaping after the burst. Every sample opened up on the convex side of the bend. Axial grooves have formed on the surface of the E110G samples under tension. The high-speed camera was used to capture the cracking and the burst. Prior to the burst, a high temperature spot was observed at the position where the crack would initiate. The timescale of the crack propagation was 0.2 ms, the crack tip was estimated to be at least 100 °C hotter than the rest of the sample. The burst pressure was determined between 700 °C and 900 °C at different pressurization rates.