Abstract
Nuclear fuel claddings can balloon and rupture at high temperatures under internal gas pressure in case of design basis accidents like loss-of-coolant-accident (LOCA). The thermal phenomena surrounding the ballooning and cracking was investigated in a series of experiments performed using zirconium alloy cladding tubes at the Centre for Energy Research in Hungary.The experiments were recorded using a high-speed infrared camera in the intermediate infrared range and the temperatures were measured in-situ. It was found that before the rupture of the cladding tube, the ballooned cladding surface forms a bulge and heats up locally, a hot spot appears. During crack propagation, the crack tip temperature is significantly warmer than the rest of the tube. The hot spot temperature and the rate of temperature increase was determined. The infrared emission coefficient of the slightly oxidized zirconium cladding samples in the intermediate infrared range was also determined. The emissivity coefficient that fit our measurements was found to be around ε = 0.89.
Highlights
A nuclear fuel cladding should maintain good performance without serious degradation under normal operating conditions, and under various accident conditions, such as loss-of-coolant accident (LOCA)
The room temperature as background noise was at 22 ◦C, reflection background was set to 950 ◦C and the emission coefficient of the sample was set to ε = 0.89 in the software to fit the value given by the thermocouples
In order to find out if the local temperature increase is due to a change in the shape of the sample or a real increase in temperature, we examined the infrared images of Sample 6 where the crack had formed in view of the camera and we could see inside the tube (Fig. 7, frame f;)
Summary
A nuclear fuel cladding should maintain good performance without serious degradation under normal operating conditions, and under various accident conditions, such as loss-of-coolant accident (LOCA). The integrity of fuel cladding can be compromised by ballooning (diameter increment with wall thickness reduction) and burst (crack formation and propagation) caused by the pressure difference between the inner and outer sides of the cladding. The ballooning and burst of nuclear cladding tubes has been inves tigated for decades. The effect of dissolved hydrogen (Suman, 2020), secondary hydridization (Hozer et al, 2018) and hydrogen distribution around the burst (Kozsda-Barsy et al, 2018) were investigated, some of the samples were subjected to metallographic measurements (Stuckert et al, 2013). Szepesi: Optical measurement of the high temperature ballooning of nuclear fuel claddings. The previous measurements (Nagy et al, 2017, 2018) gave an extensive and accurate picture of thermo-mechanical phenomena during the ballooning and burst.
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