Characterization of Sodium Vapor Formation During Simulated Fuel-Coolant Interaction Using Real-Time X-ray Imaging

In the unlikely event of an unprotected transient overpower or unprotected loss-of-coolant flow, overheating of the reactor core may lead to partial core meltdown in sodium-cooled fast reactors. Hence, studying the interaction of high-temperature melt with sodium, known as the molten fuel–coolant interaction (MFCI), is significant. In this study, to understand MFCI, experiments were conducted by dropping the high-temperature melt generated by a thermite reaction into a stainless steel interaction vessel containing sodium, maintained at 400°C. The sodium vapor generated during the MFCI process due to the melt–sodium superheat was captured using real-time X-ray imaging.For characterizing the sodium vapor from a typical X-ray image, a standard phantom was designed with cylindrical cavities that simulate sodium vapor. While the lateral dimensions of the vapor could be estimated from two-dimensional images, information on the depth had to be assessed from changes in the image intensity. Towards estimating the vapor depth, two approaches were adopted, one based on a mathematical approach arrived at by analyzing the X-ray image of the voids in sodium phantom in stainless steel interaction vessel and another based on normalizing the intensity distribution of the image, which was used to estimate the reduction in the sodium path length (RISL) while vapor was formed during MFCI.These two approaches, which were initially adapted to estimate the depth of the argon bubbles in the simulated experiments, were found to be in close agreement with each other. However, the difference between the lateral dimension estimated from the full-width at half-maximum and the RISL estimated by these two approaches showed a constant ratio. Similarly, the lateral and depth dimension of the sodium vapor formed during the MFCI process were evaluated.This paper presents the details of the phantom used, the methodology adapted for estimating the linear attenuation coefficient of sodium at 400°C, the determination of the RISL, and the correction ratio estimated for mapping of the sodium vapor depth.