Analyzing air-ingress currents in Gas-cooled nuclear reactors using Kramers Moyal expansion

During a loss of coolant accident in High Temperature Gas-cooled Nuclear Reactors, a break in the coolant system will allow air ingress into the high temperature reactor and can negatively impact the core thermal behavior. Air-ingress rates are supposed to be influenced by molecular diffusion during initial stages and global natural convection at later stages where reactor cavity air is circulated through the core due to temperature differential. But the transition times predicted by the diffusion theory are much larger than experimental results at high temperature, thus role of convection currents even before the transition stage is suspected. Velocity time-series data from hot wire anemometer located at the opening is analyzed to quantify the role of convection vs diffusion modes. It was evident with the pdfs of velocity data that kurtosis increases with increase in temperature. Kramers Moyal analysis was performed to disentangle the drift and diffusion coefficients of the velocity time series measured in a test section designed to mimic the loss of coolant accident in an HTGR. Relative values of these coefficients together with the probability density functions of the velocity fluctuations were compared at different temperatures.