Transition from molecular diffusion to natural circulation mode air-ingress in high temperature helium loop

Abstract

High Temperature Gas-cooled Reactors (HTGRs) have significantly robust passive heat removal capabilities owing largely to conduction and radiation heat transfer through graphite-fuel matrix and reactor vessel wall respectively. However, these capabilities may be significantly impacted if air was to replace Helium in the high temperature graphite under accident scenarios. In case of break in the coolant system high pressure Helium will escape into the reactor cavity leading to depressurization of the reactor. This will allow air from reactor cavity to enter the plenum or core of HTGRs via different mechanisms-diffusion, gravity currents, or natural circulation. Experimental studies were conducted in a geometrically scaled setup with a shape of small English letter 'h', to depict the role of upper plenum in this process. Main focus of these studies was to understand and observe the transition time from diffusion to natural circulation. Thermal camera is used to identify this transition and flow transducer is used to measure the flow rates. The experimental results on incipience time of natural convection after a chamber pre-filled with Helium is opened to atmosphere are qualitatively similar to observations reported in literature using 'inverse U' shaped experimental setup, but also show distinct quantitative effect of extended leg on the transition times. Prior to onset of natural circulation (ONC), molecular diffusion plays the significant role in air-ingress. However, at higher temperatures convection currents may be influencing air ingress and in-turn ONC times.

Fluid Stratification Separate Effects Analysis, Testing and Benchmarking

“…It may be seen in contemporary works that velocity jumps on the order of <1.0 m/s may be expected [37] [18] [38]. Therefore, one may see that, in experimental application, this may be of limited utility to determine ONC; however, useful interrogation of bulk flow may still be possible under such uncertainties.…”

Statistical Mechanics-Based Surrogates for Scalar Transport in Channel Flow

“…For example, motion of the eddies near the grid plates in the fuel rod bundle is approximated by a 0D equation. In other examples, mixing and stratification in large volumes, such as containments, can significantly impact the overall safety [1][2][3]. For example, hydrogen release into reactor containment under severe accident conditions of light water can lead to stratification in the gaseous space of reactor containment due to the lighter weight of hydrogen.…”