Abstract
For a High-Temperature Gas-cooled Reactor (HTGR), there is a probability that the helium pressure boundary (HPB) suffers a breach that could lead to the depressurization of the system. The helium is discharged into the cavity section of the vented low-pressure containment building, and the cavity is eventually vented under specific conditions. Under the postulated accident event, the nuclear reactor can undergo significant damage if air makes its way from the breach towards the reactor core. Even though the probability of such an event is very low, this scenario has gained the attention of regulators, plant designers, and operators because of the possible catastrophic consequences. The University of Idaho - Idaho Falls Campus designed and built a 1/20th scaled-down HTGR based on the preliminary design of the Gas Turbine Modular Helium Reactor to analyze the gas dynamics of helium-air interaction and venting of the air located within the reactor cavity following a break on the HPB.
In this study, a sensitivity study is executed to analyze the air and helium concentration within the containment building as a result of a break in the HPB. This effort aims to shed light on the gas dynamics within the vented low-pressure containment of an HTGR during the accident, as mentioned above. Additionally, this study evaluates the system behavior under varying conditions to reduce the oxygen concentration at the location of the break to reduce the probability of air ingress. Some of the varying conditions evaluated are the time of active ventilation, break size and location, and ventilation location.
The experimental results presented in this study indicate that an active ventilation time of 22 seconds allows the system to vent most of the air from the cavity section compared to 50, 65, and indefinitely time scales. Additionally, the experimental results indicate that leaving the ventilation duct system open for too long results in lower temperatures in the cavity section. The break size also influenced the oxygen concentration, where the system vents more air with small breaks than relatively large ones. The location and orientation of the break have little effect on the temperature and oxygen concentration measurements. Nonetheless, it did significantly influence the velocity of the gases being vented. The location of the ventilation system did significantly influence the oxygen concentration. The placement of the ventilation system near the bottom floor of the power conversion vessel (PCV) containment building results in a higher oxygen concentration in the cavity region of the reactor pressure vessel and a lower concentration in the PCV cavity region. Contrarily, a lower oxygen concentration in the pressure vessel cavity and higher in the PCV is the outcome when the ventilation duct is placed near the roof of the containment building of the PCV. Velocities as a result of the initial depressurization and natural circulation were recorded. Experimental results indicate that the velocities at the bottom of the axial cross-vessel are higher than at the top.