The future HTGR has been designed in JAEA. The HTGR uses the helium gas as the coolant in primary cooling system. The reactor has many merging points of the higher-temperature helium gas and the lower-temperature helium gas in the cooling system.
Previously, the reactor inlet coolant temperature was controlled lower than the specific one in the HTTR constructed in JAEA. It was confirmed that this event was caused by the lack of mixing of helium gas thermally at the measurement point of the reactor inlet coolant temperature. From this operational experience, it is needed to clear the thermal mixing characteristics of the helium gas at the annulus of the co-axial double-walled piping in HTGR from the viewpoint of the appropriate temperature control in future HTGR.
In this paper, thermal-hydraulic analysis on the helium gas at the annular flow path of the co-axial double pipe with T-junction was conducted to clarify the thermal mixing behavior of the helium gas.
It is shown that the thermal mixing behavior is not so much affected by the flow rate helium gases. Moreover it is difficult to mix the helium gas with the smaller height of the annular flow path. This is caused by smaller contact area of higher- and lower-temperature helium gas and the lack of helium gas flow in radial direction.
It is confirmed that it is difficult to mix the higher- and the lower-temperature helium gas in the annular flow path of the co-axial double-walled piping by using the hydraulic behavior, and it is necessary to arrange the mixing promotor in the annular flow path in order to mix the higher- and the lower-temperature helium gas.