Experimental and numerical investigation of natural circulation stability of the SHUNCL thermal-hydraulic loop

Nowadays, the application of natural circulation due to safety margins that provide for cooling the nuclear reactor cores has drawn the attention of researchers and designers in this context. Employing the benefits of natural circulation demands a thorough study of various phenomena such as thermo-fluid dynamics of steam-liquid interaction with the associated challenges that the most significant one is instabilities due to the severe dependency of mass flow rate on the applied heat flux. For this purpose, SHUNCL1 thermal hydraulic loop has been designed and constructed under atmospheric conditions and an experimental scenario has been arranged to investigate the mass flow rate variation due to heat flux. It is observed that the system behavior particularly the mass flow rate shows a considerable sensitivity to the heat flux variation such that in specific limit of heat flux implementation, the system behavior undergoes severe fluctuations and never attains to a stable condition. In addition to the experimental study, a numerical simulation of the SHUNCL loop has been prepared using the RELAP5 code in order to more investigate the effect of thermal hydraulic parameters on the instability of the operating system. System pressure, inlet subcooling, chimney length, orificing diameter and the heat section length are analyzed. Also, stability boundary is obtained based on two dimensionless numbers, phase change (Npch) and subcooling (Nsub) and it was observed that the occurred instability in the SHUNCL thermal hydraulic loop is of the type-1 dynamic instability.