Numerical investigation of the effect of inlet subcooling on flow instabilities in a parallel channel natural circulation boiling system

Some of the modern reactor concepts are based on natural circulation boiling systems. These systems are prone to flow instabilities and parallel channel systems can exhibit peculiar types of flow oscillations. In the present work, the existence and the nature of instabilities in a parallel channel natural circulation test facility (PCNCTF) has been investigated through numerical simulations using the system code RELAP5/MOD3.2. The numerical model is validated using experimental data of PCNCTF. The effect of inlet subcooling on flow oscillations in parallel channels is studied for different system pressure and input powers. Time variations of mass flow rate and void fraction in the channels are examined and the nature of oscillations is analyzed using phase portraits and Fourier power spectra. It is found that there exists a particular range of inlet subcoolings (at a fixed system pressure and input power), above or below which instabilities occur. Mixed mode of oscillations at low degrees of inlet subcoolings and superimposed density wave oscillations are observed at high inlet subcoolings.