Experimental and theoretical investigations on steady state and stability behavior of natural circulation systems operating with supercritical fluid

Supercritical water (SCW) is being considered as a coolant in some advanced nuclear reactor designs on account of its potential to offer high thermal efficiency, compact size, elimination of steam generator, separator and dryer, making it economically competitive. The elimination of phase change results in elimination of the CHF phenomenon. Supercritical water natural circulation loops are capable of generating density gradients comparable to two-phase natural circulation loops. SCW under natural circulation is also considered as a viable option for heat removal in some advanced nuclear reactor designs. Hence, the behavior of steady state natural circulation with supercritical fluids is of interest for a number of new reactor systems. Besides stable steady state, operation with unstable natural circulation is undesirable as it can lead to mechanical vibration of components and failure of control systems. Since SCW experiences drastic change in its thermo-physical properties (e.g. density) near the pseudo-critical temperature, natural circulation may be susceptible to density wave oscillations and ledinegg excursions. A test loop has been set up in BARC to study steady state and stability behavior of natural circulation with supercritical fluids. The loop has been designed to operate with SCW as well as supercritical carbon-dioxide. The experiments have been conducted in the test loop with supercritical carbon-dioxide. The steady state and stability predictions made by in-house developed non-linear computer code (NOLSTA) have been compared with experimental data. The present paper describes the experimental results and analysis in detail. (author)