Investigation into steady state and stability behavior of natural circulation systems operating with supercritical fluids

首都大学東京博士(工学)Supercritical Water Reactor (SCWR) is one of the next generation (Generation-IV) reactor concept. Supercritical water is being considered as a coolant in SCWRs on account of its potential to offer high thermal efficiency, compact size, elimination of steam generator, separator, dryer and recirculation system making it economically competitive. Several SCWR designs with forced circulation of primary coolant have been proposed in the past, however most of Generation-IV designs are generally not expected to be available for commercial construction before 2030. Supercritical water natural circulation loops are capable of generating density gradients comparable to two-phase natural circulation loops. Hence, natural circulation is also considered as a viable option of heat removal in SCWRs. Safety is a key issue in the design of advanced reactors and considerable emphasis is given on passive safety. Cooling a reactor at full power with natural instead of forced circulation is generally considered as enhancement of passive safety. 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. Since supercritical water (SCW) or any other supercritical fluid experiences steep change in its thermodynamic and transport properties (particularly density) in the pseudo-critical temperature region, supercritical water reactors may be susceptible to density wave instability. Due to drastic change in its properties, the heat transfer behavior is also quite different from sub-critical convective heat transfer. The experimental studies on natural circulation with supercritical fluids is very limitedly available in open literature and details of stability of supercritical water natural circulation loop have not been revealed yet. Elucidation of such phenomenon and development of numerical codes are required. Hence, a test facility called Supercritical Pressure Natural Circulation Loop (SPNCL) has been set up at Bhabha Atomic Research Centre (BARC), India. The facility is a uniform diameter rectangular closed natural circulation loop which can operate with either supercritical water or supercritical carbon dioxide. The studies revealed that instability was observed for a very narrow window of power near the pseudo-critical temperature of operation. Experimental instability data can only be simulated with in-house developed codes by including pipe wall thermal capacitance models which is very important for stability analysis of natural circulation at supercritical conditions.首都大学東京, 2014-10-16, 博士（工学）doctoral thesi