Coupled-Neutronics-Thermal hydraulics Stability Appraisal of Supercritical Forces Flow Channels Following LPM

The possible appearance of instabilities is of major concern for the operation of Supercritical Water Reactor (SCWR), owing to the substantial density variation experienced by the fluid during its passage through the cooling channels. Any supercritical fluid suffers drastic variation in all thermophysical properties around the pseudocritical point, which is the primary reason for instigating dynamic instabilities in such channels. The present study aims towards the thorough exploration of the same while coupling the thermal-hydraulic characteristics with related fuel rod dynamics and neutronics, through a reduced-order model. A lumped-parameter-based approach is adopted to get a simple, yet robust, mathematical framework. The entire flow region is divided into three zones, namely, a heavy fluid region, a light-fluid region and an intermediate heavy and light fluid mixture region. The zones are separated by time-dependent boundaries, imposing variable lengths on them. Equations for the conservation of mass, momentum and energy are integrated over each zone, thereby developing a set of ODEs and algebraic equations, with coolant exit enthalpy and length of each zone being the primary variables. A two-node lumped parameter representation is considered for the fuel rod, to incorporate the concerning heat transfer behaviour into the calculation. The US reference design of SCWR is selected as the prototype, with an operating pressure of 25 MPa. An increase in hated length and reduction in hydraulic diameter is found to destabilize the system. The Inlet orifice coefficient stabilizes the system, whereas the exit orifice coefficient destabilizes. The effect of fuel time constant and enthalpy reactivity coefficient have also been studied.