D.S. Pilkhwal

Linear and Nonlinear Stability Analysis of a Supercritical Natural Circulation Loop

Supercritical water (SCW) has excellent heat transfer characteristics as a coolant for nuclear reactors. Besides it results in high thermal efficiency of the plant. However, the flow can experience instabilities in supercritical water cooled reactors, as the density change is very large for the supercritical fluids. A computer code SUCLIN has been developed employing supercritical water properties to carry out the steady-state and linear stability analysis of a SCW natural circulation loop (SCWNCL). The conservation equations of mass, momentum, and energy have been linearized by imposing small perturbation in flow rate, enthalpy, pressure, and specific volume. The equations have been solved analytically to generate the characteristic equation. The roots of the equation determine the stability of the system. The code has been benchmarked against published results. Then the code has been extensively used for studying the effect of diameter, heater inlet temperature, and pressure on steady-state and stability behavior of a SCWNCL. A separate computer code, NOLSTA, has been developed, which investigates stability characteristics of supercritical natural circulation loop using nonlinear analysis. The conservation equations of mass, momentum, and energy in transient form were solved numerically using finite volume method. The stable, unstable, and neutrally stable points were identified by examining the amplitude of flow and temperature oscillations with time for a given set of operating conditions. The stability behavior of loop, predicted using nonlinear analysis has been compared with that obtained from linear analysis. The results show that the stability maps obtained by the two methods agree qualitatively. The present paper describes the linear and nonlinear stability analysis models and the results obtained in detail.

Linear and Non-Linear Stability Analysis of a Supercritical Natural Circulation Loop

Supercritical water has excellent heat transfer characteristics as a coolant for nuclear reactors. Besides it results in high thermal efficiency of the plant. However, the flow can experience instabilities in supercritical water reactors, as the density change is very large for the supercritical fluids. A computer code SUCLIN has been developed employing supercritical water properties to carry out the steady state and linear stability analysis of a SCW natural circulation loop. The conservation equations of mass, momentum and energy have been linearised by imposing small perturbation in flow rate, enthalpy, pressure and specific volume. The equations have been solved analytically to generate the characteristic equation. The roots of the equation determine the stability of the system. The code has been benchmarked against published results. Then the code has been extensively used for studying the effect of diameter, heater inlet temperature and pressure on steady state and stability behavior of a Supercritical Water Natural Circulation Loop (SCWNCL). A separate computer code NOLSTA has been developed which investigates stability characteristics of supercritical natural circulation loop using non-linear analysis. The conservation equations of mass, momentum and energy in transient form were solved numerically using finite volume method. The stable, unstable and neutrally stable points were identified by examining the amplitude of flow and temperature oscillations with time for a given set of operating conditions. The stability behavior of loop, predicted using non-linear analysis has been compared with that obtained from linear analysis. The results show that the stability maps obtained by the two methods agree qualitatively. The present paper describes the linear and nonlinear stability analysis models and the results obtained in detail.© 2009 ASME

Experimental Investigations on the Isolation Condenser Performance in Integral Test Loop

The proposed Advanced Heavy Water Reactor (AHWR) is a light water cooled and heavy water moderated pressure tube type boiling water reactor based on natural circulation. AHWR adopts several passive concepts with a view to simplify the design and to enhance safety and public acceptability. One such feature is passive decay heat removal using isolation condenser (IC) system during a station blackout. A scaled Integral Test Loop (ITL) was set up in BARC to simulate the overall system behavior studies for Advanced Heavy Water Reactor (AHWR). This facility simulates the Main Heat Transport System (MHTS), Emergency Core Cooling System (ECCS) and Isolation Condenser system (ICS) system, Feed Water System (FWS) and the associated controls. Power to volume scaling philosophy has been adopted for the design of the ITL systems. To evaluate the performance of IC, experiments have been carried out in ITL. The test results have been simulated using RELAP5/ MOD3.2. This paper deals with the experiments conducted, nodalization scheme adopted for ITL in RELAP5/MOD 3.2 simulation, transient predictions made and the results obtained in detail.Copyright

Investigations to Arrive at a Stable Start-Up Procedure for AHWR

Natural circulation is being adopted as a mode of core heat removal in several nuclear reactors that are under development. This is due to the passive nature of natural circulation that enhances the system safety and reliability. However, major concern in the design of natural circulation based reactor systems is to avoid the flow instabilities that may occur under certain operating conditions, i.e. unstable operational regime. Therefore various reactor operational transients such as start-up, power raising, setback and also the steady state operating points must fall within the stable regime. The choice of operating procedures needs to be made judiciously and which also needs to be validated and supported by experiments. Advanced Heavy Water Reactor (AHWR) being developed in India is a pressure tube type natural circulation boiling water-cooled reactor, wherein major part of the power is generated by thorium. Experiments and analytical studies have been performed to arrive at a rational start-up procedure for AHWR. Experimental results obtained in simple rectangular natural circulation loops as well as in a scaled down facility have revealed the importance of external pressurization to avoid the flashing and Type-I instabilities that occur at low pressure during start-up.Copyright

A Comparative Study of Single-Phase, Two-Phase and Supercritical Natural Circulation in a Rectangular Loop

A one dimensional theoretical model has been used to analyze the steady state and stability performance of single-phase, two-phase and supercritical natural circulation in a uniform diameter rectangular loop. Parametric influences of diameter, inlet temperature and system pressure on the steady state and stability performance has been studied. In the single-phase liquid filled region, the flow rate is found to increase monotonically with power. On the other hand the flow rate in two-phase NCS is found to initially increase, reach a peak and then decrease with power. For the supercritical region also, the steady state behaviour is found to be similar to that of two-phase region. However, if the heater inlet temperature is beyond the pseudo critical value, then the performance is similar to single-phase loops. Also, the supercritical natural circulation flow rate decreases drastically during this condition. With increase in loop diameter, the flow rate is found to enhance for all the three regions of operation. Pressure has a significant influence on flow rate in two-phase region marginal effect in supercritical region and practically no effect in the single-phase region. With increase in loop diameter, operation in the single-phase and supercritical regions is found to destabilize whereas the two-phase loops are found to stabilize. Again, pressure has a significant influence on stability in the two-phase region.Copyright