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.
The simulation of start-up of natural circulation boiler based on the Astrom-Bell model
