The objective of the present study is to predict the existence of metastable regime (multiple steady states) and effects of various parameters on it, in a parallel channel system under natural circulation. To examine the basic behavior of such a system, a three parallel channel system with non-uniform heat inputs and a common downcomer, was investigated in detail both analytically and using RELAP5/Mod 3.2 computer code. The results showed the existence of a metastable regime for a given set of heat input ratio. A study was also performed to investigate the effect of diameter, channel height and orificing at the inlet of heater, on the metastable regime. A Non-Dimensional Heat Flux Ratio (NH) has been proposed to predict the flow reversal. The heat flux level at which flow reversal occurs has been predicted for different rates of power rise and power reduction (with different input heat flux). It was found that the power level at which the flow reversal occurs depends upon the operating procedures (i.e. power rise and power reduction). The flow direction in different channels will depend on the prevailing density difference among channels and downcomer. It was found that, the flow direction in a channel, where power variation is taking place, follows the maximum buoyancy hydraulic path.

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