Numerical Study of Startup Instabilities in a Parallel-Channel Natural Circulation Boiling System
S. P. Lakshmanana and Manmohan Pandeyb
Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781 039, India.
alakshmanan@iitg.ac.in
bmanmohan@iitg.ac.in
ABSTRACT
During the startup of a natural circulation boiling water reactor (NCBWR), there is a transition from the stable single-phase regime (initial stage of a startup) to the stable two-phase regime (final stage) through intermediate regimes where the phenomena of geysering and flashing are highly dominant. Studies that are focused in understanding such transients are mostly done on a single channel natural circulation system. In the present work, startup transients in a parallel-channel natural circulation boiling water reactor have been simulated numerically using a thermal hydraulic safety code, RELAP5. The parallel-channel RELAP5 model is an extension of a single-channel model developed and verified in a previous work. The existence of in-phase and out-of-phase flashing instabilities in the parallel channel system is investigated through simulations under equal and unequal initial flow and power boundary conditions between the channels. The effect of flow resistance on Type-I oscillation is explored by starting the reactor under different inlet and outlet resistance between channels. It is inferred that for non-identical condition between channels, the flow fluctuations are out-of-phase.
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