A reduced order model of BWR dynamics with subcooled boiling and modal kinetics: application to out of phase oscillations

Abstract In this paper we develop a reduced order model that includes the sub-cooled boiling. New additional equations have been obtained for the following dynamics magnitudes: the effective inception length, the average void fraction in the sub-cooled boiling region, the average void fraction in the bulk boiling region, the mass fluxes at the boiling boundary and the channel exit, and so on. Each channel has three nodes, one of liquid, one with sub-cooled boiling, and one with bulk boiling. The reduced order model includes also a modal kinetics with the fundamental mode and the first sub-critical one, and two channels representing both halves of the reactor core. The model displays out of phase oscillations when enough feedback gain is provided. The feedback gain that is necessary to self sustain these oscillations is approximately one half the gain that is needed when the sub-cooled boiling node is not included.

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