Parametric studies on startup transients in multiple parallel channels of a natural circulation boiling system

Abstract

The nature of startup transients in a natural circulation boiling system with multiple parallel channels are governed by the interplay between numerous geometric and operating parameters. Present work focuses on numerically ascertaining the relative influence of such variables, with primary objective being to identify possible options of suppressing concerned oscillations. A scaled-down version of AHWR, comprising of three parallel channels, has therefore been simulated using RELAP5. Both geysering and type-I instabilities have been observed in all three channels on the initiation of two-phase operation, commonly separated by a span of near-stable reversed flow in one of the channels. Adoption of higher initial pressure has been found to have a strong stabilizing effect, as it can completely eliminate geysering and can substantially shorten the duration of type-I instability. Increase in inlet loss coefficient and decrease in either core outlet or riser outlet loss coefficients can also subside type-I oscillations, without having any noteworthy effect on geysering. Channels with smaller diameter can also reduce both amplitude and duration of type-I fluctuations, albeit at the expense of a deteriorated steady-state flow rate. The effects of wall roughness and thermal capacitance have also been explored. The influence of initial pressure has been found to be significantly stronger than the others, particularly in removing the geysering instabilities. An augmented initial pressure is capable of neutralizing the system even when the channels are subjected to unequal power, thereby identifying itself as a very potent option for practical implementation.