A CANDU multiple-channel thermosyphoning flow stability model

This paper describes an algebraic linear model (dubbed MMOSS for Model of Multiple-channel OScillationS) developed to explain and predict pass-to-pass out-of-phase oscillatory behaviour in a CANDU-type multiple-channel figure-of-eight loop under two-phase thermosyphoning conditions. MMOSS was developed for a figure-of-eight system of any number of channels. The system characteristic equation was derived from a linearization of the conservation equations for mass, momentum, and energy. MMOSS is a simple generalization of a previous model for a single-channel system. In this paper, the MMOSS characteristic equation is solved for a system of N identical channels. This simplification provides valuable physical insight and some reasonably accurate results. Furthermore, the simplification is not unrealistic because, in a multiple-channel system, a group or groups of channels are nearly similar. From the MMOSS characteristic function for the RD-14M facility under two-phase thermosyphoning conditions, a flow stability map is generated as a function of loop integrated void, secondary side pressure, and channel power. MMOSS predicts that the flow is stable above about 4 MPa secondary side pressure. At a given pressure below this pressure, the flow is unstable over a range of the void. This range is larger the lower the pressure. These predictions agree reasonably well with the results of the RD-14M experiments. (author) 11 refs., 5 figs