Stability of natural-circulation flow in a CANDU-type fuel channel

This paper describes the results of natural-circulation flow experiments in a full-scale representation of a single CANDU reactor fuel channel and its associated feeders. The objective was to investigate the stability of natural-circulation flow for various channel powers, system pressures and inlet flow subcooling levels. The results show that the flow becomes unstable for two-phase flow conditions at the channel outlet. Two types of periodic behaviour are observed: (1) an intermittent flow instability that occurs at low channel exit qualities, caused by periodic formation and expulsion of two-phase region from the channel, and (2) an oscillatory flow instability that occurs at high channel exit qualities, characterized by large-amplitude sinusoidal-type oscillations in the channel flow. The paper describes the parametric trends with power, pressure and degree of subcooling. The dynamic instability is analyzed using a linear stability model. (orig.)