THERMOSS-II: A Model for Thermohydraulics of CANDU Fuel Channel with Subcooled Stagnant Initial Conditions

Abstract

A two-fluid model, called THERMOSS-II (for THERMohydraulic Model Of Standing Start) has been developed to predict the duration of flow stagnation and stratification and fuel heatup in a CANDU reactor fuel channel in the standing start phenomenon.This phenomenon refers to a subcooled, stagnant initial channel condition and may occur in CANDU fuel channel following a small loss-of-coolant accident and subsequent refill of the channel with emergency core coolant. From this condition, the channel coolant boils and stratifies and the steam flows to the end fittings where it condenses heating up the end fittings. Eventually, the steam reaches a vertical feeder pipe resulting in channel flow and refill.THERMOSS-II generalizes the previous THERMOSS model to account in a simple manner for highly non-uniform temperature distribution in the end fitting body during the end fitting heatup. This temperature distribution and the consequently lower duration of channel flow stagnation were observed in a recent series of standing start tests conducted in a modified cold water injection test facility with reactor-like channel-end fitting geometry.