Calandria Tube Sagging Prediction of Operating CANDU Reactor Fuel Channels

A CANDU type nuclear reactor is composed of a calandria vessel, fuel channels and vertical or horizontal tubes. Pressure tubes and calandria tubes are parts of fuel channels, which are pressure boundaries of primary heat transport system and moderator system, respectively. Because the fuel channel is located horizontally, the pressure tubes and calandria tubes are sagged by vertical dead weight loads and irradiation creep deformations during operation. Because this contact was not considered in the design stage, it is not allowed during reactor operation. In this study, calandria tube sagging predictions are conducted using material irradiation creep equations and mechanical model of fuel channel of CANDU reactor, considering in-service inspection results of pressure tube sagging. By solving the equations iteratively, Sagging predictions were evaluated for all related channels of a Wolsong CANDU unit. The prediction results of pressure tube sagging are shown to agree well with the measurement results. This calculation method using mechanistic solutions and measurement results can increase the reliability of sagging prediction and help construct detail plan for aging management of CANDU fuel channels.