Reduction of pressure tube/calandria tube contact conductance

One of the ways to increase the passive safety of a CANDU reactor is to decrease the contact conductance between the pressure and calandria tubes during loss of coolant/loss of emergency core cooling (LOCA/LOECC) situations. For severe accident scenarios, we predict the pressure tube would overheat, deform and contact its surrounding calandria tube. Upon contact, heat stored in the pressure tube would be transferred across the interface to the calandria tube, conducted through the wall, and then transferred from the outer surface of the calandria tube into the surrounding moderator. The sudden transfer of heat could exceed the critical heat flux and cause film boiling on the calandria tube surface. During film boiling, the transfer of heat through the calandria tube to the moderator would be severely reduced, the temperature of the calandria tube would rise and fuel channel integrity could be at risk. A reduction in the pressure tube/calandria tube (PT/CT) contact conductance would help prevent the calandria tube from going into film boiling. This report summarizes the results of a test that demonstrated an order of magnitude reduction in the contact conductance. This test incorporates a wire screen in the fuel channel annulus next to the inner surface of the calandria tube. The screen effectively reduced the heat transfer rate during PT/CT contact, and the calandria tube remained in nucleate boiling throughout the experiment. This reduction in contact conductance significantly reduces or eliminates the risk of the calandria tube being forced into film boiling during a postulated LOCA/LOECC. A demonstrated assurance of nucleate boiling on the calandria tube surface would improve the passive safety of a CANDU fuel channel under postulated accident conditions. (Author) (12 figs., 3 refs.)