Creep sag of Zircaloy-2 pressure tubes in CANDU-PHW reactors

In CANDU-Pressurized Heavy Water reactors, the horizontal fuel channels sag as a result of creep caused by the distributed load of the fuel and coolant, elevated operating temperature, and fast neutron flux. Creep sag has been monitored in Zircaloy-2 pressure tubes installed in the NPD, Pickering Units 1 and 2, and Douglas Point reactors using direct measurement of pressure tube curvature. Using the sag measurements and additional length and diameter change data, a computational creep analysis was set up to predict the fuel channel sag in a Douglas Point type reactor. The model predicts the time and length of pressure tube/calandria tube contact as a function of annular spacer location. The effect of removing the fuel after contact has occurred was assessed and shown to result in breaking of the contact only if the fuel is removed very shortly after contact first occurs. (author). 4 refs., 6 tabs., 5 figs