Deformation Behaviour of Zr‐2.5%Nb Under Biaxial Stress State at High Temperatures

ABSTRACT The CANDU (CANadian Deuterium Uranium) nuclear reactors are pressure tube reactors. The fuel channels of the reactor comprise a calandria tube and a smaller diameter pressure tube (pressure boundary) concentric within the calandria tube. The pressure tube, made from a zirconium alloy, Zr‐2.5wt.%Nb, contains the nuclear fuel and pressurized coolant. In some scenarios, such as a postulated loss of coolant accident, the horizontally oriented pressure tube gets heated to high temperatures (above 650°C) and deforms into contact with the calandria tube either through primarily downward deformation under its weight (sagging) or primarily radial deformation under internal pressure (ballooning). This deformation establishes a more direct path to remove heat from the fuel to the heavy water moderator that surrounds the calandria tube. A method was developed to apply stereo digital image correlation paired with thermography images to measure deformation of prototypical biaxial stress tests and Zr‐2.5wt.%Nb pressure tubes demonstrating ballooning behaviour and compare experimental measurements to predictive deformation models developed from tests on uniaxially stressed specimens. The experiments were performed at an internal pressure of 3.5 MPa and a heating rate of 7.5°C/s representing low heating rates of a pressure tube in a CANDU accident scenario and provided transient surface deformation and temperature for comparison against models. The model predictions were in good agreement with the experimental measurements especially when the uncertainty in the temperature measurements is considered.