In the CANDU nuclear reactor, pressure tubes made from Zr-2·5 Nb contain the fuel and the heat-transport heavy water. At each end, the tube is connected to a stainless steel end fitting by a rolled joint. A thin Zircaloy tube surrounds the pressure tube and the annular space between the tubes contains an inert gas. If a crack develops in a pressure tube at a rolled joint, unstable fracture is avoided by the application of Leak-Before-Break (LBB). In some early CANDU reactors, a few pressure tubes leaked through delayed hydride cracks. High residual stresses, produced when tubes were incorrectly rolled into end fittings, contributed to the cracking. The cracks were easily detected by their leakage into the gas annulus, the reactor was safely shut down and the leaking tubes were identified and replaced, thus providing a clear demonstration of LBB. To ensure the margins against unstable fracture continue to be adequate, we must demonstrate the validity of leak-before-break. The method depends on showing that the time available to detect a crack is much greater than the time required to detect the crack. The time available is estimated from measurements of crack velocities, crack lengths at instability and crack lengths at penetration of the tube wall. A leak is detected by the dewpoint and the rate of dewpoint increase of the gas in the annulus. Action time for operators is shown to have a usable margin before tube rupture, supporting continued use of LBB as an operating criterion.