The CANDU industry in Canada has successfully demonstrated LBB for pressure tubes using deterministic methodologies for decades; however, fracture toughness properties of the pressure tubes downgraded by an increase in hydrogen concentration challenges current assessments. The deterministic approach is supported by fracture mechanics methodologies with experimentally determined material toughness properties. Leak-Before-Break must be demonstrated for pressure tubes in operating CANDU reactors with hydrogen concentration above the terminal solid solubility of hydrogen dissolution at a sustained temperature. The primary focus is pressure tube failure from a volumetric flaw after crack initiation and subcritical crack growth by Delayed Hydride Cracking (DHC) at the operating temperature.
In response to assessed limitations in the current LBB approach, a new probabilistic LBB framework consisting of Level 1 and Level 2 methodologies has been proposed by the CANDU industry. Even though the probabilistic approach is permitted by CSA Standard N285.8-10, the probabilistic LBB methodology has yet to be accepted by the Canadian Nuclear Safety Commission. Similarly, other nuclear regulators have yet to accept the probabilistic LBB methodologies.
