Nuclear Technology / Volume 176 / Number 3 / December 2011 / Pages 442-451
Technical Note / Thermal Hydraulics
Advanced technology may be used to exclude the dynamic effects of postulated pipe ruptures from structural design consideration. However, it must first be demonstrated that the probability of pipe rupture is extremely low under conditions consistent with the design bases for the piping. Demonstration of a low probability of pipe rupture requires a deterministic fracture mechanics analysis that evaluates the stability of postulated small, through-wall flaws in piping and the ability to detect leakage through the flaws long before the flaws could grow to unstable sizes. The concept/methodology underlying such analyses is referred to as leak before break (LBB). LBB could be accepted as a technically justifiable approach for eliminating postulated double-ended primary system pipe ruptures equal to the pressurizer surge line size or larger. Large or double-ended reactor coolant system pipe ruptures equal to the pressurizer surge line size or larger need not consider the dynamic effects of pipe whipping that may result from their failure, following LBB approval of these piping systems. However, LBB may not be applied for the demonstration of adequate emergency core cooling (i.e., calculation of post-loss-of-coolant-accident peak clad temperature and cladding oxidation). Similarly, LBB may not be applied to the determination of containment building pressure and temperature responses to postulated primary and secondary system pipe ruptures or for the environmental qualification of mechanical and electrical equipment. This conclusion has resulted from extensive research and development and rigorous evaluations by the U.S. Nuclear Regulatory Commission, the German RSK, and the commercial nuclear power industry and its organizations since the early 1970s. The LBB concept can be applied to an axial flaw in a pipe, to a circumferential crack, or to when a flaw is stable under normal operating conditions and remains stable when there is a sudden dynamic event (i.e., seismic loading) as a time-dependent inertial LBB analysis. These analyses are deterministic and could be extended to probabilistic evaluations as well. This technical note describes the evolution of the LBB concept, application, issues, and resolutions raised in the process of regulatory actions globally.
In this technical note, prior LBB studies in Europe and the United States, performed by various authors and organizations including the International Atomic Energy Agency, are also reviewed and presented. Also included are LBB options and licensing issues raised in the process of regulatory actions in the United States, along with the outlook and perspectives for LBB in the new generation of nuclear power plants.