Issue 50

I. G. F. Silva et alii, Frattura ed Integrità Strutturale, 50 (2019) 46-53; DOI: 10.3221/IGF-ESIS.50.06 47 K EYWORDS . Leak-Before-Break; LBB; Piping material; Material performance; PWR reactor; Fracture mechanics. I NTRODUCTION uclear power plants use nuclear fission as a source of heat for energy production. Currently, 450 nuclear reactors are in operation in the world, producing about 400 GW of electrical capacity; of these units in operation, 298 are PWR (Pressurized Water Reactor). In addition, 55 nuclear reactors are under construction, 45 of them being PWR [1]. The operating principle of a PWR nuclear plant is based on the removal of heat from the reactor core through a closed circuit of high pressure water, called the primary circuit. The water heated under high pressure in the primary circuit passes through a steam generator where it heats and turns into steam the water of the secondary circuit. This steam moves a turbine that drives an electric generator. The electricity generated reaches final consumers through distribution networks. Pipes that are part of the PWR reactors are commonly manufactured from stainless steels or high toughness low alloy steels that are resistant to unstable defect growth. A crack in the piping should cause a leakage in a considerable amount, allowing its identification and quantification, before a growth can occur that would lead to a sudden rupture of the pipe. This is the essence of the Leak-Before-Break (LBB) concept. A fundamental step in the application of the LBB concept is the evaluation of the stability of postulated through-wall crack in a given pipe system. Because they are built using ductile materials, this evaluation is made through the concepts of Elastic-Plastic Fracture Mechanics (EPFM). Through a fundamentally technical justification, the LBB concept has been widely applied in nuclear installation projects in several countries [2, 3, 4, 5]. LBB CONCEPT ccording to the United States Atomic Energy Commission [6], the GDC (General Design Criterion) 4 requires that structures, systems, and components important for safety be designed to accommodate the environmental and dynamic effects associated with normal operation, and even with postulated accidents. It indicates that dynamic loading from the effects of these conditions, including missiles, pipe whipping, and discharging fluids must be verified in the designs. In the case of primary circuit piping, the regulators of nuclear activity required, in the early 1970s, that nuclear plant designs take into account the hypothesis of a sudden rupture of the complete transversal section of a pipe (DEGB - Double-Ended Guillotine Break), causing a loss of coolant (LOCA-Loss-of-coolant Accident). This requirement, when applied to a high energy system, as the primary circuit of the reactor, demanded the consideration of two dynamic effects caused by sudden pipe rupture, whipping and jet effects. Thus, to protect the essential safety equipment of a nuclear plant, it was necessary the use of devices as pipe whip restraints and jet impingement shield. For about a decade, the nuclear industry has sought solutions to disconsider the dynamic effects of a DEGB. So, the LBB methodology emerged as a technically justifiable approach to remove such analysis of the design bases of nuclear plants, leading to the realization of several studies between the 1970s and 1980s and acceptance by the regulators of nuclear activity [7]. The USNRC (United States Nuclear Regulatory Commission) established in the early 1980's the Pipe Break Task Group whose work culminated in the publication NUREG-1061 [8], which outlined the feasibility of applying the LBB concept. Based on this document, the document NUREG-0800-SRP 3.6.3 [9] was elaborated, where is characterized the deterministic evaluation procedure for application of the LBB concept proposed by the USNRC. These documents became a basic reference for the implementation of the criteria associated to the LBB concept in the American plants and, by extension, in other countries. Since LBB is demonstrated, such ruptures can be excluded from the design bases, and the dynamic efforts resulting from LOCA are not considered in the structural analysis of the plant systems. As a result, it is no longer necessary to design components, equipment or piping supports of primary circuit to cope with such dynamic loads. Another advantage of the application of LBB is that pipe whip restraints and jet impingement shield, both required to protect important equipment N A