Evaluation of Dynamic J-R Curve for Leak Before Break Design of Nuclear Reactor Coolant Piping System

Abstract

Because safety is of paramount importance in the nuclear industry, numerous efforts have been made to guarantee structural integrity against sudden accidents. In the past, design against a Double Ended Guillotine Break (DEGB) was accomplished through the construction of massive pipe whip restraints and jet impingement shields to minimize the secondary damage to other structures in close proximity to ruptured piping. However, through long-term operating experience, the commercial nuclear industry has recognized that, for most damaged piping, fluid leakage from through-wall cracks occurs prior to a DEGB accident. Hence, if the leakage can be detected reliably at an early stage of fracture, a DEGB accident can be prevented by shutting down the reactor prior to the DEGB. LeakBefore-Break (LBB) design is based on this concept. For a piping system where LBB design is applied, a leak detection monitoring system must be installed to detect crack initiation while construction of massive pipe whip restraints and jet impingement shields become unnecessary. Thus, LBB design focuses on the ability to detect cracks for structural integrity while DEGB design focuses on preventing secondary damage. Since the mid-1980s, the LBB design concept has been widely applied on nuclear high energy piping systems. In Korea, the LBB design concept based on U.S. nuclear regulatory commission (USNRC) standard review plan 3.6.3 and NUREG-1061 has been applied to reactor coolant piping systems ever since the Yong-Gwang units 3 & 4 nuclear power plants were approved in 1994 (J.B.Lee & Choi, 1999). The LBB design applied to nuclear piping systems is based on the premise that a piping break accident can be prevented by detecting leakage from a through-wall crack by leak detection instrumentation prior to a DEGB accident. To meet LBB design criteria, the nuclear piping material must have excellent fracture toughness characteristics so that a sudden break will not occur even if the piping has a large through-wall crack that corresponds to a detectable leakage rate. For LBB design, material properties for stress – strain curves and J-R curves as a function of resistance to stable crack extension at service temperatures are needed. The stress – strain curve is for use in the determination of detectable leakage crack length and the elastic-plastic finite element analysis of the piping with a through-wall crack. The J-R curve is for use in the crack stability evaluation of piping under normal operating loads and safe shutdown earthquake loads. In the Korean standard nuclear power plant, shown in Fig. 1, carbon steel with stainless steel cladding is used for the hot leg pipe and the