In the mid 1980’s, the US Nuclear Regulatory Commission (NRC) began to accept the concept of leak-before-break (LBB) as a means of enhancing the safety of nuclear power plants. The LBB concept has permitted the removal, or non-installation, of many of the pipe-whip-restraint devices and jet-impingement shields originally designed to mitigate the dynamic effects of a postulated instantaneous pipe rupture. The LBB concept has enhanced the overall plant safety through increased access to critical systems for in-service inspection and monitoring, and is based on enhanced knowledge of pipe loads, material properties, and cracked-pipe behavior. One of the stipulations in the NRC’s Standard Review Plan 3.6.3 for Leak-Before-Break (LBB) assessment is that the piping system under consideration not be susceptible to an active degradation mechanism. However, it is known that Primary Water Stress Corrosion Cracking (PWSCC) is now occurring in a number of systems that have previously been granted LBB exemptions to remove pipe-whip restraints and jet impingement shields.
In an effort to address this apparent inconsistency, an assessment tool is being developed that can be used to evaluate compliance with the 10CFR50 Appendix A, General Design Criterion 4 (GDC-4) requirement that primary system piping exhibit an extremely low probability of rupture. That tool (the xLPR [eXtremely Low Probability of Rupture] probabilistic fracture mechanics [PFM] code) will be comprehensive with respect to known challenges, vetted with respect to scientific adequacy of models and inputs, flexible enough to permit analysis of a variety of in-service situations, and adaptable to accommodate evolving and improving knowledge.
To support this new probabilistic tool (xLPR code), technical issues relating to the LBB problem must be resolved before the uncertainties in the parameters are known. Throughout the 1980’s and 1990’s, extensive research was conducted by NRC staff through their contractors on the stability of cracks in nuclear piping. The vast majority of these experiments and the developed methodologies focused on idealized shaped cracks in similar metals welds and their base metals. However, with the occurrence of PWSCC in dissimilar metal (DM) welds, i.e., nickel based welds between carbon steel and stainless steel base metals, the crack stability characteristics are unknown.
To fill in the lack of knowledge of DM weld behavior, the US NRC conducted an experimental research program to develop data on the stability characteristics of complex-shaped cracks in DM welds. These experimental results will be used to determine if the current analysis methodologies used in previous LBB applications can be used to predict the load-carrying capacity of such cracked pipe. If they cannot, adjustments will be made to improve the accuracy of such methodologies.
In order to address the objective of the NRC’s test program, a series of DW weld pipe fracture experiments was conducted. In addition to the pipe fracture experiments, a series of material characterization tests were conducted on both the base metal materials, as well as the weld materials. This paper will discuss some of the unique challenges in conducting these types of experiments and will present a sampling of the results from these experiments.