Fasteners used in operating nuclear power plants are subjected to specific design rules. The regulatory basis for these rules is contained in Part 50 of Title 10 of the Code of Federal Regulations (10 CFR Part 50). The Code specifies that safety-related components in an operating nuclear power plant be designed in accordance with the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. Section III of the Code sets design rules for tensile loads, bending loads, fatigue stresses, and stress-corrosion-cracking load limits for fasteners larger than 1 in. (2.54 cm) in diameter. The ASME Code also permits design of fasteners by analysis using generally accepted methods such as finite element analysis in place of the ASME Code recommended methods. Section XI of the ASME Code describes inspection requirements for fasteners. Typical inspection methods covered by this code are visual inspection, magnetic particle inspection, liquid penetrant inspection and ultrasonic inspection.
There has been a great deal of interest in safety-related fasteners in nuclear power plants because of the many fasteners that failed in the 1970s and early 1980s. In the early 1980s, the U.S. Nuclear Regulatory Commission (NRC) established Generic Safety Issue 29, “Bolting Degradation or Failure in Nuclear Power Plants.” The industry responded to this generic safety issue by forming a Joint Task Group on Bolting that included the Atomic Industrial Forum, the Materials Properties Council, and the Electric Power Research Institute (EPRI). Generic Safety Issue 29 was resolved in October 1991 as a result of the work of the Joint Task Group on Bolting and numerous NRC actions.
Typical examples of fastener degradation will be discussed in this paper along with the regulatory response that was taken to minimize the probability of reoccurrence.