The use of Carbon Fiber Reinforced Polymer (CFRP) in nuclear industry is very limited and has not been used for nuclear safety related application until recently. In 2019, a new ASME Boiler and Pressure Vessel (BPV) Code Case N-871 was approved for internal repair of buried Class 2 and 3 nuclear safety related piping using CFRP for Service Levels A, B, C and D for a service period of 50 years. However, US Nuclear Regulatory Commission (NRC) has not yet accepted the Code Case. This Code Case provides a very detail guidance on materials, fabrication, installation, and design of CFRP repair for Class 2 and 3 nuclear safety related piping.
There are two aspects in designing CFRP repair for nuclear safety related piping application in terms of service duration — short-term use condition and end-use (end of service life) condition. The final design must satisfy both conditions in terms of available safety margin for the entire period of service. In contrast to end-use condition, some of CFRP materials’ degradation phenomena such as time effect factor due to sustained loading, material adjustment factors due to environmental exposure do not affect design calculation for short-term use. However, while end-use condition design criteria may control the final CFRP repair design, it is equally important to understand the available safety margin for short-term use.
In this paper, the available safety margin in ASME BPV Code Case N-871 was evaluated for internal CFRP repair of nuclear safety related piping for short-term use by conducting a full-scale hydrostatic test. The full-scale hydrostatic test was conducted on 40” outside diameter steel pipe. The pipe contains a postulated flaw (cut-out) with an internal CFRP repair designed according to ASME BPV Code Case N-871. The internal CFRP repair consisted of three unidirectional CFRP layers applied over the flaw 360° around the circumference at the inside surface of the pipe. Two glass fiber reinforced polymer (GFRP) layers were also applied as dielectric barrier and watertightness layer. The design pressure and temperature for the hydrostatic test was 84 psi and 72F. The pipe with internal CFRP repair was pressurized until failure (leak) occurred. The details of CFRP repair installation and hydrostatic test are presented in this paper. Finally, the available safety margin for short-term use was evaluated. Detailed analysis of strain gage data, bulging measurements and the video revealed that potential failure mechanism in the full-scale hydrostatic test of pipe with an internal CFRP repair and a postulated flaw (cut out).