A Methodology to Predict Critical Flaw Size in HDPE Piping and Joints in Nuclear Safety-Related Applications [PE]

The ASME Boiler and Pressure Vessel Codes Committee (BPVC) has recently published Revision 1 of Code Case N-755 that describes the requirements for the use of High Density Polyethylene (HDPE) pipe for the construction of Section III, Division 1 Class 3 buried piping systems for service water applications in nuclear power plants. The Code Case was developed by Special Working Group–PE Pipe (SWG-PP) within Section III (Design) of the Boiler and Pressure Vessel Code (BPVC). The United States Nuclear Regulatory Commission (USNRC) has not as yet approved this Code Case for generic use in regulatory decisions. Since 2007, Emc2 has been conducting confirmatory research on the issues for the USNRC involving structural integrity and service life of both HDPE pipe and fusion joints in safety-related service water applications. As is well established, the limiting failure mode for the long term performance of HDPE pressure piping is slow crack growth (SCG) which is governed by the sustained stress levels (pressure) and increases exponentially with elevated temperature. Emc2 has undertaken a comprehensive approach to studying SCG in both parent HDPE material involving a combination of experiments and analysis. This paper outlines a fracture mechanics based methodology to analyze stress rupture data from notched pipe tests to establish the critical flaw size for crack initiation. This approach may be use to predict the allowable flaw size for given service loading conditions and pipe geometry to establish safety margins during the expected service life of PE piping.